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4-2
SNA Over Frame Relay

DICK THUNEN

Today most telecommunications carriers provide frame relay services that allow the IBM Systems Network Architecture (SNA) user to reap a number of benefits, including:

  Investment protection in SNA devices.
  Lower line costs compared to dedicated links.
  Up to 40% increases in network utilization through frame relay’s multiprotocol support.
  Sustained integrity and control of the SNA network with NetView and simple network management protocol (SNMP) management.
  Integration of SNA and multiprotocol LANs.
  High-performance access networking for Advanced Peer-to-Peer Networking (APPN) and a migration path to asynchronous transfer mode (ATM) backbones.

Traditional IBM host networks connect users to mainframes via SNA or bisynchronous multidrop lines. These are usually low-speed analog lines that represent a single point-of-failure between user and host. Even though these networks subject network managers to the complexities of dealing with a multitude of leased lines, many organizations continue to maintain their IBM host networks because of the mission-critical applications they support.

IBM Corp. introduced X.25 as a cost-effective alternative to private lines. Many network planners have chosen not to implement it, however, because of higher user-response times from network overhead delays caused by every node in the X.25 network performing error detection/correction, message sequencing, and flow control. Frame relay, however, performs these functions only at the network access points using an end-to-end protocol; thus frame relay uses the network more efficiently.

IBM has developed a set of SNA frame relay products for packet-based, wide area networks (WANs). Frame relay is an integral element of the evolution of SNA networks into the future with full support for APPN and ATM.

FRAME RELAY TECHNOLOGY: AN OVERVIEW

Frame relay is a relatively new technology offering virtual private-line replacement. As a network interface, it traces its origins to integrated services digital network (ISDN).

When ISDN was being developed, two transport services were envisioned: circuit-mode services for voice and transparent data, and packet (i.e., X.25 and frame relay) mode for data. Frame relay has since evolved into a network interface in its own right, independent of ISDN. It is now specified as a set of American National Standards Institute (ANSI) and International Telecommunications Union (ITU) standards.

The User Perspective

Although services are typically available with transmission rates from 64K bps to T1/E1 (1.53/2.05M bps), frame relay is defined as an access interface up to T3 or 45M bps. By contrast, the typical synchronous data link control (SDLC) multidrop line is a 4.8K or 9.6K bps analog line. The transmission of a typical two-page text document on a frame relay network takes 1/4 second at 64K bps and 1/100 second at 1.53M bps. Transmission of the same two-page text document on a SDLC multidrop line takes 3 1/3 seconds at 4.8K bps and 1 1/6 seconds at 9.6K bps.

To the user, a frame relay network appears simple and straightforward. Users connect directly to destinations on the far side of the network. Frame relay provides logically defined—links commonly called data link connection identifiers (DLCIs), permanent virtual circuits (PVCs), or permanent logical links (PLLs)—for a permanent virtual connection.

For example, user A is connected across the frame relay network through separate PVCs to both user B and user C. The PVCs are multiplexed across user A’s frame relay interface. Frame relay networks guarantee bandwidth to each PVC, but allow unused bandwidth to be shared by all active users. The guaranteed bandwidth of a PVC is specified as the committed information rate (CIR) of the PVC. A user’s traffic can have transmission data rates in excess of the CIR, referred to as the burst rate of the PVC.

User B appears to user A with frame relay address DLCI 100, and user A appears to user B with DLCI 80. A PVC connects user A’s frame relay interface through the frame relay network to user B’s frame relay interface. Each user’s DLCI numbers have local significance only. User A has a second PVC with its own DLCI number connecting to user C. In addition, each user has a local management interface, typically on DLCI 0 (see Exhibit 4-2-1).


Exhibit 4-2-1.  Frame Relay Permanent Virtual Circuit (PVC) Topology


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