8. Frame-Relay Technology

Frame Relay Technology
a Tutorial
By Inderdeep Singh

Today's LANs and computing equipment have the potential to run at much higher speeds and transfer very large quantities of data. With the diversity and complexity of today's networks, management can be a mammoth task if you don't have the proper tools. Each environment is a unique combination of equipment from different vendors. Frame Relay, which is a relatively new wide area networking method, is gaining in popularity. It uses a packet-switching technology, similar to X.25, but is more efficient. As a result, it can make your networking quicker, simpler, and less costly.

Frame Relay Network pic.jpg (38313 bytes)

Frame Relay was developed to solve communication problems that other protocols could not: the increased need for higher speeds, an increased need for large bandwidth efficiency, particularly for clumping ("bursty" traffic), an increase in intelligent network devices that lower protocol processing, and the need to connect LANs and WANs.

Like X.25, Frame Relay is a packet-switched protocol. But the Frame-Relay process is streamlined. There are significant differences that make Frame Relay a faster, more efficient form of networking. A Frame-Relay network doesn't perform error detection, which results in a considerably smaller amount of overhead and faster processing than X.25. Frame Relay is also protocol independent-it accepts data from many different protocols. This data is encapsulated by the Frame-Relay equipment, not the network.

The intelligent network devices connected to a Frame-Relay network are responsible for the error correction and frame formatting. Processing time is minimized, so the transmission of data is much faster and more efficient.

In addition, Frame Relay is entirely digital, which reduces the chance of error and offers excellent transmission rates. Frame Relay typically operates at 56 kbps to 1.544 mbps.

What does Frame Relay do?

Frame Relay sends information in packets called frames through a shared Frame-Relay network. Each frame contains all the information necessary to route it to the correct destination. So in effect, each endpoint can communicate with many destinations over one access link to the network. And instead of being allocated a fixed amount of bandwidth, Frame-Relay services offer a CIR (committed information rate) at which data is transmitted. But if traffic and your service agreement allow, data can burst above your committed rate.

Why choose Frame Relay?

Since Frame Relay has a low overhead, it's a perfect fit for today's complex networks. You get several clear benefits: First, multiple logical connections can be sent over a single physical connection, reducing your internetworking costs. By reducing the amount of processing required, you get improved performance and response time. And because Frame Relay uses a simple link layer protocol, your equipment usually requires only software changes or simple hardware modifications, so you don't have to invest a lot of money to upgrade your system.

Since Frame Relay is protocol independent, it can process traffic from different networking protocols like IP, IPX, and SNA.

Frame Relay is an ideal choice for connecting Wide Area Networks (WANs) that have unpredictable, high-volume, and bursty traffic.  Typically, these applications include data transfer, CAD/CAM, and client-server applications.

Frame Relay also offers advantages for interconnecting WANs. In the past, setting up WANs required the use of private lines or circuit switching over a leased line. Single, dedicated lines are not needed to make each WAN-to-WAN connection with Frame Relay, reducing costs.

Permanent Virtual Circuits.

Essentially, a permanent virtual circuit (PVC) is your dedicated connection through the shared Frame-Relay network replacing a dedicated end to-end line. A PVC is needed for each site in the network, just as a private line is. But in a Frame Relay network, the bandwidth is shared among multiple users. So any single site can communicate with any other single site without the need for multiple dedicated lines.

PVCs function via a Local Management Interface (LMI), which provides control procedures. The control procedures function in three ways: link integrity verification initiated by the user device, network status report giving details of all PVCs, and network notification of whether a PVC's status changes from active to inactive or vice versa. Data-Link Connections (DLCs) are PVCs pre-configured by both sides of the connection. The DLC identifier (DLCI) is used as the logical address for frame-layer multiplexing.

What do I need to get started?

First, you need a Frame-Relay Bearer Service (FRBS), which is offered by the local telephone company. You'll sign up for a committed information rate (CIR), which might be 64 kbps. That means you're guaranteed the data will go through your PVC at this rate. But, depending on network traffic and what type of line you have, such as a fractional T1 line capable of 128 kbps, you may actually get higher transmission rates thanks to 2-second bursts of speed across the network. At peak times when there is a lot of congestion, you may only transmit at 64 kbps.

Next, you need Frame-Relay equipment. Since Frame Relay doesn't provide protocol conversion and error detection/correction, the end-user devices need to be intelligent. Typically you can access the Frame-Relay service through Frame-Relay devices, such as Frame-Relay Assembler/Dis-assemblers (FRADs), frame routers, bridges, or switches.

