Message Switching

Home , Circuit switching, Message switching, Store and forward

Dr. Farid Farahmand, Ph.D, Central Connecticut State University

Introduction 1 Message-Switched Protocols 7 Basic Ideas in Message Switching 1 Performance of Message-Switched Networks 9 Message Switching Characteristics 2 Delay Performance 9 Message-Switched Networks 2 Node Delay 9 Network Elements 2 Comparison of Message, Packet, and Store-and-Forward Switches 3 Circuit Switching 11 Message-based Protocols and the OSI Model 4 Conclusion 12 Message-Switched Networks and Glossary 12 their Applications 5 References 12 Store-and-Forward Networks 6 Further Reading 13

INTRODUCTION Prior to advances in packet switching, message switching was introduced as an effective alternative to circuit A communication network consists of a collection of switching. In message switching, end-users communi- devices (or nodes) that wish to communicate and inter- cate by sending each other a message , which contains the connect together. The primary objective in any commu- entire data being delivered from the source to destina- nication network is simply moving information from one tion node. As a message is routed from its source to its source to one or more destination nodes. Based on the destination, each intermediate switch within the net- techniques used to transfer data, communication networks work stores the entire message, providing a very reliable can be categorized into broadcast and switched net- service. In fact, when congestion occurs or all network works. In broadcast networks, data transmitted by one node resources are occupied, rather than discarding the traffi c, is received by many, sometimes all, of the other nodes. the message-switched network will store and delay the In switched-communication networks, however, the data traffi c until suffi cient resources are available for success- transferred from source to destination is routed through ful delivery of the message (Davis, 1973). The message the switch nodes. The way in which the nodes switch data storing capability can also lead to reducing the cost of from one link to another as it is transmitted from source transmission; for example, messages can be delivered at to destination node is referred to as a switching technique . night when transmission costs are typically lower. Three common switching techniques are circuit switch- Message switching techniques were originally used in ing, packet switching, and message switching. data communications. Early examples of message switch- In circuit switching, the end-users are interconnected ing applications are paper tape relay systems and telex using dedicated paths. The most common example of a networks. Electronic mail (e-mail) and voice mail are also circuit-switched communications network is the plain examples of message switching systems. Today, message old telephone service (POTS) network. One major issue switching is used in many networks, including ad hoc with circuit switching is that it can be rather ineffi cient, sensor networks, satellite communications networks, and particularly in data communications (Stallings, 2004). military networks. This is because in a circuit-switched network the channel capacity is dedicated for the entire duration of a connection, even if no information is being transferred. Further- Basic Ideas in Message Switching more, in a circuit-switched network, the actual time to Message-switched data networks are hop-by-hop sys- setup and tear down the path may in fact be longer than tems that support two distinct characteristics: store-and- the data transfer time between end-users. Refer to Chap- forward and message delivery. ter 73 for a complete discussion on circuit switching. In a message-switched network, there is no direct con- In packet switching, data is divided into smaller units, nection between the source and destination nodes. In called packets, and then transmitted through the network. such networks, the intermediary nodes (switches) have The minimum length of the packet is determined by the the responsibility of conveying the received message network and varies from network to network. Packets are from one node to another in the network. Therefore, each transported over the network between nodes. Each inter- intermediary node within the network must store all mes- mediate node queues the packet for a brief time while it sages before retransmitting them one at a time as proper determines the route the packet should take to reach the resources become available. This characteristic is often next node. For this reason, a packet-switched network referred to as store-and-forward (Stallings, 2004) . In mes- is sometimes called hold-and-forward network (Tomasi, sage switching systems (also called store-and-forward 2004) . Please refer to Chapter 72 for a complete discus- systems), the responsibility of the message delivery is sion on packet-switched networks. on the next hop, as the message travels through the path

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toward its destination. Hence, to ensure proper delivery, regulation, and message storage when messages cannot each intermediate switch may maintain a copy of the be delivered. However, there are several drawbacks asso- message until its delivery to the next hop is guaranteed. ciated with message switching techniques. For example, In case of message broadcasting, multiple copies may be since messages are entirely processed and stored indefi - stored for each individual destination node. nitely at each intermediate node, switches require large The store-and-forward property of message-switched storage capacity (Davis, 1973). networks is different from queuing , in which messages In general, message-switched networks are relatively are simply stored until their preceding messages are slow. This is because all messages must be entirely stored, processed. With store-and-forward capability, a message processed, and retransmitted. This slow switching scheme will only be delivered if the next hop and the link con- may lead to poor performance, particularly in wide area necting to it are both available. Otherwise, the message networks (WAN). Although, using various scheduling is stored indefi nitely. For example, consider a mail server techniques, time critical messages can be given higher that is disconnected from the network and cannot receive transmission priority, it is always possible that a short the messages directed to it. In this case, the intermediary message falls in line behind a very long one. Consequently, server must store all messages until the mail server is con- in message switching, messages initiated by different us- nected and receives the e-mails. The store-and-forward ers can expect large delay variance. technology is also different from admission control tech- Large overall delay and high delay variations make niques implemented in packet-switched or circuit- message-switched networks less suitable for real-time and switched networks. Using admission control, the data interactive applications, such as voice communications or transmission can temporarily be delayed to avoid over- multimedia games. Instead, message-switched networks provisioning the resources. Hence, a message-switched are very attractive for supporting applications which can network can also implement an admission control mech- tolerate high delays, yet very little loss. Furthermore, mes- anism to reduce network’s peak load. sage switching can be an alternative solution for networks The message delivery in message-switched networks where continuous end-to-end connectivity cannot be includes wrapping the entire information in a single guaranteed, such as ad hoc sensor networks. A detailed message and transferring it from the source to the des- discussion on advantages and disadvantages of mes- tination node. The message size has no upper bound; sage switching compared to circuit switching and packet although some messages can be as small as a simple da- switching can be found in Subsection Comparison of tabase query, others can be very large. For example, mes- Message, Packet, and Circuit Switching in this chapter. sages obtained from a meteorological database center can contain several million bytes of binary data. Practical limitations in storage devices and switches, however, can MESSAGE-SWITCHED NETWORKS enforce limits on message length. In this section we fi rst describe different elements of a Each message must be delivered with a header. The message-switched network and examine their basic func- header often contains the message routing information, tionalities. Then we focus on store-and-forward switch including the source and destination, priority level, ex- node architecture and explain its building blocks. piration time. Figure 1 shows an example of a message datagram with possible information embedded in the message header. Network Elements It is worth mentioning that while a message is being Figure 2 shows a generic message-switched network stored at the source or any other intermediary node in the allowing message transport from one end-user to another. network, it can be bundled or aggregated with other mes- This network consists of transmission links (channels), sages going to the next node. This is known as message store-and-forward switch nodes, and end stations (Tomasi, interleaving . One important advantage of message inter- 2004) . The transmission links differ depending on the leaving is that it can reduce the amount of overhead gen- transmission technology and the physical transmission erated in the network, resulting in higher link utilization. media over which the communications take place. The store-and-forward switch nodes provide end stations Message Switching Characteristics with access to transmission links and other switch nodes. Message switching offers a number of attractive benefi ts, They allow data storage and transport of messages from including effi cient usage of network resources, traffi c one node to another until they reach their destination

