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Floating in a Frame Relay Cloud

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The efficiency of a corporation’s communication links has become increasingly important as the demand for LAN and WAN connectivity increases. Frame relay has moved to the forefront of fast-packet technologies and is quickly gaining acceptance as the solution to expensive leased lines and slow packet-switching networks. Frame relay excels in its ability to offer significant savings over private lines, providing greater bandwidth while still reducing telecommunications complexity for network managers. But what is frame relay, and can you implement it on an AS/400?

In this article, I take a quick look at frame relay on the AS/400. I explain what frame relay is and how it’s set up and configured on your AS/400 and offer you some tips for improving its performance.

Understanding the Frame Relay “Cloud”

Frame relay is a high-speed communications technology that is currently being used in hundreds of networks worldwide. In addition to providing LAN-to-LAN connectivity, frame relay can deliver WAN interconnection of Token-Ring, Ethernet, or Fiber Distributed Data Interface (FDDI) networks while still providing solutions for legacy SNA networks. It provides users with higher transmission speeds than X.25, as frames are switched to their destination with a delay of only a few bytes at a time.

Frame relay defines how frames, or packets, of information are routed through a fast-packet network. One of the key features of packet technology is that it contains the destination address in addition to the data. In a packet-switching network, each packet is transmitted individually and can even follow different routes to its ultimate destination. After all packets that form the message have arrived, they are recompiled to form the original message. Frame relay has the ability to deliver low overhead and reliable data transfer with little delay. It accomplishes all of this because frame relay operates at the physical and data link layers (layers one and two) of the Open Systems Interconnection (OSI) model for networking. That is, it operates below the network layer (layer three) of the model. Consequently, throughput is dramatically increased because each frame requires much less processing.

The high speed you can achieve makes frame relay a valuable asset in the connection of WANs of two or more LANs. Quite often, these connections exist in

geographically dispersed locations that are separated over long distances. A frame relay network, then, is often represented in diagrams as a cloud. This “network cloud” analogy exists for a couple of reasons. First, it is a good representation of the geographic distance that a frame relay network has the ability to close. Second, a frame relay connection is not a physical path but a logical path.

The AS/400’s Head Is in the Network Cloud

You can easily configure the AS/400 to take advantage of what frame relay has to offer. Starting with V3R2, the system provides support for the following frame relay connections:

• Direct supports communication using SNA, TCP/IP, and IPX. This support allows for multiple networks to communicate using the frame relay network as a backbone while excluding the need for expensive T1 lines.

• Bridged allows the AS/400 to communicate over a frame relay network through a remote bridge. These connections allow your AS/400 to appear as if it were locally attached to the LANs that are connected to the bridge.

Frame relay is connection-oriented with virtual circuits. A virtual circuit is essentially a connection between devices that acts as though it may be a physical connection between two hosts in a packet-switching network. Each virtual circuit is defined between endstations (the frame relay terminating equipment or FRTE). The switching is accomplished by the frame relay frame handler (FRFH). The frame handler can also be viewed as the network node in a frame relay network.

The connection from the FRTE to the FRFH is made through a Data Service Unit (DSU) and a Channel Service Unit (CSU). The DSU is a device that performs protective and diagnostic functions for a telecommunications line. The CSU is a device that connects a terminal to a digital line. Typically, the two devices are packaged as a single unit in a DSU/CSU. An effective analogy is to view this pair as a high-powered modem. Together, they convert the FRTE physical interface to the FRFH interface that the frame relay network uses. The three physical interfaces supported by the AS/400 to attach to the DSU/CSU are RS-449(V.36), X.21, and V.35.

On your AS/400, you use the Create Network Interface (frame relay) command (CRTNWIFR) to specify the physical interface (the Interface parameter) and the data transmission rate (the Linespeed parameter) of the line being used. The actual type of physical interface depends on the cable being used. To allow a frame relay connection to your AS/400, you must use an input/output processor (IOP). This IOP must be used in conjunction with the physical interface that attaches to the frame relay DSU. Consult Figure 1 for more information on data parameters and physical specifications for different types of AS/400 IOPs.

The NWI Clock Parameter specification has three values associated with it. *MODEM is the default value. You can use *LOOP to improve performance if your modem supports it, and you can use *INVERT if the modem does not support looped clocking. If you choose a transmission rate of 512,000 bps, you may need to specify *LOOP or *INVERT clocking.

One Quick Example

You can integrate frame relay into SNA with very few modifications and minimal disruption. If you decide to run SNA over a frame relay network, frame relay acts as the interface to a separate network. At the interface between the SNA equipment and the frame relay network, many virtual links are multiplexed over a single physical connection. The SNA equipment must be able to multiplex these links over a single real link. Figure 2 shows an SNA direct connection of two AS/400 systems, one in St. John’s and one in Ottawa. You can configure a direct connection using SNA, like the scenario above, by entering the commands shown in Figure 3. The CRTNWIFR command specifies that you

will create a frame relay communications interface, while Create Line Description (frame relay) specifies the physical connection that you will create between the AS/400 and the network. In this example, I define Advanced Program-to-Program Communications (APPC) controller and device descriptions by using the Create Controller Description (APPC) command (CRTCTLAPPC) and the Create Device Description (APPC) command (CRTDEVAPPC).

You can use the Data Link Connection Identifier (DLCI) to identify the logical connections between nodes. Each permanent virtual connection (PVC) is identified by a DLCI number that is assigned by the frame relay network. You can specify the DLCI number for your AS/400 on the DLC identifier (NWIDLCI) parameter of its line description commands or by specifying the number in the DLCI parameter of the CRTNWIFR command. The AS/400 will support up to 256 of these virtual circuits for each physical connection.

