Designing Large-Scale LAN/WAN, Part 2

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DATA COMMUNICATIONS MANAGEMENT DESIGNING LARGE-SCALE LAN/WANS, PART II

John R. Vacca

INSIDE Designing Large-Scale IP LAN/WAN Systems; Designing SRB LAN/WAN Systems; Designing SDLC, SDLLC, and QLLC LAN/WAN Systems; Designing APPN Internetworks; Designing DLSw+ Internetworks; Designing ATM; Designing Packet Service LAN/WAN Systems; Designing DDR Internetworks; Designing ISDN; Designing Switched LAN Systems; Designing LAN/WAN Systems for Multimedia

DESIGNING LARGE-SCALE IP LAN/WAN SYSTEMS The following discussion provides a very quick overview of the key de- cisions one must make when selecting and deploying routing protocols for large-scale IP LAN/WAN systems. This discussion lays the foundation for subsequent discussions regarding speciﬁc routing protocols.

Describing LAN/WAN Topology The complete set of routers, and the networks that connect them, de- scribe the physical topology of a LAN/WAN system. A logical topology is also included in a LAN/WAN. Different routing protocols establish the logical topology in different ways. A logical hierarchy is not used by some routing protocols. Within a given LAN/WAN system environment (and to establish a logical topology), such protocols use addressing to segregate speciﬁc areas or do- mains. For such nonhierarchical, or ﬂat, protocols, no manual topology creation is required. Through the establishment of a backbone and logical areas, other protocols require the creation of an explicit hierarchical topology. Examples of routing protocols that use a hierarchical structure are the OSPF and In- termediate System-to-Intermediate System (IS-IS) protocols. According to Cisco, the explicit topology in a hierarchical scheme takes precedence PAYOFF IDEA over the topology created through This article provides details on how to utilize LAN addressing. and WAN technologies available today to design large-scale LAN/WANs.

The addressing topology should be assigned to reﬂect the hierarchy if a hierarchical routing protocol is used. Also, the addressing implicitly creates the topology if a ﬂat routing protocol is used. There are two recom- mended ways to assign addresses in a hierarchical LAN/WAN. The simplest way is to give each area (including the backbone) a unique LAN/WAN address. An alternative is to assign address ranges to each area. Logical collections of contiguous LAN/WANs and hosts are areas. All the routers having interfaces on any one of the included LAN/WANs are also areas. Each area runs a separate copy of the basic routing algorithm. Therefore, each area has its own topological database.

DESIGNING SRB LAN/WAN SYSTEMS The following discussion addresses implementation issues that can affect large-scale, router-based SRB LAN/WANs, SRB-related technology, and features that provide support for SRB requirements. Speciﬁc topics include typical SRB environments and multiport bridging.

Using Typical SRB Environments Three types of user environments use SRB:

1. Any-to-any (ﬂat): End users at one site need to access end stations at another site. 2. Many end stations to few end stations (hierarchical): In a hierarchical SNA network, end users from multiple access sites need connectivity to a host site through a limited number of front-end processors (FEPs). 3. Many end stations to several end stations (distributed): Many users need to access a limited number of servers or a limited number of devices, such as an AS/400.

The following discussion evaluates SRB environment design issues in relation to these user environments.

Multiport Bridging A two-port, ring-to-bridge-to-ring combination was the fundamental design of an SRB, as initially created by IBM. A half-bridge conﬁguration that consisted of a ring-to-wide-area-network (WAN) combination fol- lowed by a second WAN-to-ring half-bridge combination was also created by IBM. Multiport routers adopt an implementation that allows SRBs to include multiple rings on a single internetworking node to support more than two rings. The virtual ring capability accomplishes this. According to Cisco, a virtual ring is a conceptual entity that connects two or more

