Frame Relay—A Shared Wide Area Network Service

Frame Relay is a network access method offered by local and long distance telephone companies. First implemented in 1992, Frame Relay is a public network offering that enables customers to transmit data between LANs in multiple locations (see Figure 6.9). It also is used to access the Internet. By using Frame Relay, organizations do not have to plan, build and maintain their own duplicate paths to each of their sites. Multiple users share the Frame Relay networks.

While Frame Relay has potential congestion problems, it has the following advantages:

• The network is managed by a long distance provider, not the end user. This is critical for companies that want to concentrate on their main business, not on maintaining their networks.

• Less hardware is required at each location than that used for private, dedicated networks. Fewer modems and multiplexers to connect sites together are needed. Each site need only be connected to the Frame Relay network, not to each location in the network.

• Capacity on Frame Relay is more flexible than that of private lines. Many fast-growing small companies, such as high-tech businesses, like the flexibility provided by Frame Relay to add capacity easily.

• Frame Relay has its own internal backup routes so that customers do not have to provide multiple routes for reaching each location.

Frame Relay was developed as a replacement for X.25. Frame Relay networks are faster than older X.25 packet networks because they do not perform extensive error checking in the network. X.25 packet networks checked each packet many times as it traveled through the packet network. This slowed down the transmissions. With Frame Relay service, it is up to customers to perform error checking and checks for dropped packets in their own routers. (See Chapter 1 for a description of routers.)

Connections to Frame Relay—Frame Relay Access Devices and Access Line Speeds

The line that connects each customer to the Frame Relay network is called an access line. It provides access from the user equipment to the Frame Relay network. Each site that uses the Frame Relay services leases a circuit, a telephone line, from its equipment to a port on the Frame Relay switch.

Access lines to Frame Relay networks run at various speeds depending on the amount of traffic generated at each site. Sites at different locations in the same organization can be configured with access lines at different speeds. Some Frame Relay vendors also offer dialup (e.g., ISDN) access to their networks for customers with small sites. Dialup connections require modems to dial a telephone number to transmit data. Dedicated connections are “always on.” Dialup access services also are used as a backup in case the dedicated access lines to the Frame Relay network fail. Some of the options for access lines are:

• 56 Kbps

• 128 Kbps

• 256 Kbps

• 384 Kbps

• T-1—1.54 megabits

• T-3—44 megabits

Some customers share their T-1 lines for voice and Frame Relay access. For example, 22 channels of the 24 T-1 channels may be terminated on the telephone system for voice traffic. The other two channels are used to carry Frame Relay traffic to the network service provider's Frame Relay port. This saves customers money on leasing access to the Frame Relay network.

Equipment on the customer premise converts the traffic from the local area network packets into frames compatible with the Frame Relay network. This equipment is called a Frame Relay access device (FRAD). It is often a card within the router. Each frame has bits called the flag, telling the network when the user data (frame) starts and when it ends. There also are addressing and destination bits in the frame for billing and routing purposes so that the Frame Relay provider knows where to route and bill each frame.

Customers' frames are sent to ports on high-speed switches that carry data from site to site within the carrier's Frame Relay network. The main technology the networks are run on is ATM (asynchronous transfer mode).

Frame Relay for Transmitting Voice

Organizations use Frame Relay to replace private lines for voice traffic between sites. Voice over Frame Relay is improving but is not as good as the voice transmitted on the standard public network.

The technologies used to transmit voice on Frame Relay are voice compression and silence suppression. Silence is suppressed so that pauses between words are used to transmit data and voice from other users. In addition, the voice itself is compressed, or made smaller, so that it does not require as much network capacity. Finally, voice traffic is given a high priority so that delay in voice conversations is minimized. The ATM switches (discussed later) located in core Frame Relay networks are able to prioritize packets containing voice traffic.

If the Frame Relay network becomes highly congested, voice quality can be degraded because packets are dropped or delayed. For this reason, some customer equipment has the ability to monitor traffic levels on the Frame Relay network and send it over the public switched network if there is congestion.

Frame Relay Pricing—Ports, Circuits and Committed Information Rate

Frame Relay service is priced at fixed monthly fees based on the following four elements, plus the cost of the telephone line used to connect each site to the Frame Relay service:

• The permanent virtual circuit (PVC) is a logical predefined path or link through a carrier's network. For example, if San Francisco and Tucson sites need to exchange data, the carrier defines a permanent virtual circuit between these two locations. PVCs are priced at fixed monthly fees.

• The switched virtual circuit (SVC): Unlike permanent virtual circuits, SVC charges are based on usage. Temporary connections are set up between points on a Frame Relay network. SVCs can be used to carry voice traffic if volumes are low. Thus, users only pay for what they use instead of incurring fixed monthly fees associated with permanent virtual circuits.

• The Frame Relay port is the entry point, on a Frame Relay provider's switch, to the Frame Relay network. Multiple permanent virtual circuits can use one port. Ports are available in variable speeds such as T-1, 56 Kbps, 256 Kbps and 512 Kbps.

• The committed information rate (CIR) is the minimum guaranteed number of bits-per-second throughput, typically half the capacity of the port the customer is guaranteed to be able to send from each site. Some customers save money by using zero committed information rate. Customers can “burst,” sending data at the maximum speed of their Frame Relay port. For example, customers connected to a 1.54 megabit per second port can send data at up to 1.54 megabits per second even if their committed information rate is zero. (This assumes they have a 1.54 megabit per second access line.)

Potential Congestion on Frame Relay

Customers rely on carriers not to oversell capacity on their Frame Relay networks. Once end users have the service, they depend on their carrier's managing capacity, using the best telecommunications switches and providing them with reports on the success of their transmissions. If the carrier's network is oversubscribed, the carrier might drop frames. Organizations with mission-critical data communications or high levels of security requirements often hire the staff and spend the money to manage their own private networks.

Many carriers now offer customers the ability to monitor traffic on Frame Relay networks via connections to the Internet. Customers are able to see traffic levels in terms of packets they are sending and how close to capacity they are. This helps them know if they have ordered the correct speed of permanent virtual circuits between sites, ports into the network and access lines.

Interfacing Between Carriers' Frame Relay Networks

When different public Frame Relay networks send data to each other, they use a standard protocol defined by the Frame Relay Forum called Network-to-Network Interface (NNI). This is useful when customers have locations not served by their Frame Relay network.