Content Overview Frame Relay was originally developed as an extension of Integrated Services Digital Network (ISDN). It was designed to enable the circuit-switched technology to be transported on a packet-switched network. The technology has become a stand-alone and cost-effective means of creating a WAN.Frame Relay switches create virtual circuits to connect remote LANs to a WAN. The Frame Relay network exists between a LAN border device, usually a router, and the carrier switch. The technology used by the carrier to transport the data between the switches is not important to Frame Relay. The sophistication of the technology requires a thorough understanding of the terms used to describe how Frame Relay works. Without a firm understanding of Frame Relay, it is difficult to troubleshoot its performance. Frame Relay has become one of the most extensively used WAN protocols. One reason for its popularity is that it is inexpensive compared to leased lines. Another reason Frame Relay is popular is that configuration of user equipment in a Frame Relay network is very simple. This module explains how to configure Frame Relay on a Cisco router. Frame Relay connections are created by configuring routers or other devices to communicate with a Frame Relay switch. The Frame Relay switch is usually configured by the service provider. This helps keep end-user configuration tasks to a minimum. Students completing this module should be able to:
Content 5.1 Frame Relay Concepts 5.1.1 Introducing Frame Relay Frame Relay is an International Telecommunication Union Telecommunications Standardization Sector (ITU-T) and American National Standards Institute (ANSI) standard. Frame Relay is a packet-switched, connection-oriented, WAN service. It operates at the data link layer of the OSI reference model. Frame Relay uses a subset of the high-level data link control (HDLC) protocol called Link Access Procedure for Frame Relay (LAPF). Frames carry data between user devices called data terminal equipment (DTE), and the data communications equipment (DCE) at the edge of the WAN. Originally Frame Relay was designed to allow ISDN equipment to have access to a packet-switched service on a B channel. However, Frame Relay is now a stand-alone technology.A Frame Relay network may be privately owned, but it is more commonly provided as a service by a public carrier. It typically consists of many geographically scattered Frame Relay switches interconnected by trunk lines. Frame Relay is often used to interconnect LANs. When this is the case, a router on each LAN will be the DTE. A serial connection, such as a T1/E1 leased line, will connect the router to a Frame Relay switch of the carrier at the nearest point-of-presence for the carrier. The Frame Relay switch is a DCE device. Frames from one DTE will be moved across the network and delivered to other DTEs by way of DCEs. Computing equipment that is not on a LAN may also send data across a Frame Relay network. The computing equipment will use a Frame Relay access device (FRAD) as the DTE. Web Links Frame Relay http://www.cisco.com/univercd/cc/ td/doc/ cisintwk/ito_doc/ frame.htm
Content 5.1 Frame Relay Concepts 5.1.2 Frame Relay terminology The connection through the Frame Relay network between two DTEs is called a virtual circuit (VC). Virtual circuits may be established dynamically by sending signaling messages to the network. In this case they are called switched virtual circuits (SVCs). However, SVCs are not very common. Generally permanent virtual circuits (PVCs) that have been preconfigured by the carrier are used. The switching information for a VC is stored in the memory of the switch. Because it was designed to operate on high-quality digital lines, Frame Relay provides no error recovery mechanism. If there is an error in a frame it is discarded without notification. The FRAD or router connected to the Frame Relay network may have multiple virtual circuits connecting it to various end points. This makes it a very cost-effective replacement for a mesh of access lines. With this configuration, each end point needs only a single access line and interface. More savings arise as the capacity of the access line is based on the average bandwidth requirement of the virtual circuits, rather than on the maximum bandwidth requirement. The various virtual circuits on a single access line can be distinguished because each VC has its own Data Link Channel Identifier (DLCI). The DLCI is stored in the address field of every frame transmitted. The DLCI usually has only local significance and may be different at each end of a VC. Interactive Media Activity Drag and Drop: Frame Relay Terminology When the student has completed this activity, the student will be able to correctly identify frame relay terminology. Web Links Frame Relay http://www.cisco.com/univercd/cc/ td/doc/cisintwk/ito_doc/frame.htm
Content 5.1 Frame Relay Concepts 5.1.3 Frame Relay stack layered support Frame Relay functions by doing the following: The physical layer is typically EIA/TIA-232, 449 or 530, V.35, or X.21. The Frame Relay frame is a sub-set of the HDLC frame type. Therefore it is delimited with flag fields. The 1-byte flag uses the bit pattern 01111110. If the Frame CheckSum (FCS) does not match the address and data fields at the receiving end, the frame is discarded without notification.
Content 5.1 Frame Relay Concepts 5.1.4 Frame Relay bandwidth and flow control The serial connection or access link to the Frame Relay network is normally a leased line. The speed of the line is the access speed or port speed. Port speeds are typically between 64 kbps and 4 Mbps. Some providers offer speeds up to 45 Mbps.Usually there are several PVCs operating on the access link with each VC having dedicated bandwidth availability. This is called the committed information rate (CIR). The CIR is the rate at which the service provider agrees to accept bits on the VC. Individual CIRs are normally less than the port speed. However, the sum of the CIRs will normally be greater than the port speed. Sometimes this is a factor of 2 or 3. This statistical multiplexing accomodates the bursty nature of computer communications since channels are unlikely to be at their maximum data rate simultaneously. While a frame is being transmitted, each bit will be sent at the port speed. For this reason, there must be a gap between frames on a VC if the average bit rate is to be the CIR. The switch will accept frames from the DTE at rates in excess of the CIR. This effectively provides each channel with bandwidth on demand up to a maximum of the port speed. Some service providers impose a VC maximum that is less than the port speed. The difference between the CIR and the maximum, whether the maximum is port speed or lower, is called the Excess Information Rate (EIR). The time interval over which the rates are calculated is called the committed time (Tc). The number of committed bits in Tc is the committed burst (Bc). The extra number of bits above the committed burst, up to the maximum speed of the access link, is the excess burst (Be). Although the switch accepts frames in excess of the CIR, each excess frame is marked at the switch by