EXHIBIT 1 — A Multiport Bridge Using a Virtual Ring Concept to Permit Multiple Ring Interconnection

physical rings together, locally or remotely. Exhibit 1 illustrates the concept of multiport bridges and a virtual ring. Virtual rings can be expanded across router boundaries as a concept. Cisco recommends that several access points can be connected to a central router with an FEP by a large virtual ring. Simple bridging, multiport bridging, and connections to both local and remote virtual rings are supported by routers. Required to communicate with remote rings is a virtual ring conﬁguration. The half-bridge conﬁgu- ration is not supported. The IBM half bridge does not use the concept of virtual rings. Two IBM half bridges use two rings. The virtual ring advantage is in a topology that features many SRBs. In such an arrangement, only a single unit is required at a central site. A property not found in physical ring topologies is what makes up remote virtual rings. The LAN/WAN administrator determines the logical connectivity. Two options are available: partially meshed topologies (sometimes called redundant star topologies) or fully meshed topologies. In a partially meshed topology, a single central location (such as an FEP Token Ring) is connected to all access locations. Each access location is

logically connected to the central FEP rings and is not connected to any other ring. Partially meshed topologies using virtual rings do not permit direct communication between remote rings. However, communication is allowed from the central ring to the remote rings, which also allows communication among remote rings through the central ring.

DESIGNING SDLC, SDLLC, AND QLLC LAN/WAN SYSTEMS This part of the article very brieﬂy describes three techniques designed to enable LAN/WAN system capabilities for SNA-based network architectures:

1. SDLC via STUN 2. SDLLC implementation 3. QLLC conversion

Tunneling SDLC via STUN SDLC via serial tunneling (STUN) encapsulates SDLC frames into Internet Protocol (IP) packets and routes the encapsulated packets over IP-supported LAN/WAN media. According to Cisco, the SDLC frame is transmitted without modiﬁcation. Also, the information within the frame is transparent to the LAN/WAN. All SNA physical unit (PU) types are supported.

Implementing SDLLC Serial-attached devices using the SDLC protocol are allowed to communicate with LAN-attached devices using the Logical Link Control, type 2 (LLC2)1 protocol via the SDLLC function. To consolidate the traditionally disparate SNA/SDLC networks onto a LAN-based, multiprotocol, multimedia backbone network is the basic purpose of the SDLLC function. Routers use the SDLLC feature to forward the LLC2 trafﬁc through remote source-route bridging (RSRB) over a point-to-point or IP LAN/WAN, terminate SDLC sessions, and to translate SDLC to the LLC2 protocol. According to Cisco, routers support SDLLC over all such media through IP encapsulation because a router-based IP LAN/WAN can use any arbitrary media, such as FDDI, Frame Relay, X.25, or leased lines. Exhibit 2 illustrates a general SDLLC media translation LAN/WAN inter- network arrangement.2

Converting QLLC A data-link protocol deﬁned by IBM that allows SNA data to be transport- ed across X.25 networks is known as QLLC. According to Cisco, each SDLC physical link is replaced by a single virtual circuit with QLLC. Also, according to Cisco, QLLC conversion is a feature of its IOS Software3 that causes the router to perform all of the translation required

EXHIBIT 2 — SDLLC Media Translation

to send SNA data over an X.25 network so that IBM devices that are connected to a router do not have to be conﬁgured for QLLC.

DESIGNING APPN INTERNETWORKS Because APPN has many of the characteristics of the LAN networks and still offers the advantages of an SNA network, with APPN, one can consolidate the two networks (an SNA subarea network and an interconnected LAN network). The following are the major beneﬁts of using APPN:

• APPN provides an effective routing protocol to allow SNA trafﬁc to ﬂow natively and concurrently with other protocols in a single network. • APPN supports subarea4 applications as well as newer peer-to-peer applications over a single network. • Connections are peer-to-peer, allowing any end user to initiate a connection with any other end user without the mainframe (VTAM) in- volvement. • Traditional SNA class of service (COS)/transmission priority can be maintained.

One feature has remained critical to many users: COS,5 as SNA has evolved. On an SNA session basis (on the backbone), this feature provides trafﬁc prioritization. This, in turn, allows a single user to have sessions with multiple applications, each with a different COS. In APPN, this feature offers more granularity and extends this capability all the way to the end node rather than just between communication controllers.

