![]() |
|||
![]()
|
![]() |
![]() Click Here! |
![]() |
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:
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).
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.
The frame relay switch, or node, uses the following two-step review process to forward frames across the network:
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. IBMs 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.
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. IBMs 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:
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:
|
![]() |
|
Use of this site is subject certain Terms & Conditions. Copyright (c) 1996-1999 EarthWeb, Inc.. All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of EarthWeb is prohibited. Please read our privacy policy for details. |