Message Datagram Size (in bytes) = M + H

Header User Message (H bytes) (M bytes)

Message Header Possible Contents Message Source Message Expiration Time Message Destination Message Arrival Time Intermediary Nodes Visited Message Id Message Priority Error Control (e.g., Checksum or CRC) Figure 1: An example of a message datagram structure

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Terminal (1) Terminal (2)

SW A SW B Message

Store-&- Forward Switch

SW C SW D

Email Server Mainframe Figure 2: A message-switched network

nodes. The end stations can be individual computer ter- The input processing controller can also check for any minals, servers, and mainframes. Many networks may be errors in the message. Upon detection of any errors, it equipped with message gateways, which can convert one requests message retransmission. Examples of message message protocol to another. errors include transmission errors and invalid addressing. Accepted messages with valid headers are stored in storage devices. Any stored message can be accessed or requested for retransmission using a specifi c id or Store-and-Forward Switches serial number. After delivery, a copy of the message will In general, a switch is a connectivity device that makes be stored for future retransmission. These messages may forwarding decisions. A store-and-forward switch per- be retained for many hours, days, or even weeks. The forms forwarding functionalities similar to a switch in a lookup table maintains the forwarding information and packet-switched network. However, a store-and-forward keeps track of which messages are waiting for delivery or switch forwards a message only if suffi cient resources have already been delivered. are available and the next hop is accepting new data. When the output phase begins, the output processing Hence, the switching strategy in store-and-forward controller retrieves the proper message from the storage switches is different from the common switching strategy device and after assigning a header, passes it to the out- used in packet-switched networks in which the incom- put buffer queue and line coder block for appropriate for- ing data is forwarded as soon as its destination address is matting and transmission. The output buffer queue can determined. be used to control the message transmission rate. Figure 3 shows the basic functional blocks of a generic The output processing controller is also responsible for store-and-forward switch node (Shafritz, 1964). In the deciding how to forward the message to the next hop. In following paragraphs we briefl y outline the basic func- general, forwarding decisions used in message-switched tionalities of each block. networks are similar to routing schemes implemented in The message received by a store-and-forward switch packet-switched networks. For example, forwarding deci- is fi rst decoded and queued. When the input processing sions can be based on the least congested link, or shortest controller becomes available, it receives the message and end-to-end path (Tanenbaum, 2002) . A number of routing analyzes its header for its destination, associated id, and techniques in packet-switched networks are discussed in priority level. Chapter 72 .

Message in Line Input Input Buffer Decoder Processing Queue RX Controller

Primary/ Statistics Secondary Storage Lookup Table Devices

Message out Line Output Output Buffer Coder Processing Figure 3: A generic store-and-forward Queue TX Controller switch node architecture