Identifying Types of APPN Nodes An APPN LAN/WAN has three types of nodes: local entry networking (LEN) nodes, end nodes (EN), and network nodes (NN). According to Cisco, the control point (CP), which is responsible for managing a node’s

resources and adjacent node communication in APPN, is key to an APPN node. The APPN control point is the APPN equivalent of the signaling connection control part (SSCP).6

Using Local Entry Networking (LEN) Nodes LEN nodes are pre-APPN, peer-to-peer nodes. They can participate in an APPN LAN/WAN using the services provided by an adjacent network node. The CP of the LEN node manages the local resources but does not establish a CP-CP session with the adjacent network node. Session part- ners must be predeﬁned to the LEN node, and the LEN node must be predeﬁned to the adjacent network node. LEN nodes are also referred to as SNA node type 2.1, physical unit (PU) type 2.1, or PU2.1.

Using End Nodes End nodes contain a subset of full APPN functionality. They access the LAN/WAN through an adjacent network node and use the adjacent network node’s routing services. An end node establishes a CP-CP session with an adjacent LAN/WAN node, and then uses that session to register resources, request directory services, and request routing information.

Managing Network Nodes Network nodes contain full APPN functionality. The CP in a network node is responsible for managing the resources of the network node along with the attached end nodes and LEN nodes. The CP establishes CP-CP sessions with adjacent end nodes and network nodes. It also maintains network topology and directory databases, which are created and updated by dynamically gathering information from adjacent network nodes and end nodes over CP-CP sessions. In an APPN environment, network nodes are connected by transmission groups (TGs), which in the current APPN architecture refers to a single link. Conse- quently, the LAN/WAN topology is a combination of network nodes and transmission groups.

DESIGNING DLSw+ INTERNETWORKS This part of the article very brieﬂy describes Data Link Switching Plus (DLSw+). DLSw+ routers are referred to as peer routers, peers, or part- ners. The connection between two DLSw+ routers is referred to as a peer connection. A DLSw+ circuit compromises the data-link control connection between the originating end system and the originating router; the connection between the two routers (typically a Transport Control Proto- col [TCP] connection); and the data-link control connection between the target router and the target end system. A single peer connection can car- ry multiple circuits.

Circuits between SNA physical units (PUs) or between NetBIOS clients and servers are supported by DLSw+. The SNA PU connectivity supported is PU 2.0/2.1-to-PU 4 (attached via any supported data-link controls), PU 1-to-PU 4 (SDLC only), PU 4-to-PU 4 (Token Ring only), and PU 2.1- to- PU 2.1 (any supported data-link control).7

DESIGNING ATM This part of the article very brieﬂy describes current Asynchronous Trans- fer Mode (ATM) technologies that LAN/WAN designers can use in their networks today. It also brieﬂy focuses on the role of ATM in LAN/WAN systems.

Identifying the Role of ATM in LAN/WAN Systems Today, 97 percent of computing power resides on desktops. This power is growing exponentially. Distributed applications are increasingly bandwidth-hungry, and the emergence of the Internet is driving most LAN architectures to the limit. Voice communications have increased significantly with increasing reliance on centralized voice mail systems for verbal communications. The LAN/WAN system is the critical tool for information flow. LAN/WAN systems are being pressured to cost less yet support the emerging applications and higher number of users with increased performance. Local and wide-area communications have remained logically separate to date. Bandwidth is free and connectivity is limited only by hardware and implementation cost in the LAN. The LAN has carried data only. In the WAN, bandwidth has been the overriding cost, and such delay-sensitive traffic as voice has remained separate from data. New applications and the economics of supporting them, however, are forcing these conventions to change. The first source of multimedia to the desktop that immediately breaks the rules is the Internet. More predictable LAN and WAN performance is required by such Internet applications as voice and real-time video. In addition, the Internet also necessitates that the WAN recognize the traffic in the LAN stream, thereby driving LAN/WAN integration.

