Frame Relay

Frame relay was designed for digital transmission through a reliable medium. There are many frame relay standards in relation to the data-link encapsulation type and Local Management Interface (LMI) functions used by modern frame relay carriers.

Frame Relay is a high-performance WAN protocol that can provide digital internetwork WAN connections up to 2.048 Mbps (and sometimes higher) in many parts of the world. Frame relay uses virtual circuits to connect sites and provide scalability by the definition of guaranteed data pipes (using a committed information rate). Frame relay is so popular because its scalable bandwidth offerings across digital pathways. Using standard Frame Relay configurations is a simple way of minimizing connectivity problems in large Frame Relay networks.

Frame relay was designed for digital transmission through a reliable medium, typically fiber optic, compared to X.25 that was originally designed for analog transmission through an unreliable medium such as standard telephone line.

The following is the main feature of frame relay:

1. Frame relay provides error detection but not error recovery
2. Frame relay can provide data transfer up to 1.54Mbps
3. Frame relay have a variable packet size (called a frame)
4. Frame relay can be used as backbone connection to LANs
5. Frame relay can be implemented over a variety of connection lines (56K, T1, T3)
6. Frame relay operates at Physical and Data link layers of the OSI model

When you sign up for a frame relay service, you are assigned a level of service called a Committed Information Rate (CIR). CIR is the maximum guaranteed data transmission rate you will receive on the frame relay network. When network traffic is low, you will likely be able to send data faster than the CIR. As network traffic increases, priority is given to the data coming from the customers with a higher CIR, and the effective rate may drop.

As frame relay networks assume a reliable transmission medium, each switch performs error checking but not error correction. The most common source of errors on a frame relay is not corrupt or lost packets, but rather network congestion. As network traffic increases, frame relay switches begin to drop the packets to keep up with the load.

The following is the concept how data is transmitted over a frame relay network:

1. Routers connect to a frame relay switch either directly or through a CSU/DSU
2. Frame relay networks simulate an “always on” connection with PVCs
3. Sending routers send data immediately without establishing a session
4. Frame relay switches performs error checking but not correction
5. Corrupted packets are simply dropped without notification
6. Packets travel through the frame relay cloud without acknowledgement
7. Error correction is performed by sending and receiving devices
8. Frame relay switches begin dropping the packets when congestion occurs
9. Congestion is the most common cause of packet loss on a frame relay network
10. Packets are discarded based on information in the Discard Eligible (DE) bit
11. Frame relay switches send backward explicit congestion notification (BECN) message to slow data transfer rates.
    

Frame relay addressing

Frame relay uses Data-Link Connection Identifiers (DLCIs) for each virtual circuit.

• The DLCI ranges between 16 and 1007
• The DLCI represents the connection between two frame relay devices
• The frame relay service provider assigns the DLCI when the virtual circuit is set up
• Each DLCI is unique for the local network but not for the entire WAN

Local Management Interface (LMI)

LMI is a set of management protocol extensions that automates many frame relay management tasks. LMI is responsible for managing the connection and reporting connection status.

1. Maintain the link between the router and the switch
2. Gather status information about other routers and connections on the network
3. Enable dynamic DLCI assignment through multicasting support
4. Make DLCIs globally significant for the entire network

Cisco routers support three LMI types: Cisco; ANSI and Q933a.

When you connect a router to the frame relay network, the router interface has a direct line to the frame relay switch at the service provider. Although there is only one physical path between the router and the switch, frame relay supports multiple virtual circuits. When configuring a frame relay connection or circuit, there are two options:

1. Point-to-point, which simulates a leased line – a direct connection with a destination device.
2. Multipoint, which configures each circuit to communicate with more than one destination device. The same circuit is used for multiple conversations.

You can configure the router with multiple sub-interfaces that allows configuring multiple virtual circuits, each with different configuration parameters.


When configuring a router for a frame relay, the DLCI number acts like a Data Link or physical device address. Because frame relay support multiple upper layer protocols, you will need to associate logical, network layer destination addresses with the DLCI number used to reach that address. For multiple connections, you have the following configuration options.

1. Dynamically associate DLCIs with inverse ARP protocol to dynamically discover destination addresses associated with a specific DLCI
2. Manually map addresses to DLCIs by identifying the address of each destination device, and associates each address with DLCI. Although more work, results are less prone to errors than when using inverse ARP.

If the interface or sub-interface uses a point-to-point connection, you do not need to associate Network layer addresses with DLCIs. This is because the interface and the corresponding DLCI has only one possible connection.

Frame relay minimum standard

There are many frame relay standards in relation to the data-link encapsulation type and Local Management Interface (LMI) functions used by modern frame relay carriers.

For the corporation, the following is the minimum requirement standard for frame relay:

1. The preferred serial connection type is a V.35 physical interface.
2. The IETF mode for frame relay encapsulation should be used for new services to ensure service interoperability.
3. The ANSI mode for LMI-type should be used for new services to ensure service interoperability.
4. The use of point-to-point sub-interfaces for all new frame relay configurations is required to minimize known network connection problems.

Frame relay networks commonly connecting business units of a large organization via WAN cloud which are regarded as private organization network. Each business usually connects to the internet via a properly firewall security configuration with the addition of network protection software. For large network environment, besides the network security software, the network management software is also a common organization need to support the management of the information security.

See also:

• WAN Technologies
• Router connection
• The best wireless routers