SNA Connections Using Frame Relay

The first consideration in running SNA over a Frame Relay network⁴⁶ is that FR is the interface to a separate network. At the interface between a piece of SNA equipment and a Frame Relay network, many virtual links are multiplexed over a single physical connection. The SNA equipment must be able to multiplex many virtual links over a single real link. SNA equipment already does this for X.25 and for LAN network connections, but the FR connection is little different.

SNA connections over FR networks⁴⁷ promise to be much more stable and more efficient than similar connections over X.25 networks.

Error recovery

Because there is a link control running end-to-end across the FR network, network errors will be recovered by the link control.

When SNA is used over X.25 this does not happen. An error in the network causes the catastrophic loss of all sessions using that virtual circuit.⁴⁸

Interface Efficiency

Because there is no packetisation or packet level protocol to perform, the FR network interface is likely to use Significantly less resource within the attaching SNA product than is required to interface to X.25.

Network Management

FR has a signaling channel which allows the exchange of some network management information between the device and the network. X.25 has no such ability.

Nevertheless, at the current state (1991) of the definition it is not possible to provide full seamless integration of SNA network management with the management of the FR network over which it will run. To some extent the FR network (like X.25 networks) will form a "black hole" in the SNA network management system.

Multidrop

Although it is not in the FR standard, there is a potential ability to use FR for limited "multidrop" connection for devices located in close physical proximity to one another. This is a real problem in X.25 since the X.25 interface is strictly poi nt-to-poi nt, so that if there are many devices in the same location many links to the X.25 network are necessary.

Using FR, a relatively simple "interface splitter" device could be constructed which would allow the connection of multiple FR devices to the same network link attachment.

46 Information in this section is derived from an early SNA prototype implementation. The description is conceptual and may not accurately reflect the operational detail of any product.

47 There is no such thing as a Frame Relay network. Frame Relay is a network interface protocol not an internal network protocol. The same is true of the phrase "X.2S network". X.2S is an interface not a network protocol. However, it is convenient to refer to "FR networks" and to "X.2S networks" to mean "networks that support FR interfaces" and "networks

that support X.2S interfaces". In the case of Frame Relay, standards work is under way to specify the interconnections ( between nodes within a network using Frame Relay protocols but the work is not yet complete. Nevertheless, the vast

majority of FR networks proposed in 1992 do not use FR protocols internally.

48 Except in the case of the IBM System/36 and IBM AS/400 which are able to use an end-to-end link protocol (called ELLC) across the X.2S network. ELLC uses the elements of procedure of LAPB as an end-to-end network protocol.

802.2 Logical Link Control

I

Frame Relay support in SNA is an extension of the LAN support. Figure 58 shows how Frame Relay may be situated logically in relation to the two most important LAN architectures. There are differences of course.

1. The LAN environment at the Media Access Control level (MAC layer) is a connection less environment. You can send data to any device at any time provided you know the correct address to send to.

Frame Relay provides connections across the network. In the early networks these connections must be predefined although later there will be an ability to set up connections on demand (switched virtual circuits).

2. The LAN environment provides a broadcast function but Frame Relay does not.

3. A Frame Relay network can provide congestion information to attaching devices where a LAN typically does not.

4. The network management environment is quite different.

5. An end station in a LAN environment must support source routing protocols in order to allow communication across "source routing" bridges. In Frame Relay this is not needed.

These differences are either irrelevant or can be fairly easily accomodated.

Most important are the similarities:

1. Both SNA and Frame Relay are connection oriented systems. When SNA uses a LAN connection it usually builds a switched connection across the LAN.

2. Both LAN and Frame Relay take a whole data frame and transport it to another user. The functions of framing, addressing, error detection (but not recovery) and transparency are handled by the network attachment protocol (MAC layer).

3. In both LAN and Frame Relay a single physical attachment to a device may contain many virtual links to other devices.

The link control protocol used across the LAN is called IEEE 802.2. This is just another link control protocol like SOLC, LAPB and LAPO (all forms of HOLC).

The difference is that SOLC, LAPB and LAPO perform the functions of framing, transparancy (via bit-stuffing), error detection and addressing. In the LAN environment these functions are provided by the MAC protocol and in Frame Relay they are provided by the Frame Relay link control. IEEE 802.2 is simply a

Chapter 8. High Speed Packet Networking 155

link control protocol that leaves the responsibility for framing, addressing etc. to the. MAC function. Thus,B02.2 provides exactly the function that is needed for Frame Relay. In addition 802.2 uses an addressing structure that allows multiple Service Access Points (SAPs), which provide a way of addressing multiple independent functions within a single device.

Thus the link control used by SNA across a Frame Relay connection is IEEE 802.2 (the LAN link control). In addition SNA devices use the congestion information provided by the Frame Relay network to control the rate of data presented to the network (flow control) .

• ~--- Frame Relay Frame - - - ' - - - . .

+-- 802.2 LPDU ---~.

Address Control User Data FCS Flag

FLAG OLCil OLCi

OSAPI SSAP

(Variable) byte 0 byte 1

Figure 59. Frame Relay Format as Used in SNA. The LPDU format is exactly the same as that used in the LAN environment.

The HOLC family of link controls uses a "rotating window" scheme to provide delivery confirmation. Multiple blocks may be sent before the sender must wait for an acknowledgement from the receiver. This means that several blocks may be in transit at any time and compensates for propagation delays.

In IEEE 802.2 this rotating window mechanism is used for flow control across a LAN. Since most LANs contain bridges there is a need to control the flow of data in the case where the bridge becomes congested. The mechanism is exactly suited to use across a Frame Relay network.

The flow control mechanism operates at the sender end only and the receiver is not involved in its operation. The transmitter is allocated a number (n) of blocks it may send before it must wait for an acknowledgement. The receiver

acknowledges blocks by number and an acknowledgement of block number 3 (for example) implies correct receipt of blocks numbered 1 and 2 (but in practice most receivers acknowledge every block).

When the transmitter is notified by the FR network of congestion along its forward path (by receiving the BECN bit in the header of a received frame)49 it reduces its send window size to 1. In the uncongested state there may be n frames in the network between transmitter and receiver. When congestion is detected this is immediately reduced to 1.

As operation continues if the transmitter has had m (an arbitrary number) responses without seeing another congestion notification then the send window n is increased by one. This will continue to happen until the maximum transmit window size n is reached.

49 The transmitter's "forward" path is the "backward" path of blocks being received. Hence it is the BEeN bit that a transmitter must examine.

(

The described mechanism is about as much as any end user device can do. It is really a delivery rate control. However, it must be noted that if FR networks are to give stable operation then they will need flow and congestion controls

internally.

8.2.8 Disadvantages

As discussed above (section 8.2.5, "Characteristics of a Frame Relay Network"

on page 151), faster links are required than would be needed in a better controlled network (such as an SNA network) to handle the same amount of traffic. This is partly because the network cannot know about priorities within the link and partly because of the variability in frame lengths allowed.

In the environment where the cost of wide area links is dropping very quickly, this may not be important.