Supporting Multiservice LAN/WANs One of the emerging technologies being used for integrating LANs and WANs is known as ATM. ATM can support any traffic type in separate or mixed streams, delay-sensitive traffic, and nondelay-sensitive traffic. According to Cisco, ATM can also scale from low to high speeds. From LAN to private branch exchange (PBX), it has been adopted by all the industry’s equipment vendors. With ATM, network designers can integrate LANs and WANs, support emerging applications with economy in the enterprise, and support legacy protocols with added efficiency.

DESIGNING PACKET SERVICE LAN/WAN SYSTEMS This part of the article focuses very briefly on the implementation of packet-switching services and addresses system design in terms of hierarchical LAN/WAN system design. It also outlines the overall issues that influence the ways in which packet-switched LAN/WAN systems are designed.

Designing Hierarchical LAN/WANs To modularize the elements of a large-scale system into layers of internetworking8 is the objective of a hierarchical LAN/WAN system design. The access, distribution, and backbone (or core) routing layers are the key functional layers in this model. In essence, a hierarchical approach strives to split LAN/WANs into subnetworks so that trafﬁc and nodes can be more easily managed. According to Cisco, hierarchical designs also facilitate scaling of LAN/WAN systems because new subnet- work modules and internetworking technologies can be integrated into the overall scheme without disrupting the existing backbone. Exhibit 3 illustrates the basic approach to hierarchical design. Three basic advantages tilt the design decision in favor of a hierarchical approach:

• scalability of hierarchical LAN/WAN systems • manageability of hierarchical LAN/WAN systems • optimization of broadcast and multicast control trafﬁc

EXHIBIT 3 — Hierarchical Packet-Switched Interconnection

Supporting the Scalability of Hierarchical LAN/WAN Systems A primary advantage that supports using a hierarchical approach to packet-service connections is known as scalability. Because they allow one to grow a LAN/WAN system in incremental modules without running into the limitations that are quickly encountered with a ﬂat, nonhierarchical structure, hierarchical LAN/WAN systems are more scalable. Hierarchical LAN/WAN systems raise certain issues that require careful planning however. The complexity inherent in a hierarchical design (par- ticularly when integrated with a meshed topology), and the need for additional router interfaces to separate layers in the hierarchy are issues that include the costs of virtual circuits. One must match the hierarchy of LAN/WAN systems with a comple- mentary approach in your regional topologies to take advantage of a hierarchical design. The packet services that one implements, as well as requirements for fault tolerance, cost, and overall performance, are what design speciﬁcs depend on.

Managing Hierarchical LAN/WAN Systems Several management advantages are offered by hierarchical designs. First of all, one has LAN/WAN system simplicity. This is where adopting a hierarchical design reduces the overall complexity of a LAN/WAN system by partitioning elements into smaller units. Also, this partitioning of elements makes troubleshooting easier, while providing inherent protection against the propagation of broadcast storms, routing loops, or other potential problems. Second, there is design flexibility. This is where hierarchical LAN/WAN system designs provide greater flexibility in the use of WAN packet services. Most LAN/WAN systems benefit from using a hybrid approach to the overall system structure. In many cases, leased lines can be implemented in the backbone, with packet-switching services used in the distribution and access LAN/WAN systems. Finally, there is router management. With the use of a layered, hierarchical approach to router implementation, the complexity of individual router configurations is substantially reduced because each router has fewer neighbors or peers with which to communicate.

Optimizing Broadcast and Multicast Control Traffic One is required to implement smaller groups of routers by the effect of broadcasting in packet-service LAN/WANs. The routing updates and No- vell Service Advertisement Protocol (SAP) updates that are broadcast between routers on a packet-switched data network (PSDN)9 are typical examples of broadcast traffic. An excessively high population of routers in any area or layer of the overall LAN/WAN system might result in traffic

bottlenecks brought on by broadcast replication. A hierarchical scheme allows one to limit the level of broadcasting between regions and into the backbone.