Frame Routers.

Frame routers translate existing data communications protocols for transmission over a Frame-Relay network, then route the data across the network to another frame router or other Frame-Relay compatible device. Frame routers can handle many types of protocols, including LAN protocols. They're used in environments that require T1 or slower network access speeds. Each router supports one of many physical data interfaces and can provide several user ports.

Bridges, Routers, and FRAID.

You can also use bridges, routers, or FRADs (Frame-Relay access devices). These devices aggregate and convert data into Frame-Relay packets.

Bridges are easy to configure and maintain, and they usually connect a branch off ice to a hub location.

Routers can handle traffic from other WAN protocols, re-route a connection if a line fails, or provide support for flow control and congestion control.

FRADs format outgoing data into the format required by a Frame-Relay network, and some even function as routers. They work well in applications where a site already has bridges and routers or when sending mainframe traffic over a Frame-Relay network.

What's next for Frame Relay?

While Frame Relay offers many benefits, a host of problems have to be overcome before it can be used effectively as a carrier for voice, fax, or video traffic. Until recently, the advancements were vendor-specific solutions that offered no interoperability. Recently ratified industry standards have addressed such issues as compression, packetization, and prioritization.

This move towards standardization has been led by the Frame-Relay Forum (FRF) and the International Telegraphic Union (ITU).

In February 1998, the ITU ratified an umbrella standard for simultaneous transmission of voice, data, and video traffic over I P. Known as H.323, this standard incorporates other newly adopted criteria, such as G.729 and G.723. These standards specify algorithms for compressing voice traffic (which usually travels over a full 64-kbps telco circuit) down to 8 kbps for (Voice Over Frame Relay) VOFR.

The FRF recently agreed to ratify two new procedures for VOFR. FRFA 1

specifies a process for connecting PBXs over Frame Relay to carry voice, data, and fax traffic over one PVC. FRF. 12 addresses packetization and (consequently) prioritization. It standardizes a procedure for Frame Relay to break down larger frames into a series of smaller ones.

This technique helps alleviate network congestion problems that occur during peak usage periods when larger data blocks queue up ahead of time-sensitive voice traffic. In lieu of a formal (Quality of Service) QoS protocol, such as that implemented by asynchronous transfer mode (ATM), FRFA 2 relies on smaller-sized packets to ensure predictable delay patterns and therefore maintain the quality and integrity of voice transmissions. Instead of traffic-snarling data packets clogging up the circuitry, smaller, fragmented data frames are interleaved with delay-sensitive traffic, reducing jitter and delay and clearing the path for voice calls.

Currently, (resource reservation protocol) RSVP is the only industry standard specifically designed to support traffic prioritization. While RSVP is rather limited compared to ATM's QoS capabilities, it is a dynamic mechanism that helps keep traffic flowing by activating automatically whenever voice packets are present on the line.

Frame relay data packet pic.GIF (45289 bytes)

The future.

As new standards continue to emerge, we predict you'll see more VOFR in data centers that rely heavily on international communications, where the potential for savings looms largest

Frame Format

This diagram illustrates the 'Two-Byte' frame format:

Two-byte frame

Frame Relay normally modifies the HDLC header from a 1 byte address field to a 2 byte address field, as seen above. You can have a 3 or 4 byte format as well. In addition, there is no Control field!
  • Starting Delimiter Flag - 0x7E
  • High order 6 bits of DLCI address
  • Command/Response used by higher order applications for end-to-end control
  • Extended Address bit which indicates whether this octet is the last one in the header. 0 means more to follow and 1 means that this is the end.
  • Low order 4 bits of DLCI address
  • Forward Explicit Congestion Notification (FECN) bit
  • Backward Explicit Congestion Notification (FECN) bit
  • Discard Eligibility (DE) bit
  • Extended Address bit. In this case, this is a 1, but there can be more address bits to follow to give 17 bits (3-byte address field) or 24 bits (4-byte address field).
  • Data - can be up to 16,000 octets, but the 16-bit FCS generally limits the whole frame size to 4096 octets.
  • Frame Check Sequence (FCS)
  • Ending Delimiter Flag - 0x7E

Valid 10 bit DLCI addresses are:

0 Reserved for ANSI Annex D and CCITT Annex A link management
1 - 15 Reserved
16 - 1007 Any PVC
1008 - 1018 Reserved
1019 - 1022 Reserved for LMI multicast
1023 Reserved for LMI link management
Additional address bytes allow for DLCI addresses greater than 1024, however these are not common.