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The exact characteristics of message-switched networks is working again. The switch can also reroute the message vary depending on features supported by the store-and- to other alternative recipients acquiring the same mes- forward switch nodes in the network. In the following sage. Message interception can also be considered for paragraphs, we elaborate on storage requirements and cases where the destination node has moved and it is no other features commonly provided by store-and-forward longer in its previous location. In such cases, assuming switch nodes. the system can locate the user, the message will be redi- The storage devices used in a store-and-forward switch rected to the new location. node have large storage capacities. This is because mes- A store-and-forward switch that can also act as a sages delivered to each switch are often long and may be message gateway is often called a transactional switch stored in the switch indefi nitely until suffi cient resources (Tomasi, 2004) . In addition to storing messages and per- are available in order to forward them to the next node. forming message routing, a transactional switch can In cases where a message has to be delivered to multi- change the message format and bit rate and then convert ple destination nodes, multiple copies of the messages it to an entirely different form. Hence, through message may be stored for a potentially long duration. A copy reformatting, message switching can effectively allow for of the transmitted message may be stored for possible translating messages between dissimilar high-level appli- retransmission until message delivery to the next hop is cations and protocols. confi rmed. Moreover, the storage capacity must be large enough to avoid any types of data overfl ow and there- Message-based Protocols and the fore data loss. The actual storage capacity of the switch is based on the desired performance and can be estimated OSI Model through statistical analyses. Clearly, as the message In order to better understand functionalities of different arrival rate increases, the amount of data stored in the message-based protocols, in this section we conform these switch becomes greater, leading to larger storage require- protocols to the seven-layer Open Systems Interconnec- ment. Larger storage capacity, on the other hand, leads tions/Reference Model (OSI/RM). Different layers of the to requiring faster processors and more effi cient search OSI model are depicted in Figure 4 . As we mentioned be- algorithms. fore, message transmission and message switching over Each store-and-forward switch node is typically pro- the physical layer are performed one hop at a time. Each grammed to detect, report, and record any fault conditions, intermediate store-and-forward switch node receives including presence of excessive noise, system tampering, the entire message, stores it, and forwards it to the next and link failure. Switch nodes must also become aware hop. A network using this technique is called a store-and- of each other’s status. For example, in order to notify its forward network. neighboring nodes that the node is alive , each node can pe- Data-link layer protocols in message-switched net- riodically send each neighboring node the serial number works deal with the algorithms (e.g., error detection of the last message it received from that node. and message fl ow regulation) for achieving reliable and A common capability of store-and-forward switches is effi cient message passing between two neighboring store- supporting message priority. Based on parameters such and-forward switch nodes. Another important function as delay (or loss) tolerance or message length, every mes- of such protocols is providing proper services to the net- sage can have a particular priority level. Messages with work layer (Tanenbaum, 2002) . Such services include the higher priority must be treated fi rst and forwarded in following: (a) unacknowledged connection-less service minimum time. Supporting message priority can add (the sender sends the message randomly and expects considerable complexity to message storage strategies, no response back from the receiver); (b) acknowledged requiring separate listing for each priority group. Fur- connection-less service (the sender sends the message thermore, the system must allow high-urgency messages randomly and expects some kind of response back from to interrupt others during their transmission. The inter- the receiver); (c) connection-oriented service (prior to rupted messages will be automatically retransmitted as sending the message, a direct circuit must be established soon as possible, with no further action by the sender. between the two adjacent nodes). A system that does not support message priority can Depending on the data-link protocol, different retrans- simply handle incoming messages on a fi rst-in-fi rst-out mission mechanisms, if any, may be implemented. A basis. simple approach is to send an acknowledgement frame A store-and-forward switch can also perform various back to the sender. In this case, the sender can assume statistical analysis, billing operations, and status reports. proper reception of the message by the downstream node For example, it can examine each message for its number and remove its copy of the message. In some networks of characters in order to charge the sending party. It may the retransmission occurs when a negative acknowledg- also analyze message headers to gather statistical infor- ment or simply nothing is received. mation on various network links. The status report can The main functionality of the network layer in mes- include the status of facilities, error statistics, and message switching is to provide routing algorithms, such as sage counts. source routing and shortest-path routing. Based on the An important function of a store-and-forward switch routing procedure, the intermediate store-and-forward is that it can intercept a message for various reasons. For node must determine the next hop and forward the mes- example, if the destination node is temporarily unavaila- sage there. The network layer is also responsible for sup- ble, the switch can store the message until the destination porting message broadcasting and message multicasting.

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Store & Forward Switch Node Conversion Source Destination Application Layer 7 7 7 7 (e.g., SMTP, X.400)

Presentation Layer 6 Middle 6 6 6 ware (e.g., MOM) Session Layer 5 5 5 5

Transport Layer 4 4 4 4

Network Layer 3 3 3 3 (e.g., Message Routing)

Data Link Layer 2 2 2 2 (e.g., Error Detection, Message Flow Regulation)

Physical Layer 1 1 1 1 (e.g., Message Transmission and Message Switching) Message Message

Figure 4: Store-and-forward technology can be implemented at different layers of the OSI model

In general, message-switched networks do not require MESSAGE-SWITCHED NETWORKS AND an offi cial transport protocol. In such networks, each pair of interconnected store-and-forward switch node can THEIR APPLICATIONS negotiate the use of any desired transport protocol (or The basic concept of store-and-forward implemented in none at all), as long as the underlying communication is message switching has been around since the early use of reliable. smoke signals and beacons. The postal delivery system in Because in message switching the entire message is the eighteenth century, which was not signifi cantly differ- delivered and stored at the next hop, message-based pro- ent from the current postal system, also used store-and- tocols can offer features beyond the transport layer. These forward techniques. In this system a piece of mail (e.g., protocols are typically application-aware and hence, they a parcel) is dropped off at the local post offi ce. The mail can operate in various transmission environments. For will be picked up and taken to the next post offi ce clos- example, they are capable of tolerating temporary node est to the destination when the delivery service becomes unavailability or converting message format. Two com- available. Eventually, the mail is delivered to the recipient mon examples of message-based protocols are X.400 and or stored in the destination post offi ce, waiting for pickup SMTP, which will be discussed in Store-and-Forward by the recipient. We should note that in the postal deliv- Electronic Mail later in this chapter. ery system example, proper delivery to the next station is The store-and-forward technology can also be imple- typically assumed. mented in middleware . Middleware is a software that In the late eighteenth century, semaphore networks provides a translation mechanism, interconnecting became popular. These networks used visual signals application software across a network, such as a server/ (fl ags, lamps, heliographs) to convey messages between client network, and creating a single-system image. One operators in towers. Each tower, in turn, would relay the category of middleware that utilizes store-and-forward message to the next until the message was delivered. technology and provides program-to-program commu- The store-and-forward method was also implemented nications by message passing is called Message-Oriented in telegraph message switching centers, commonly Middleware (MOM). We discuss MOM in Message-Based known as torn-tape centers (Davis, 1973). In these cent- Client/Server Systems later in this chapter. ers operators tore message tapes from tape punches on Figure 4 shows that an incoming message into the incoming circuits and stored them in output baskets store-and-forward switch node can be entirely reformat- according to their outgoing address. Then, when the dis- ted prior to its retransmission. In many systems a copy patcher clerk became available, the messages were keyed of each message will be stored until its reception by the in an appropriate circuit to the next center. Telegraphy next hop is acknowledged. Message storage can be per- messages were also called telegrams or cable grams. formed at different layers, including the network layer The development of mechanical teleprinters, also or application layer. Data-link and possibly other higher known as teletypewriters or teletypes, resulted in an layer protocols are responsible to provide message status increase in the message carrying capacity of telegraph information. Note that middleware mediates between top lines. Furthermore, by using the teleprinter terminals layers of the OSI reference model. the operators were no longer required to know special