DESIGNING DDR INTERNETWORKS According to Cisco, several functions are provided by its IOS Dial-on-De- mand Routing (DDR). First of all, to provide the image of full-time connectivity using dialer interfaces, DDR spoofs routing tables. When the routing table forwards a packet to a dialer interface, DDR then ﬁlters out the interesting packets for establishing, maintaining, and releasing switched connections. LAN/WAN internetworking is achieved over the DDR maintained connection using Point-to-Point Protocol (PPP)10 or other WAN encapsulation techniques, such as High-Level Data Link Control (HDLC),11 X.25,12 or Serial Line Internet Protocol (SLIP).13

Understanding the DDR Design Stack A DDR stacked approach is similar to the model provided by the OSI for understanding and designing internetworking. It can be used to design DDR networks.

Forming the Dialer Clouds The dialer media or dialer cloud can generically be labeled the LAN/WAN or vice versa. They are both formed by the interconnected DDR devices. The intended interconnected devices are included in the scope of the dialer cloud. And, it does not include the entire switched media (the entire ISDN spans the globe and is beyond the scope of the dialer cloud). The exposure to the ISDN must be considered when designing security. The fundamental characteristics of dialer clouds are as follows:

1. Dialer clouds are collective bundles of potential and active point-to- point connections. 2. For outbound dialing on switched circuits (such as ISDN), network protocol address to directory number mapping must be conﬁgured. 3. Inactive DDR connections are spoofed to appear as active to routing tables. 4. On active connections, dialer clouds form non-broadcast multiaccess (NBMA)14 media similar to Frame Relay. 5. Unwanted broadcast or other trafﬁc causing unneeded connections can be prohibitively expensive. Potential costs on tariffed media (such as ISDN) should be closely analyzed and monitored to prevent such loss.

Every stage of DDR internetworking design is affected by the characteristics of dialer clouds. Very robust and cost-effective LAN/WAN sys-

tems can be developed by a solid understanding of LAN/WAN protocol addressing, routing, and ﬁltering strategies.

DESIGNING ISDN LAN/WAN system problems are not solved using ISDN by itself. ISDN can provide the LAN/WAN system designer with a clear data path over which to negotiate PPP links by using either DDR or user-initiated sessions. A Public Switched Telephone Network to provide LAN/WAN system connectivity requires careful consideration of network security and cost containment. This part of the article includes overviews of the following ISDN design issues:

• ISDN connectivity • datagram15 encapsulation • DDR: Dial-On-Demand Routing • security issues • cost containment issues

Connecting ISDN Physical PRI and BRI interfaces provide connectivity to ISDN. A multi- plexed bundle of B and D channels is provided by a single PRI or BRI interface. The B channel provides bearer services such as high bandwidth data (up to 64 Kbps per B channel) or voice services. The D channel provides the signaling and control channel and can also be used for low-bandwidth data applications. A groomed local loop that is traditionally used for switch to analog phone service is being discontinued because BRI service is now provided. Two 64-Kbps B channels and one 16-Kbps D channel (2B+D) are de- livered to the subscriber by BRI. PRI service is provided on traditional T1 and E1 leased lines between the customer premise equipment (CPE) and the ISDN switch: T1-based PRI provides 23 B channels and one D channel (23B+D); and E1-based PRI provides 30 64-Kbps B channels and one 64-Kbps D channel (30B+D). Very stringent requirements exist on the physical equipment and ca- bling in the path from ISDN switch to ISDN CPE by the provisioning of both PRI and BRI services. According to Cisco, within ISDN service pro- vider enterprises, typical installations can require additional lead times as well as require working with dedicated support groups.

Encapsulating The Datagram Some method of datagram encapsulation is needed to provide data connectivity when DDR (or a user) creates an end-to-end path over the IS- DN. PPP, HDLC, X.25, and V.120 are available encapsulations for ISDN designs. X.25 can also be used for datagram delivery over the D channel.