Frame Relay Traffic Flow

The Local Access Rate is the clock speed of the port and is the rate that data traffic travels in and out of the port.

The financial costs for Frame Relay are based on Access Line speeds, Linking up the line and the Committed Information Rate (CIR).

The CIR is based on the expected volume of the traffic and can never exceed the line speed. An example is a customer buying a 9.6K CIR on a 64K access line. The customer will be guaranteed 9.6K speed but could burst up to 64K if the need arises, for which he may be charged or frames may be dropped. A customer could go for a zero CIR which would mean that all traffic would be marked as Discard Eligible (DE), so that in a period of congestion, these frames would more likely be dropped. The CIR is calculated as an average rate over a period of time called the Committed Rate Measurement Interval (Tc) and given by the formula CIR = Bc/Tc.

The Committed Burst (Bc) is maximum number of bits that a switch is set to transfer over any Tc. The formula Bc = Tc x CIR demonstrates that the higher the Bc to CIR ratio is the longer the switch can handle a burst.

The Excess Burst (Be) is the maximum number of uncommitted bits over and above the CIR, that the switch will try to forward.

The Excessive Information Rate (EIR) is the average rate over which bits will be marked with DE and is given by the formula EIR = Be/Tc. If you do not know Tc, then because Tc = Bc/CIR, you can substitute Tc into the EIR formula to give EIR = CIR * Be/Bc.

The Peak Rate is CIR + EIR.



The Forward Explicit Congestion Notification (FECN) notifies downstream (destination) nodes of congestion when set to '1'; whereas Backward Explicit Congestion Notification (BECN) notifies upstream (source) nodes of congestion when set to '1'. Statistical Time Division Multiplexing is used to provide a measure of this congestion. Discard Eligibility (DE), when set to '1', indicates non-priority traffic and is therefore eligible for discard in congested periods. Any traffic that goes above Bc is marked as DE.

As an aside, when measuring bandwidth you need to realise that you are measuring packets going into a buffer rather than packets going through a physical link so as a percentage you may sometimes see values over 100%.

In a IP environment TCP is used to provide reliable transport of data. The TCP window size slowly increases in size until a packet is dropped, then the window size rapidly shrinks before slowly growing again dynamically. This is often called Slow Start. In the diagram above, switch A gets congested due to traffic from all users that are supplied by the carrier, not just the client illustrated. It is a good idea to configure the edge router to listen to the FECNs and BECNs before the Frame Relay switches decide to drop packets, some of which may be yours. If your packets are dropped then the IP traffic will go through low Start and this adds to the congestion problem if Slow Start occurs continually.

Random Early Detect (RED) is a facility that picks on individual users at different times and drops their packets so forcing them to go through slow start at different times. This helps to spread out congestion and give control of when packets are dropped back to the client.

Link Management Interface (LMI)

Data Link Control Management Interface (DLCMI) software on the router checks for Access Line Integrity, new or deleted DLCIs (PVCs) and status of current DLCIs. There are three types of DLCMIs; ANSI T1 617D, Link Management Interface (LMI) and CCITT Annex A (Q.933a). The DTE and DCE need to be configured with the same one and therefore only needs to be the same locally between the router and the switch.

You can configure a router as a partial Frame Relay switch for testing purposes (hence the options, LMI switch, Annex D switch and Annex A switch), using modem eliminators or X.21/V.35 crossover cables will enable you to simulate a Frame Relay switch on one router connecting to a normally configures router. Also, 'DLCMI none' could be chosen if you wished to statically configure all the PVCs. With Rev 1 LMI the switch (DCE) dynamically sends DLCI information and addressing to the DTE (router) and we do not have to set up the DLCI numbers.

By default the router sends a Full Status Message every 6th poll and the switch responds with a Full Status Response. These polling intervals must be the same on the router as on the switch otherwise a no response to poll could result in the line being brought down. DLCI 1023 is the LMI control DLCI at the switch.

The A bit in the PVC status field of an LMI response is cleared when a particular PVC becomes inactive, the local switch can then inform other switches that this PVC is down.

If you have a Cisco-based Frame Relay network, then you have the capability of utilising Enhanced LMI which is used by routers to request QoS information from Frame Relay switches. This QoS information can then be used for FRTS and ensures that you do not have mismatches in configuration between the router and the switch.