Frame Relay

Frame Relay is a packet-switching technology that exists at the Data Link layer of the OSI model and is one that has become increasingly common as a WAN solution since the early 1990s. Unlike with leased lines and circuit-switched networks, the available bandwidth on a provider’s Frame Relay network is shared amongst many subscribers. This sharing of resources leads to significantly lower costs than traditional leased lines.

Many people tend to be confused by packet-switching technologies like Frame Relay. Mostly this is a result of trying to understand how data actually gets from one location to another. On packet-switched networks (like Frame Relay), data streams are separated through the use of “virtual” rather than dedicated hardware circuits. In other words, a logical path is defined between endpoints, through a provider’s packet-switched network. Many virtual circuits will be defined for different customers, and will be multiplexed over the shared physical links of the network. As an example, consider the figure below. It shows two different companies, each connecting two offices over a provider’s Frame Relay network. Notice that between Frame Relay switches X and Y, both of their virtual circuits traverse a common physical link. The data that one company passes between their own offices is completely separate from the data of the other company – all data stays within each company’s dedicated virtual circuit only.

Figure: PVCs of two different companies traveling over the same Frame Relay network, and at times, common links.

Two main types of virtual circuits can be defined on a Frame Relay networks – permanent virtual circuits (PVCs) and switched virtual circuits (SVCs). A PVC functions somewhat similar to a leased line, in that a service provider defines a path through the packet switched network to each customer location. In cases where companies wish to have “always-on” connectivity between locations using Frame Relay, PVCs are usually defined.

A switched virtual circuit (SVC) functions somewhat differently, almost like a circuit-switched connection. SVCs are not permanent, and can instead be created as required across a packet-switched network. For example, an SVC could be created between a company’s head office and a remote location. For the duration of the connection, data would travel across the path defined by the SVC. However, if the circuit was terminated and a new SVC established at a later time, data might travel over a completely different path.

Frame Relay networks are referred to as being non-broadcast multi-access (NBMA). What this means is that, by default, broadcast traffic will normally not be passed over a virtual circuit without explicit configuration. This is an important consideration when dealing with the use of broadcast-based routing protocols like RIP or IGRP in a Frame Relay environment. You’ll look at how broadcast traffic can be handled on Frame Relay networks later in this section.

ISDN and the OSI Model

ISDN maps to the three lower layers of the OSI model – Network, Data Link, and Physical. However, different protocols exist at the Data Link and Network layers for B and D channels, as illustrated in the figure below. The functions handled at each OSI layer are described below.

  • Physical Layer. The ISDN Physical layer is concerned with the actual sending and receiving of bits over variety of interfaces. For example, the I.430 standard is responsible for providing communication over S/T reference points, while the ANSI T1.601 standard defines communication over U interfaces in North America.
  • Data Link Layer. ISDN D channels use the Link Access Procedure for D Channels (LAPD – Q.921) to frame signaling and control data at Layer 2. On the B channel, data can be framed in a variety of ways, including via PPP and HDLC.
  • Network Layer. ISDN D channels handle call setup, termination, and maintenance at the OSI Network layer using the Q.931 protocol, which implements common signaling standards. On B channels, ISDN uses common Network layer protocols like IP, IPX, AppleTalk, and so forth.

Figure: ISDN protocols and their relationship to the OSI model for B and D channels.

WAN Technologies and the OSI Model

WAN technologies are considered to exist and function at the three lower layers of the OSI model – Physical, Data Link, and Network. While not all WAN technologies have elements that function at the Network Layer, some (like X.25 and ISDN) do. The figure below provides an overview of how the WAN technologies that you’ll look at in this chapter map to the OSI model.

Figure: A high-level overview of how various WAN technologies map to the Physical, Data Link, and Network layers of the OSI model.

TCP/IP and the OSI Model – The Application (Process) Layer

The Application/Process Layer is where TCP/IP applications and services reside. You’re more than likely familiar with many of these, since you probably interact with many TCP/IP applications on a daily basis – a web browser using HTTP, or your email client connecting to a POP3 server are but two simple examples.

The list below outlines some of the more common Application layer protocols that you should be familiar with.

  • Telnet. Telnet is used to create a terminal session with a remote host, providing command-line access to the target system running a telnet server (daemon).
  • FTP. The File Transfer Protocol is used to reliably transfer files between an FTP client and server using TCP.
  • SMTP. The Simple Mail Transfer Protocol is used for the exchange of email between systems.
  • DNS. The Domain Name Service is a distributed database that is queried to resolve (or translate) names such as www.2000trainers.com to an IP address.
  • SNMP. The Simple Network Management Protocol is a lightweight network protocol that allows information to be gathered about network devices. Examples include information about utilization, hardware configuration, and so forth.
  • TFTP. The Trivial File Transfer Protocol is used to transfer files between a client and a TFTP server over UDP. You’ll learn more about TFTP later, since it’s the protocol used to transfer files to and from a Cisco router.

TCP/IP and the OSI Model – The Network (Internet) Layer

If you recall, the Internet layer’s primary responsibilities are determining a path between networks (routing), as well as network addressing. The addressing that takes place at the Internet layer is often referred to as logical addressing. These addresses aren’t “burned-in” like Ethernet MAC addresses, but instead are assigned by an administrator. The addressing protocol of the TCP/IP stack is the Internet Protocol (IP).

Note that TCP/IP routing protocols such as RIP, OSPF, and others also exist at the Internet layer. These will be looked at in Chapter 8, when routing is covered in detail.