PPP is used as the encapsulation by most LAN/WAN internetworking designs. To establish data links, provide security, and encapsulate data trafﬁc, the Point-to-Point Protocol (PPP) is a powerful and modular peer-to-peer mechanism. PPP is negotiated between the LAN/WAN internetworking peers each time a connection is established. PPP links can then be used by LAN/WAN protocols such as IP and IPX to establish system connectivity. PPP solutions can support bandwidth aggregation using MultiLink PPP to provide greater throughput for LAN/WAN system applications.

Understanding DDR: Dial-On-Demand Routing Designers must determine how ISDN connections will be initiated, maintained, and released when building LAN/WAN internetworking applications. According to Cisco, DDR is a sophisticated set of its IOS features that intelligently establishes and releases circuit switched connections as needed by LAN/WAN system trafﬁc. DDR can spoof LAN/WAN system routing and directory services in numerous ways to provide the illusion of full-time connectivity over circuit switched connections.

Understanding Security Issues It is imperative to design and confirm a robust security model for protect- ing your LAN/WAN because system devices can now be connected to over the public switched telephone network (PSTN).16 According to Cisco, its IOS uses the authentication, authorization, and accounting (AAA) model for implementing security. ISDN offers the use of caller-ID and dialed number identification service (DNIS)17 information to provide additional security design flexibility.

Evaluating Cost Containment Issues To avoid the cost of full-time data services (such as leased lines or Frame Relay) is a primary goal of selecting ISDN for a LAN/WAN system. To en- sure that WAN costs are controlled, it is very important to evaluate data trafﬁc proﬁles and monitor ISDN usage patterns. Dialer Callback can also be implemented to centralize billing.

DESIGNING SWITCHED LAN SYSTEMS When purchasing a technology for campus networks, LAN/WAN designers had only a limited number of hardware options in the past. Routers were for the data center or main telecommunications operations, and hubs were for wiring closets. According to Cisco, in traditional shared- media environments, the increasing power of desktop processors and the requirements of client/server and multimedia applications, however, have driven the need for greater bandwidth. These requirements are

EXHIBIT 4 — The Evolution from Shared to Switched LAN/WAN Systems

prompting LAN/WAN designers to replace hubs in their wiring closets with switches, as shown in Exhibit 4. With dedicated bandwidth to the desktop for each user, this strategy allows LAN/WAN managers to protect their existing wiring investments and boost network performance. A similar trend exists in the LAN/WAN backbone. This coincides with the wiring closet. Here, the role of Asyn- chronous Transfer Mode (ATM) is increasing as a result of standardizing protocols, such as LAN emulation (LANE), that enable ATM devices to co- exist with existing LAN technologies. LAN/WAN designers are collapsing their router backbones with ATM switches, which offer the greater backbone bandwidth required by high-throughput data services.

DESIGNING LAN/WAN SYSTEMS FOR MULTIMEDIA In campus LAN and WAN environments, networked multimedia applications are rapidly being deployed. From the enterprise perspective, as the next generation of productivity tools, LAN/WAN multimedia applications, such as network TV or videoconferencing, hold tremendous promise. The use of digital audio and video across enterprise LAN/WAN infrastruc- tures has tremendous potential for internal and external applications. The World Wide Web is a good example of network multimedia and its manifold capabilities. More than 92 percent of personal computers sold are multimedia ca- pable. A wide range of audio- and video-based applications have been

brought to the desktop by a hardware revolution that has initiated a software revolution. It is not uncommon for computers to run video editing or image processing applications (such as Adobe Premiere and Photo- shop and AutoCAD) in addition to basic productivity applications (word processing, spreadsheet, and database applications). A new class of multimedia applications that operate in LAN/WAN environments has spawned the proliferation of multimedia-enabled desktop machines. These LAN/WAN multimedia applications leverage the existing network infrastructure to deliver video and audio applications (such as videoconferencing and video server applications) to end users. With these application types, video and audio streams are transferred over the LAN/WAN between peers or between clients and servers. It is important to understand both multimedia and networking, to suc- cessfully deliver multimedia over a LAN/WAN. When deploying network multimedia applications in campus LAN and WAN environments, three components must be considered:

• Bandwidth: How much bandwidth do the network multimedia applications demand and how much bandwidth can the network infrastructure provide? • Multicasting: Does the network multimedia application utilize bandwidth-saving multicasting techniques and how can multicasting be supported across the network? • Quality of service: What level of service does the network multimedia application require and how can this be satisﬁed through the network?

