Frame Relay Equipment
Frame Relay, as a service offering, depends on certain hardware and firmware in order to accomplish the interface. That equipment includes the Frame Relay Assembler/Disassembler (FRAD), the Frame Relay Network Device (FRND), and the Frame Relay switch.
- Frame Relay Assembler/Disassemblers (FRADs)
- also known as Frame Relay Access Devices, are analogous to Packet Assembler/Disassemblers (PADs) in a X.25 packet-switched network. The FRAD essentially organizes the user data into a Protocol Data Unit (PDU) that can vary in size up to < 4,096B. The FRAD then encapsulates the PDU into a Frame Relay frame, placing the necessary control information around it. FRADs can be standalone units, serving multiple hardwired devices, or they can be in the form of a printed circuit board that fits into the expansion slot of a workstation. Additionally, FRADs can be incorporated into X.25 PADs, T-carrier MUXs, or even PBXs for dedicated networks. More likely, they are incorporated into a high-end bridge or router serving a LAN.
In early 1996, several vendors announced CO-based FRADs that offer the potential to eliminate the end user investment in such equipment. Although they are limited in functionality, intended to support conversion of SDLC traffic to frame format, they appear to offer advantages to users seeking to replace dedicated networks connecting large data centers [11-6] and [11-7].
- Frame Relay Network Devices (FRNDs)
- are the link terminating equipment in the Central Office.
- Frame Relay Switches
- are nodal processors capable of switching frames at very high speed. They contain buffers that, within limits, can absorb incoming frames until the switch can act on them, and buffers that can absorb outgoing frames until the link becomes available. The switches contain very high-speed switching matrixes and internal busses. They have sufficient intelligence, in the form of routing tables to read the control information embedded in the frames in order to route the frames correctly over the PVC, which has been identified previously in the call setup process. The nodal processors also have intelligence sufficient to check for errors in the frame, although error correction processes are performed at the end-user level. Since the switches perform no error correction or protocol conversion functions, they act on the frames very quickly, thereby minimizing latency. Frame Relay nodal processors can be managed from a centralized Network Operations Center, thereby enhancing the scalability of the network [11-8].
The Frame Relay switch also may contain a voice compressor module, where voice is communicated over Frame Relay. Memotec (CX 1000) and Micom (Netrunner) switches provide this capability, using the ACELP compression algorithm. While Frame Relay is not intended for voice, it can be accommodated with varying degrees of quality.
Frame Relay Protocol: Frame Structure
The Frame Relay Protocol involves the relaying of frames of user data across the network. In an appropriate LAN, MAN, or WAN data application, the preponderance of the frame is the user data; relatively little control information is included. As illustrated in Figure 11.3, the five data fields in the ANSI T1.618 frame format comprise beginning and ending flags, an address field, an information field, and a frame check sequence (FCS). Although the Frame Relay Protocol is a subset of the HDLC (High Level Data Link Control) protocol used in other data communications environments, it lacks control information such as frame sequence number. In the Frame Relay environment, it is the responsibility of the user to identify and correct for frame sequence errors or missing frames.
Figure 11.3 Frame Relay frame.
- Flag Field
- 1 octet with fixed binary sequence 01111110 is employed to identify and separate frames. Flag fields appear at the beginning and end of the frame.
- Address Field
- 2 octets are used as a default length. Address fields include most necessary control information in the Data Link Connection Identifier (DLCI). The address field also contains a Command/Response field, Address Field Extension, Forward and Backward Explicit Congestion Notification fields, and Discard Eligibility data.
- Data Link Connection Identifier (DLCI)
- 10 bits that identify the data link and its service parameters to the network. The service parameters include frame size, Committed Information Rate (CIR), Committed Burst Size (Bc), Burst Excess Size (Be), and Committed Rate Measurement Interval (Tc). The significance of these service parameters will be discussed later in this chapter
- Command/Response (C/R)
- 1 bit is reserved for use of FRADs, rather than the Frame Relay network. Defined to facilitate the transport of polled protocols such as SNA, such protocols require a command/response for signaling and control.
- Address Field Extension (EA)
- 2 bits signal the extension of the addressing structure beyond the 2 octet default. The use of EA, which is unusual, must be negotiated with the carrier at the time the service is established. As the popularity of Frame Relay grows and as large public and private Frame Relay networks develop, provisions are in place to expand the EA field to as many as 60 bits.
- Forward Explicit Congestion Notification (FECN)
- A 1-bit field available to the network is used to advise upstream devices of congestion. The receiving device clearly recognizes that the frame carrying the FECN survived. It also is advised that subsequent frames may not be so fortunate. Should subsequent frames be discarded or corrupted in transmission, the receiving device is advised that recovery may be required in the form of requests for retransmission. Should the upstream device be in control of the rate of data transfer, it has the opportunity to throttle back.
- Backward Explicit Congestion Notification (BECN)
- A 1-bit field is used by the network to advise devices of congestion in the direction opposite to the traffic flow. If the user device is capable of reducing the frame rate, it is well advised to do so, as frames may be discarded by the network once the notification is posted.
- Discard Eligibility (DE)
- This is a 1-bit field is indicating the eligibility of the frame for discard under conditions of network congestion. Theoretically, the DE is set by the user equipment.
- Information Field
- Contains user information, either in the form of payload data or internetwork control information passed between devices such as routers. Although the Information Field may be < 4,096B in length, ANSI recommendations are that the maximum size be 1,600B, consistent with most LAN traffic.
- Frame Check Sequence (FCS)
- A 2-octet CRC supporting frames with information fields < 4,096B in length).
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