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codes (e.g., Morse code). The telex, short for teleprinter statement, and a subject line. A very important aspect of exchange, was introduced as a public service using tel- AUTODIN was providing highly secure communications. eprinters connected through automatic exchanges (telex Hence, all external connections to the network from switching facilities). Using the telex network a subscriber telephone lines were subject to strong encryption tech- can type out the telex number or code for the called party, niques. wait for an acknowledgment from the called machine, For the past several decades, AUTODIN has been and then transmit. The printed message is often stored at regarded as a highly sophisticated message-switched the other end even if the called teleprinter is not staffed. network. It also had major contributions in advancing Advancements in digital computers and digital stor- data communications. For example, in 1966 AUTODIN age devices made radical improvements in message was one of the fi rst systems that allowed electronic text switching systems. During the 60s and 70s, the store-and- messages to be transferred between users on different forward technology was largely promoted by Western computers. Union, which traditionally was a message carrying or- In 1982 an enhanced program, known as AUTODIN II, ganization. Due to long distances between cities in the aiming to improve the system performance and security, United States, Western Union primarily was focusing on was terminated in favor of using packet-based ARPANET store-and-forward techniques to maximize line utilization technology. As of 2003, many AUTODIN switching sites in order to offer telex and telegraph, as well as variety of have been shut down. The intention is to replace the old computer-based services. system with the Defense Message System (DMS). DMS Today, although many major networks and systems is based on implementations of the OSI X.400 e-mail, are packet-switched or circuit switched networks, their which provides message services, through supporting delivery processes can be based on message switching. multimedia messaging, directory services, and different For example, in most electronic mail systems the delivery grades of services, to all Department of Defense users. process is based on message switching, while the network is in fact either circuit-switched or packet-switched. SITA and AFTN Networks Société Internationale de Télécommunications Aéro- Store-and-Forward Networks nautiques (SITA) was originally established in 1949 by a group of airlines. The purpose of the project was to pro- Message switching technology has been the forerunner of vide computer-based seat reservation service to facilitate today’s advanced data communications. It set the trends the sale of seats on airplanes and exchange of operational for packet switching technology and more sophisticated information (Martin, 1985). SITA organized a common networks of the twenty-fi rst century. In fact, many of the global service supported by a low-speed message switch- early data networks and communications services were ing network that was based on teletype message format actually built upon the message-switched concept and generated by teleprinters. store-and-forward technology. In the remainder of this Aeronautical Fixed Telecommunication Network section, we fi rst discuss a few of such networks, which (AFTN) is also another example of a worldwide message are considered as major breakthroughs in development switching network. After World War II, following SITA of current complex data communications networks. experience, AFTN was introduced to support exchange Then, we examine some of the common communications of messages between aeronautical fi xed stations and air- protocols that are designed based on store-and-forward crafts. AFTN is composed of aviation entities, including technology. aviation service providers, airport authorities, and govern- ment agencies. It exchanges vital information for aircraft AUTODIN Network operations such as urgency messages, fl ight safety mes- The Automatic Digital Network (AUTODIN), developed sages, meteorological messages, fl ight regularity messages, in the mid-1960s, is one of the largest and most complex and aeronautical administrative messages (Bush, 2003). message switching systems. Using store-and-forward Today, the airline industry continues to use teletype switching technology, AUTODIN provided a message- messages over SITA and AFTN as a medium for commu- based worldwide communication network for the U.S. nicating via messages. These teletype messages are military, handling the data communication needs for the machine-generated by automatic processes. The Inter- Department of Defense and other Federal organizations national Air Transport Association (IATA) is responsible (the alternative system to handle voice communications for standardizing the message format throughout the air- was AUTOVON) (FAS, 2003) . AUTODIN has also been line industry. In the last several decades, however, as the used by other major agencies including the NSA (National packet switching has become the standard means of tele- Security Agency), the DIA (Defense Intelligence Agency), communications, both SITA and AFTN networks have un- and NATO (North Atlantic Treaty Organization). dergone major reengineering and their nodes have been AUTODIN was originally considered as a dedicated replaced to support packet switching. backbone network serving primarily for secret data transmission. All switches and routers in the network were con- USENET Network trolled by a system called the AUTODIN Switching Center. USENET (USEr NETwork) began in 1979 as a bulletin The system was designed to run at 2400 bits-per-second board between two universities in North Carolina. It is (bps); however, speeds up to 9600 bps were possible. basically a public access network on the Internet and The header of each message sent by AUTODIN in- other TCP/IP-based networks that provides user news cluded source, destination, security classifi cation, priority and group e-mail.