CONCLUSION AND SUMMARY Today’s growing, fast-changing LAN/WAN systems are like growing com- munities — the trafﬁc they create tends to cause congestion and delays. To alleviate these problems, one can design higher-speed LAN/WAN technologies into a network. This article provided details on how to utilize key LAN and WAN technologies for creating a high-speed LAN/WAN environment.

John Vacca is an information technology consultant and internationally known author based in Pomeroy, Ohio. Since 1982, John has authored 29 books and more than 350 articles in the areas of Internet and intranet security, programming, systems development, rapid application development, multimedia, and the Internet. John was also a conﬁguration management specialist, computer specialist, and the computer security ofﬁcial for the NASA space station program (Freedom) and the International Space Station Program, from 1988 until his early retire- ment from NASA in 1995. John can be reached at [email protected].

Notes 1. Connection-oriented OSI LLC-sublayer protocol. 2. In Exhibit 2, the Token Ring connection (Token Ring 10) could also be an Ethernet segment that connects the FEP or 3172 and router as shown in Exhibit 2. 3. Cisco system software that provides common functionality, scalability, and security for all products un- der the CiscoFusion architecture. Cisco IOS allows centralized, integrated, and automated installation and management of internetworks, while ensuring support for a wide variety of protocols, media, services, and platforms. 4. Portion of an SNA network that consists of a subarea node and any attached links and peripheral nodes. 5. Indication of how an upper-layer protocol requires a lower-layer protocol to treat its messages. In SNA subarea routing, COS definitions are used by subarea nodes to determine the optimal route to establish a given session. A COS definition comprises a virtual route number and a transmission priority field. 6. Trillium software that supports routing and translation and management functions and data transfer without logical signaling connections. 7. Because of an idiosyncrasy in how FEPs treat duplicate source-route bridged paths, N PU 4-to-PU 4 connectivity supports only a single path between front-end processors (FEPs). In addition, remote load is not supported. 8. General term used to refer to the industry devoted to connecting networks together. The term can refer to products, procedures, and technologies. 9. Network that uses packet-switching technology for data transfer; sometimes called a PSDN. 10. Successor to SLIP that provides router-to-router and host-to-network connections over synchronous and asynchronous circuits. Whereas SLIP was designed to work with IP, PPP was designed to work with several network layer protocols, such as IP, IPX, and ARA. PPP also has built-in security mechanisms, such as CHAP and PAP. PPP relies on two protocols: LCP and NCP. 11. Bit-oriented synchronous data link layer protocol developed by ISO. 12. TU-T standard that defines how connections between DTE and DCE are maintained for remote terminal access and computer communications in PDNs. X.25 specifies LAPB (a data link layer protocol) and PLP (a network layer protocol). 13. Standard protocol for point-to-point serial connections using a variation of TCP/IP. 14. Term describing a multiaccess network that either does not support broadcasting (such as X.25) or in which broadcasting is not feasible (e.g., an SMDS broadcast group or an extended Ethernet that is too large). 15. A piece of a message transmitted over a packet-switching network. One of the key features of a packet is that it contains the destination address in addition to the data. In IP networks, packets are often called datagrams. 16. General term referring to the variety of telephone networks and services in place worldwide; sometimes called POTS. 17. DNIS is a high-traffic ,T-1 based 800 service that allows telemarketing and other call centers to receive called number identification. With this information, a call can be routed to a specific agent for help with a product.