A frame relay network routes frames through a network by examining the DLCI field and then routing each frame to an outgoing link. At each node within the network, the frame format, size, and frame check sequence (FCS) are verified. If valid, the DLCI and FCS are replaced and the frame is routed to the next node. If invalid or damaged, the frame is discarded.

Tweaking Frame Relay Performance

Performance is always of concern for your network. If you choose a frame relay connection, consider increasing your frame sizes to supply better performance. Large frame sizes may not work well for error-prone lines or networks because longer times are required to retransmit large frames when errors are encountered. When frame relay networks become congested, frames are discarded to alleviate the congestion. Therefore, large frame sizes may not work well on congested frame relay networks where many retries for transmissions are necessary. The frame size is specified in the Maximum frame size (MAXFRAME) parameter in the line and controller descriptions. Discarded frames represent an overflow situation where the data sent is greater than the Committed Information Rate, so they are never re-sent.

The frame size is usually determined by the particular frame relay network to which the system is attached. The AS/400 system allows a maximum frame size of up to 8,182 bytes. Other frame sizes (265 through 8,182 bytes) may be supported.

Send in the Clouds

When evaluating frame relay for the AS/400, network configuration, speed, and economics all play decisive roles. The ability to offer reliable performance, scalability, and significant savings while still working with existing technologies makes frame relay an attractive alternative for most networking environments. Ultimately, however, only your requirements will be able to determine your choice of technology.

2666 High-speed Communications IOP Physical Interface Cable length NWI Clock Parameter Max. Data Transmission Rate RS-449 20 ft. (6.1 m) *MODEM, *LOOP 2,048,000 bps

80 ft. (24.4 m) *MODEM 64,000 bps

*LOOP 2,048,000 bps 150 ft. (45.7 m) *MODEM 64,000 bps

*LOOP 2,048,000 bps
V.35 20 ft. (6.1 m) *MODEM, *LOOP 2,048,000 bps 80 ft. (24.4 m) *MODEM, *LOOP 64,000 bps
X.21 20 ft. (6.1 m) *MODEM 2,048,000 bps

2629-2699 and 2809-2721 High-speed Communications IOP Physical Interface Cable length NWI Clock Parameter Max. Data Transmission Rate RS-449 20 ft. (6.1 m) *MODEM, *LOOP, *INVERT 2,048,000 bps

50 ft. (15.2 m) 80 ft. (24.4 m) 150 ft. (45.7 m) *MODEM 64,000 bps

*LOOP 2,048,000 bps
V.35 20 ft. (6.1 m) , 50 ft. (15.2 m) *MODEM, *LOOP, *INVERT 2,048,000 bps 80 ft. (24.4 m) *MODEM, *LOOP, *INVERT 64,000 bps
X.21 20 ft. (6.1 m) *MODEM, 2,048,000 bps 50 ft. (15.2 m) *MODEM, *INVERT 64,000 bps

Figure 1: Here are the supported physical interace specifications.

Frame Relay Network

Figure 2: Here's a diagram of the SNA direct connection between St. John's and Ottawa systems.

Configuration for the St. John’s Connection:
CRTNWIFR NWID(FRSTJOHNS) RSRCNAME(LIN211) NRZI(*NO) INTERFACE(*RS449V36) +
CLOCK(*MODEM) LINESPEED(1536000) LMIMODE(*TE) + TEXT(‘Frame Relay NWI at St. Johns’)

CRTLINFR LIND(FROTTAWA) NWI(FRSTJOHNS) NWIDLCI(23) MAXFRAME(1590) +
EXCHID(05601300) TEXT(‘Frame Relay DLCI to Ottawa + Branch’)
CRTCTLAPPC CTLD(FROTTAWA) LINKTYPE(*FR) SWTLINLST(FROTTAWA) +
RMTCPNAME(STJOHNS) EXCHID(05633966) DSAP(04) SSAP(04) TEXT(‘APPN Connection to Ottawa Office’)
CRTDEVAPPC DEVD(FROTTAWA) RMTLOCNAME(OTTAWA) CTL(FROTTAWA) +
LOCADR(00) TEXT(‘APPC connection to AS/400 in Ottawa’)

Configuration for the Ottawa Connection:
CRTNWIFR NWID(FROTTAWA) RSRCNAME(LIN121) NRZI(*NO) INTERFACE(*RS449V36) +
CLOCK(*MODEM) LINESPEED(64000) LMIMODE(*TE) + TEXT(‘Frame Relay NWI in Ottawa’)





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CRTLINFR LIND(FRSTJOHNS) NWI(FROTTAWA) NWIDLCI(21) MAXFRAME(1590) +
EXCHID(05633966) TEXT(‘Frame Relay DLCI to St. Johns + Branch’)
CRTCTLAPPC CTLD(FRSTJOHNS) LINKTYPE(*FR) SWTLINLST(FRSTJOHNS) +
RMTCPNAME(OTTAWA) EXCHID(05601300) DSAP(04) SSAP(04) TEXT(‘APPN Connection to St Johns Office’)
CRTDEVAPPC DEVD(FRSTJOHNS) RMTLOCNAME(STJOHNS) CTL(FRSTJOHNS) +
LOCADR(00) TEXT(‘APPC connection to AS/400 in St Johns’)

Figure 3: These are the commands used to configure the frame relay connection between St. John’s and Ottawa.

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