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USENET provides a set of protocols for generating, hence reducing the number of packets being routed storing, retrieving, and exchanging news articles among through the network (Fall, 2002). Aggregated messages a widely distributed user group. The format of news mes- are often stored, scheduled, and eventually forwarded sages and e-mail messages are structurally the same. toward the destination node. The other benefi t of imple- Hence, USENET permits news messages to be carried the menting message switching is that it does not require same way as e-mail messages. The only difference is pre-established end-to-end state of the network. Using the additional header fi elds indicating which newsgroups store-and-forward technology, each node discovers its (a repository of articles posted by the users) the message available neighboring nodes and forwards them the mes- belongs to. sages. Research in challenged networks is a relatively USENET is considered as one of the fi rst peer-to-peer new area and is still being investigated. applications. In this case the peers are the servers and can be accessed by the users. A major advantage of USENET is that it reduces network traffi c and eliminates the need Message-Switched Protocols to store a copy of each article on every subscriber’s sys- In this subsection we look at some of the most common tem. This is done by putting all articles in a central data- protocols that are based on store-and-forward technol- base and allowing users to access the database to get the ogy. As a result of their vast applications, we only focus on articles they need (Tanenbaum, 2002). three specifi c systems: electronic mail systems, message- Originally, USENET relied on a message exchange based client/server systems, and wireless messaging system called UUCP (UNIX-to-UNIX Copy Program). systems. UUCP is a fl ood broadcast mechanism. Hosts send news articles they receive to other hosts, which in turn forward Store-and-Forward Electronic Mail the news on to other hosts that they feed. Usually, a host With the advancement in digital technology and digital receives duplicates of articles and must discard those data communications, sending electronic data, text mes- duplicates. This can be a time-consuming process, result- sages, documents, as well as voice and images, collectively ing in waste of bandwidth. known as electronic mail (e-mail), became very popular. Today, message delivery in USENET is primarily han- The fi rst e-mail systems simply consisted of fi le trans- dled by an Internet protocol called NNTP (network news fer protocols. Over the years, many new protocols were transport protocol). NNTP uses an interactive command developed for more ambitious e-mail systems. The major- and response mechanism that lets hosts determine which ity of these protocols have been designed based on store- articles are to be transmitted. A host acting as a client and-forward technology. In the following paragraphs, we contacts a server host using NNTP, and then inquires if briefl y discuss two e-mail delivery protocols: X.400 and any new newsgroups have been created on any of the SMTP. serving host systems. X.400. The main motivation in development of X.400 Performance-Challenged Networks was to create a common standard that would allow dif- challenged Performance-challenged networks (or simply ferent electronic mail systems to communicate together. networks ) are interconnected networks in which end-to- X.400 (also referred to as message-oriented text inter- end latency, bandwidth asymmetry, and/or path stability change systems [MOTIS]) is a set of standards developed are substantially worse compared to typical Internet en- by ITU-T (the International Telecommunications Union- vironments. Such network environments are particularly Telecommunications, formerly known as CCITT), which common in space or ocean (acoustic underwater) com- includes protocols dealing with message handling sys- munications, sensor networks, and military tactical tems (MHS). These standards are designed to exchange communications, all of which lack an infrastructure. messages (e-mails) between different store-and-forward In challenged networks the signal propagation time servers and networks. In its original form, X.400 out- is comparatively long as a result of either long physi- lines a set of basic message-handling characteristics that cal separation of end nodes, as in space, or because of establish functionalities including the following: slow transmission speed, as in water. Consequently, the total transmission time in such networks can take liter- • Store-and-forward delivery of messages to multiple ally hours or perhaps days. Protocols such as TCP/IP that recipients expect acknowledgments may not be appropriate for such environments. This is mainly because too many timeouts • Conversion of message content to allow message trans- can occur because of heavy congestions in the low capac- fer between dissimilar sending and receiving devices ity links. Another issue with challenged networks is path (fax, telex, PC) instability, which is generally the result of short node life- • Delivery-time control time or mobility. For example, in sensor networks many of the sensors may run out of battery power after some X.400 was fi rst published in 1984 (Red Book) and a sub- period of time (Fall, 2003) . stantially revised version, known as X.400/88, was pub- One approach to support interconnecting challenged lished in 1988 (Blue Book). In 1992, new features and networks and their interoperability is called delay toler- updates were added (White Book) (Betanov, 1992) . Al- ance networking (DTN), which is based on message switch- though X.400 was originally designed to accommodate ing. The use of message switching for these networks OSI model transport services, today it is typically run allows all the small request/responses to be aggregated, over TCP/IP.

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A network model defi ned by X.400 consists of a SMTP . Simple mail transfer protocol (SMTP) is con- number of basic elements including user agent (UA), sidered an application-level protocol, which runs over message transfer agent (MTA), and message store (MS), packet-based TCP/IP. SMTP was developed by IETF as shown in Figure 5 . The UA is a program that allows (Internet Engineering Task Force) and has become the the user to compose, send, and receive mail. The MTA de facto standard for mail transfer on the Internet. Vir- is the electronic post offi ce, accepting the e-mail from user tually all e-mail systems that send mail via the Internet agents and making sure it is delivered. MS, which often use SMTP to send their messages. SMTP is used for for- co-resides with MTA, maintains electronic mailboxes warding messages from one host to the other and writes for each user. When a message is delivered, it may pass them to a message store (e.g., mbox or Maildir). How- through multiple MTAs, each of which reads the message ever, SMTP does not provide the functionality of allowing address and passes it to another message transfer agent, users to retrieve mail. The post offi ce protocol (POP) has until the message reaches its destination. As indicated by been developed for retrieving messages from the massage Figure 5 , messages can be transmitted directly by MTA to store. the UA recipient or, alternatively, they can be passed on Although SMTP is the most prevalent of the e-mail to MS (the equivalent of a mailbox) which makes it possible protocols, it lacks some of the rich features of X.400. to store the message. Incoming messages can be stored A primary weakness of SMTP is the lack of support for in these mailboxes until the user logs in. A collection of non-text messages. Multipurpose Internet mail exten- all message transfer agents cooperating to relay the mes- sions (MIME) has been introduced to supplement SMTP sages until their destinations is called the message transfer allowing the encapsulation of multimedia (non-text) mes- system (MTS). sages inside of a standard SMTP message (Parker, 2002) . Figure 5 also shows some of main protocols of X.400. X.400 and SMTP have similar features but also unique These are P1, defi ning the communication between MTAs; features in themselves. Generally speaking, X.400 is a P3, standardizing the connection between the user agent more complex protocol and it makes it harder for users and MTA; P2 standardizing the (virtual) protocol between to fake e-mail addresses and contents, compared with the the UAs; and P7, describing the interaction between the situation in SMTP. For a complete discussion on SMTP user agent and message store (Tanenbaum, 2002). protocol, please refer to Chapter 88 . At its early development, X.400 was widely implemented, especially in Europe. However, as e-mails became Message-Based Client/Server Systems more popular, many of X.400 features, such as structured Many client/server systems use middleware to exchange addressing and central control, made it more complex messages. Middleware is used to simplify the complex- for everyday usage. On the other hand, advances in the ity of dealing with communication protocols and lay- Internet-based e-mail protocols, in particular simple mail ers of software. Broadly speaking, middleware software transfer protocol (SMTP), made X.400 less popular. Iron- is defi ned as the glue between software components or ically, the same features which made X.400 less popular, between software and the network or it is the slash in made it attractive for special networks such as military client/server (MRC, 2004). One category of middleware and aviation. In fact, extended versions of X.400, such that provides program-to-program communication by as military message handling systems (MMHS) and avia- message passing is called message-oriented middleware tion message handling systems (AMHS), are still under (MOM). MOM is based on message queuing and store- research and development. These protocols employ inte- and-forward technology. MOM is a non-blocking asyn- grated security capabilities, including message routing, chronous form of communication. Hence, every node in password management, and provisioning of public key the system can randomly send message to the queue or infrastructure (PKI).

