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The Frame Relay Frame

Each frame relay access station is responsible for transforming the data into frame relay packets for transport (i.e., relay) over the network. Each frame contains the following elements:

  Flag. The flag indicates the start and end of a frame relay packet.
  Frame Relay Header. The header contains the destination of the user data packet and management information.
  User data. The user data contains the data to be transported across the frame relay network.
  Frame check sequence (FCS). The FCS allows the integrity of the data to be validated.

The frame relay network receives, transports, and delivers variable-length frames. The frame relay network consists of a group of interconnected nodes (i.e., switches) that relay the data across the network on the appropriate PVC. A frame relay switch uses only the DLCI information contained in the frame relay header to forward the frame across the network to its destination (see Exhibit 4-2-2).


Exhibit 4-2-2.  Frame Relay Network Showing PVC Connecting User A and User B

The path through the network is transparent to the user. The DLCI does not include any description of how the connection transverses the network or the routing topology of the network. A frame relay network operates an Open Systems Interconnection (OSI) layer 2 router network. Each frame relay access node puts the routing information (destination DLCI) in the data link layer (i.e., frame relay header) of the frame. The frame relay network uses only this information to relay the frame across the network. (See Exhibit 4-2-3.) In other words, the frame relay network nodes look only at the frame relay header and the FCS.


Exhibit 4-2-3.  Layer 2 Router Network

The frame relay switch, or node, uses the following two-step review process to forward frames across the network:

  The integrity of the frame is checked using the frame check sequence; if an error is indicated, the frame is discarded.
  The destination DLCI address is validated, and if it is invalid, the frame is discarded. The DLCI destination address is contained in the frame relay header of the frame.

All frames that are not discarded as a result of the FCS or DLCI checks are forwarded. The frame relay node makes no attempt to correct the frame or to request a retransmission of the frame. This results in an efficient network, but requires that the user end-stations assume responsibility for error recovery, message sequencing, and flow control.

Thus, frame relay switches do not look at the user data packets, which makes the network transparent to all protocols operating at levels above OSI level 2.

RFC 1490

Because frame relay networks do not look at the contents of the user data, any format can be used to packetize the data, such as X.25 or high-level data link control (HDLC). IBM uses logical link control type 2 (LLC2) as its frame relay SNA data format.

The IBM format is based on ANSI T1.617a Annex F, which covers encapsulating protocol traffic in frame relay. This process has been approved by the Frame Relay Forum and is included in its Multiprotocol Encapsulation Agreement. IBM’s treatment of a frame relay network is based on standards and promotes interoperability with third-party implementations. IBM uses the LLC2 frame format and protocol for transporting SNA across Token Ring and Ethernet LANs.

For SNA data, the RFC 1490 header designates that it is 802.2 (LLC2) data, whether it is SNA subarea, peripheral, or APPN data, and the LLC2 destination and source addresses. This format, illustrated in Exhibit 4-2-4, is also used for NetBIOS data.


Exhibit 4-2-4.  RFC 1490 Frame Relay Frame Format

Users connected to the network using RFC 1490 frame relay data terminal equipment (DTE) have a logical view of the frame relay network as a virtual LAN. IBM’s use of RFC 1490 for its frame relay equipment provides a familiar metaphor to SNA users.

Because the frame relay network does not look at the contents of user data, it allows the multiplexing of multiple protocols across a single frame relay interface. Frame relay network access nodes are responsible for converting the user data into the appropriate RFC 1490 format for SNA and LAN traffic. In summary, a frame relay WAN:

  Provides packet-mode technology.
  Does not utilize store-and-forward.
  Relies on intelligent endpoints and high-integrity lines.
  Results in low transit delay.
  Is transparent above layer 2.

As a result, frame relay provides a cost-effective alternative to dedicated-line networks.

FRAME RELAY AS A REPLACEMENT FOR SDLC

Frame relay delivers enhanced services compared to alternative SNA WAN techniques such as SDLC. Frame relay:

  Uses the same framing and CRC bits as SDLC. This means that all front-end processor (FEP) SDLC line interface couplers (LICs), modems, and DSUs/CSUs, can be used with frame relay.
  Usually allows for frames up to 2,106 bytes in a frame relay network, but IBM’s network control program (NCP) allows for the configuration of up to 8,250-byte frames for use on high-quality, private frame relay networks. Large packets reduce network overhead and improve network performance.
  Allows network access connections from 56/64K bps to T1/E1 speeds, whereas the typical multidrop connection is 4.8/9.6K bps. User response times are directly improved by efficient network backbone connectivity.
  Is implemented in software (like SDLC), which means that no hardware changes in either the FEP or remote devices are required to move to frame relay.
  Can be managed by NetView management by NCP for both SDLC and frame relay connections. Therefore, familiar network management tools and practices can be used on the frame relay network.
  Adds multiple protocol transport. All protocols can be transported across the frame relay network; SDLC supports only SNA traffic.
  Provides SNA guaranteed bandwidth through the PVC’s committed information rate.
  Requires no host application changes to migrate from SDLC to frame relay.
  Supports point-to-point connections, like SDLC. Frame relay also provides many-to-many connections; SDLC requires a multidrop line to provide one-to-many connections.
  Provides for transparent network routing. SDLC is a single physical path connection.
  Supports burst mode, which lets users exceed their CIR of the link.


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