Message Switching 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 switch- ing 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 connec- tion, 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 1 bbid44608_ch70.inddid44608_ch70.indd 1 55/19/07/19/07 88:23:48:23:48 PPMM 2 MESSAGE SWITCHING 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