Sender Receiver

P2 UA UA

P3 P7 P3 P7 MS MS MTA MTA MTA

P1MTS P1

Presentation Layer User Agent (UA) Session Layer Message Transfer Agent (MTA) Transport Layer Message Store (MS) Network Layer Message Transfer Data Link Layer System (MTS) Physical Layer Figure 5: A general network model deﬁ ned by X.400

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taking message from queues. Note that the queue can be satellites, as opposed to communications between a sin- a part of the client or a dedicated server. Message storage gle satellite and Earth stations (Charles, 1999). Message and queuing can be persistent (logged on disk) or non- switching is also being considered for interplanetary net- persistent (in memory). Persistent messages are slower works, a high-bandwidth infrastructure designed to link but they can be recovered in case of power failures. Earth’s Internet to other parts of our solar system. The message fl ow in MOM messaging products can Research suggests that store-and-forward technol- have different types, including send-and-pray (no res- ogy can also enhance the wireless range of low-cost ponse about the message is required), send-and-nay (a short-range technologies, such as WiFi (IEEE 802.11). response is required if there is something wrong with the One approach is simply relaying the data between message), and send-and-say (response is mandatory). multiple wireless devices over long distance. A more cost- effective approach is to transmit data over short point- to-point links between mobile storage devices called Store-and-Forward Wireless Messaging mobile access points (MAP). Through the use of low- Today, many wireless applications and services are based cost WiFi radio transceivers, the data carried by the on store-and-forward technology in order to reliably MAP is automatically and wirelessly transferred at high- transfer messages between end-users. A popular example bandwidth for each point-to-point connection (Pent- is short message service (SMS). Although, SMS is being land, 2002). The motivation in utilizing this approach is supported by many digital-based mobile communications providing a very low-cost and affordable solution to systems, it was introduced as a datagram service in the resolve the last mile connectivity problem, particularly in GSM (global system for mobile communications) system. poor rural areas. SMS is a text message service that enables short text messages (up to 160 characters in length) to be transmitted and received by a cell phone. Similar to e-mail, short mes- PERFORMANCE OF sages are stored and forwarded at SMS centers, which MESSAGE-SWITCHED NETWORKS means messages can be retrieved later if the user is not In this section we examine the performance of message- immediately available (MWR, 2004). SMS messages travel switched networks in terms of delay and peak traffi c load. to the cell phone over out-of-band control channels, sepa- rated from voice channels. SMS has been broadly used in Europe and Asia for many years. In North America, Delay Performance SMS was made available initially on digital wireless net- Figure 6 depicts the transmission of a message across works built by early pioneers such as BellSouth Mobility, four nodes. Three types of delays can be identifi ed on the PrimeCo, and Nextel, among others. diagram: (a) propagation delay—the time it takes a sig- Paging systems capable of storing and delivering mes- nal to propagate from one node to the next; (b) transmis- sages (or pages) are also examples of store-and-forward sion delay—the time it takes for a transmitter to send out wireless systems. A wireless pager receiver can be acti- a message (e.g., it takes 0.1 sec. to transmit a 10,000 bit vated and alert its user via a tone or a vibrator. This is message onto a 100 kbps line); (c) node delay—the time it typically called a one-way paging system. Pager activa- takes for a node to perform the necessary processing as it tion can be done using a telephone or a PC. In store-and- switches data plus the message storage time. Hence, the forward based paging systems, if the network cannot total delay will be the sum of all propagation and trans- reach the user to deliver the message, it stores the mes- mission delays at each node plus the sum of node delays sage until the user can be reached (Taylor, 1996). Some at each switch node (Rosner, 1982). paging systems have SMS capability and allow displaying As Figure 6 indicates, prior to retransmission, the en- small alphanumeric messages. Two-way paging systems tire message must be received at each switch node. This include an acknowledgement feature that allows the user results in a large end-to-end delay, particularly if the mes- to acknowledge the receipt of the message. sage size is very large. In fact, under heavy load, when Wireless sensor networks (WSN) also use store-and- the node delay is not negligible, the total delay using mes- forward technology. WSN are mesh networks of small sage switching can be signifi cantly longer than for circuit sensor nodes communicating among themselves using switching, which requires time to setup and tear-down a RF communication. Sensor nodes can be deployed in connection. large scale (from tens to thousands) to sense the physical world, for example, monitoring, tracking, and control- ling. The data messages gathered by each sensor node Node Delay will be periodically exchanged between nodes or passed An important factor impacting node delay is unavailabil- on to intermediate nodes. Intermediate nodes can aggre- ity of the next hop or the destination node. The store-and- gate small data messages together prior to retransmitting forward switches in a message-switched network avoid them to the next node (Zhao, 2005) . repeated attempts to send messages to the next hop. Store-and-forward technology has also been consid- Hence, if the next node is not available, the message ered to be implemented in satellite communications sys- retransmission is simply delayed. Clearly, as the total tems in order to ensure higher reliability and network number of messages in the network increases, the average robustness. For example, message switching is recently node delay through the network will become longer being investigated for inter-satellite networks, where until no storage resources are available and message drop- messages are transmitted between two communication ping occurs. This is illustrated in Figure 7a .

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10 Km; 320 kbps

An Example: Terminal (1) SW A SW B Terminal (2)

Given: L (Link length) = 10 Km D t Dp M (Message size) = 1 M Byte Me = ssa H (Header size) 32 Byte ge C (Link rate) = 320 kbps S (Prop. speed) = 200 Km/s D (i) (Node (i) delay) = 0 s n (A) Dn Calculated: DT = = Dp (Prop. delay) L/S 0.05 s M es = + = sa Dt (Trans. delay (M H)/C 25.001 s ge = DT (Total delay) + ∑ = 3(Dt Dp) + Dn(i) 75.153 s (B) i Dn

M es sa ge

Time

Figure 6: Event timing diagram in message-switched networks (the example assumes all the link speeds and lengths are the same; node delays are assumed to be negligible)

Network Message Load Total available Delay Through the Network network resources Peak Load Peak Load Spread due to Storage

Storage Total available 1 2 Delay network resources

Network Load → 1- Traffic load into the network Time → 2- Traffic load delivered through the network (A) (B)

Figure 7: (a) Delay in message-switched networks as the load increases; (b) trafﬁ c characteristic as a result of store-and-forward switches

result is an increase in the message delivery time through Another important factor affecting the node delay in the network. a store-and-forward switch is network congestion. That Another way of looking at a network with store-and- is the case where switches cannot deliver as fast as mes- forward switches is to regard it as a large distributed set sages are arriving. When congestion occurs, the system of storage facilities, which gradually ﬁ ll as the load on the continues to accept new messages, as long as its storage network increases. Consequently, the peak load is shifted capacity allows. In message switching new messages will in time by an amount depending on the available storage be stored until free resources are available and the deliv- (Davis, 1973) . Figure 7b illustrates this point by showing ery is ensured; hence, no message blocking occurs as long how a peak load is reduced, but at the expense of being as sufﬁ cient storage capacity is available. As the load delayed. The area under each of the two curves repre- of the network increases, more messages will have to sents the total number of messages delivered. Clearly, this wait for available processing resources and free links. The area will be the same for both curves.

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Comparison of Message, Packet, and any required line code, transmission speed, or protocol Circuit Switching conversions between incompatible nodes. Therefore, message switching can be a potential solution for less ro- Message-switched networks provide a number of advan- bust communication environments. In circuit switching, tages over circuit-switched networks. For example, protocol conversion is only possible through protocol message-switched networks provide more effi cient use of gateways. network resources. This is a result of their inherent store- In message-switched networks, unlike circuit switch- and-forward capability. As new information enters the ing, simultaneous availability of sender and receiver network for future delivery, the message storage proc- is not required and the network can store the message ess creates a reservoir of traffi c. This reservoir tends to pending the availability of the receiver. Moreover, features remove the gaps between new message arrivals onto such as broadcasting, data auditing, and data rerouting the network. In addition, it smoothes the peak network can be supported by message-switched networks much load, leading to some degree of traffi c regulation over the more easily and typically require no major architectural network. modifi cations. When traffi c becomes heavy on a circuit-switched The difference between message switching and packet network, some connection requests are blocked; that is, switching begins with the characteristics of messages and the network refuses to accept any additional connection packets themselves. Messages are units of information requests until the load on the network decreases. In a with unrestricted lengths, whereas packets have maximum message-switched network, messages are still accepted length size. Furthermore, in a message-switched network as long as the storage capacity is available but delivery the network is responsible for the message and typically delay increases. As a result, message-switched networks no responses or feedbacks exist. Consequently, in message translate potential message blocking into potential de- switching, rather than minimizing transit time of the mes- lay. As long as the total end-to-end message delay meets sage, its guaranteed delivery is emphasized. On the other the user delay tolerance, message delaying can be far hand, packet-switched networks aim to deliver packets more convenient than message retransmission due to with minimum delay and do not hold packets for delayed blocking. delivery. Table 1 compares the main characteristics of Another attractive feature of message-switched net- message, packet, and circuit-switching technologies. works is that in these networks each intermediary node Figure 8 illustrates the transmission of data across four can perform protocol translation. Since each intermedi- nodes using circuit-switched and packet-switched net- ary node processes the entire message, it can perform works. An important characteristic of circuit switching

Table 1: Comparison of Message, Packet, and Circuit Switching (Stallings, 2004; Martin, 1985)

Circuit-Switched Networks Message-Switched Networks Packet-Switched Networks Communication is performed through No dedicated path exists. No dedicated path exists. a dedicated path. Provides real-time or continuous Too slow for real-time or Provides near real-time data transmission. transmission of data. interactive data transmission. No data storing is required. Messages are stored for later Packets are queued for delivery; they are retrieval. not stored. The switch path is established for The route is established for The route is established for each packet. the entire connection time. each message. For small messages, the data The message delivery time The packet delivery time is very short. transmission time is negligible can be substantially long. compared to the time required to setup and tear-down the connection. The connection is blocked if the No message blocking can Packet blocking can occur, however, the end-user is busy or not available. occur as long as the storage blocked packets will be retransmitted to Once the connection starts, no capacity is sufﬁ ciently large. the end-user. blocking may occur. As the network load increases, As the load increases, As the load increases, packets on average more blocking can occur. messages on average experience longer queuing delay, although experience longer delivery still very short compared to message delay. switching. The length of transmission Messages have no theoretical Packets have a maximum length. is unlimited. maximum length and can be very long.

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Terminal (1) SW A SW B Terminal (2)Terminal (1) SW A SW B Terminal (2)

D Dp Packet p Request Packet Signal Packet Packet D Packet Setup n Packet Packet Time D n Packet D Accept Packet T Signal

D t DT Message

Time Terminate Time Signal

(A) (B) Figure 8: Event timing diagram in (a) a circuit-switched network and (b) a packet-switched network; we assume no loss of data occurs

is the need to setup an end-to-end path. During this time IETF : Internet Engineering Task Force is a large, open interval (setup time), depicted in Figure 8a , the system is international community of network designers, opera- ensuring path availability. tors, vendors, and researchers, which sets standards In packet switching, shown in Figure 8b , no physical for the Internet. It is also responsible for much of the path is established in advance between sender and re- work done on TCP/IP. ceiver. Hence, unlike message switching (Figure 6 ), each Peer-to-Peer Architecture (P2P): It is a type of network node along the route may begin transmission of a packet architecture in which each workstation has equivalent as soon as it arrives. capabilities and responsibilities. Protocol : A formal and pre-agreed set of rules that govern the communications between two or more CONCLUSION entities. The protocol determines the meaning of In this chapter we examined the basic concepts of message- specifi c values occurring in specifi c positions in the switched networks. We surveyed their advantages and dis- stream, the type of error checking to be used, the data advantages and demonstrated their applications. We also compression method, how the sender will indicate described different elements of a message-switched net- that it has fi nished sending a message, and how the work and examined their basic functionalities. Although receiver will indicate that it has received a message. packet switching is becoming the dominant switching Teleprinter : A typewriter-like terminal with a keyboard scheme, message switching is still considered for many and built-in printer. It was used to communicate typed dedicated networks. messages between terminals through a simple point- to-point electrical communication channel, often just a pair of wire. A teleprinter terminal is also called GLOSSARY teletype or TTY machine. ARPANET: The Advanced Research Project Agency Telegram : A message transmitted by telegraph system. Network created by the U.S. Department of Defense. This organization is credited with creating the Inter- net in 1969. CCITT : Short for Comité Consultatif International Télé- REFERENCES phonique et Télégraphique , an organization based in Stallings, William. 2004 . Data and Computer Communi- Geneva, Switzerland, that sets international commu- cations, 7th ed. Upper Saddle River, NJ : Prentice Hall . nications standards. CCITT changed its name to Inter- Tomasi , Wayne . 2004 . Introduction to Data Communica- national Telecommunications Union (ITU), the parent tions and Networking, 1st ed. . Upper Saddle River, NJ : organization, on March 1, 1993. Prentice Hall. Datagram : A sequence of bytes that constitutes the unit Davis , Donald Watts and Derek Leslie Barber . 1973 . Com- of transmission in the network layer. munication Networks for Computers. New York : John Exchange : A room or building equipped with manual Wiley & Sons. or automatic switching equipments that can intercon- Tanenbaum , Andrew S. 2002 . Computer Networks, 4th nect incoming communication lines as required. ed. New York: Prentice Hall .

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Shafritz , Arnold B. 1964 . The use of computers in mes- 1960s and 1970s. Several comprehensive discussions on sage switching networks. Proceedings of the 1964 19th these topics are found in Davis (1973) and Martin (1985) . ACM national conference pp. 142.301 – 142.306 . A basic analytical model for a message-switched network Federation of American Scientists. 2003 . Automatic in terms of data loss and delay is provided by Rosner Digital Network (AUTODIN). http://www.fas.org (ac- (1982). Stallings (2004) , Tomasi (2004) , and Martin (1985) cessed March 1, 2006). offer a list of advantages and disadvantages of message Martin , James . 1985 . Telecommunications and the Com- switching and compare the performance between mes- puter. Englewood Cliffs, NJ: Prentice Hall. sage, packet, and circuit switching. One of the fi rst Bush , David . 2003 . The AFTN Message Switch Case detailed tutorials on store-and-forward switch node func- Study. http://www.ascolto.co.uk/Resources/ (accessed tionalities can be found in Shafritz (1964) . February 1, 2006). Routing techniques in message-switched and packet- Fall , Kevin . 2003 . A delay tolerant networking architec- switched networks are very similar. Various routing tech- ture for challenged Internets. Proc. SIGCOMM 2003 . niques are described in Tanenbaum (2002) . August 25–29, 2003. Pages: 27 – 34 . Examples of message-switched networks, including Fall , Kevin . 2002 . A message-switched architecture for SITA and AUTODIN, along with detailed discussions of challenged Internets. http://www.intel-research.net/ their technical specifi cations and evolutions, are provided Publications/Berkeley/120520021026_66.pdf (accessed in Martin (1985) . FAS (2003) is specifi cally dedicated to December 1, 2006). the history of AUTODIN. Bush (2003) offers background Middleware Resource Center . 2004 . http://www. information on AFTN. Introductory information about middleware.org/ (accessed December 1, 2006). USENET, including its commands and protocols, can be Betanov, Cemil . 1992 . Introduction to X.400 . Norwood, found in Tanenbaum (2002) . Betanov (1992) and Parker MA : Artech House. (2002) offer a comprehensive discussion on X.400 and Parker , Tim , and Karanjit B. Siyan . 2002 . TCP/IP Unleashed SMTP protocols and their differences. More informa- (Unleashed), 3rd ed. Indianapolis, IN: Sams . tion about middleware and MOM can be found in MRC Taylor, Mark , Mohsen Banan , and William Waung . 1996 . (2004) . Internetwork Mobility: The CDPD Approach. Upper Recent investigation on implementing message switch- Saddle River, NJ: Prentice Hall. ing in challenged networks and satellite communications Zhao , Feng , and Leonidas Guibas . 2005 . Wireless Sensor can be found in Fall (2003) , Fall (2002) , and Charles Networks: An Information Processing Approach . San (1999) . More information regarding the paging systems Francisco, CA: Morgan Kaufmann. and wireless sensor networks can be found in Taylor Charles , John . 1999 . Interplanetary Network Aims for the (1996) and Zhao (2005) , respectively. A number of books Stars . IEEE Computer Society , 32 ( 9 ): 16 – 8 , 21 . about short message services are listed in MWR (2004) . Pentland , Alex (Sandy) , Richard Fletcher , and Amir A. Moreover, an interesting work demonstrating how store- Hasson . 2002 . A Road to Universal Broadband Con- and-forward technology can be used to provide a low- nectivity. http://www.itu.int/council/wsis/080_Annex4. cost solution to resolve the last mile continuity problem pdf (accessed February 1, 2006). in rural areas can be found in Pentland (2002) . Message- Rosner, Roy, D. 1981 . Packet Switching . Belmont, CA: Switched Networks Lifetime Learning Pub. MobileIN Web Resources. 2004 . http://www.mobilein. com /sms.htm (accessed January 1, 2007).

FURTHER READING Many of the original publications on message switching and store-and-forward technology belong to early

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