Basic Concepts of Frame Relay Explained in Easy Language

Frame Relay has been widely used for its features and lower cost. Despite newer technologies such as VPNs and ATM, Frame Relay remains useful. This tutorial explains basic concepts of Frame Relay and its terminology: virtual circuits (VC), permanent virtual circuits (PVC), switched virtual circuits (SVC), data terminal equipment (DTE), data circuit-terminating equipment (DCE), discard eligibility (DE), access link (the connection between a device and Frame Relay network), Local Management Interface (LMI) types, LMI status enquiry, Data-Link Connection Identifier (DLCI) numbers, Forward Explicit Congestion Notification (FECN), Backward Explicit Congestion Notification (BECN), access rate, and Committed Information Rate (CIR).

Fundamentals of Frame Relay

The following image shows a network. It uses point-to-point leased-line connections to connect routers.

This network has four routers, requiring six leased lines and three serial interfaces per router. The following formula calculates the required connections.

(N × (N – 1)) / 2 [Here N is the number of routers]

For four routers, you need (4 × (4 - 1)) / 2 = 6 leased lines. With 100 routers, you would need 4950 leased lines and 99 interfaces per router, more than high-end routers provide. Frame Relay solves this by turning physical interfaces into virtual ones. With Frame Relay, you can convert one physical interface into virtual interfaces equal to the required number of connections. In this case, you can create 99 virtual interfaces on each router's physical interface. It enables larger networks with fewer physical interfaces, as shown in the next figure.

With Frame Relay, six virtual (not physical) connections are required. It reduces hardware requirements by allowing multiple connections to share a single interface.

Frame Relay Terminology

In Frame Relay terminology, virtual connection lines are called Virtual Circuits (VCs). A Virtual Circuit (VC) is a logical rather than a physical path between two devices. VCs are categorized as either Permanent Virtual Circuits (PVCs), which are always available, or Switched Virtual Circuits (SVCs), which are set up as needed and terminated when not in use. The following table lists the differences between PVCs and SVCs.

SVCs are excluded from the CCNA exam. All further references to VC or PVC in this article refer only to PVCs.

Frame Relay Network Type

A Frame Relay network can either be fully or partially meshed. It is fully meshed when all routers are directly connected to each other. If some routers are not directly connected, the network is partially meshed.

Frame Relay Terminology

Frame Relay employs various terms to describe its components and functions.

DTE

DTEs (such as routers or PCs) convert frames into signals and communicate with DCE devices. It receives all required connection parameters from the connected DCE device.

DCE

DCE devices (such as modems or CSU/DSUs) connect a DTE device to the WAN network, provide connection parameters (such as clock rate and synchronization), and manage data timing.

CSU/DSU

A CSU/DSU (DSL or cable modem) converts signals between the LAN and the WAN.

Access link

An access link is the physical connection (such as a cable or wire) that connects the DTE device (like a router or PC) to the DCE device (such as a modem or CSU/DSU).

Frame Relay cloud

The Frame Relay cloud is Telco infrastructure.

VC

A VC is a logical path between two DTE endpoints for data transfer.

Access Rate

The access rate is the maximum speed (measured in bits per second) that your connection can transmit or receive data. The access link should use this speed as its clock rate, setting the pace for data transfer.

CIR (Committed Information Rate)

The Committed Information Rate (CIR) is the minimum guaranteed bandwidth (measured in bits per second) from the provider for a Virtual Circuit (VC). During network congestion, the provider guarantees at least this bandwidth for your connection.

For example, consider three networks using a Frame Relay switch to share a single path. Network1 and Network2 have an access rate of 128 Kbps and a CIR of 64 Kbps, while Network3 has an access rate and CIR both set to 64 Kbps.

When the access rate and CIR are equal, the Frame Relay connection functions similarly to a leased line. For example, Network3 pays for a 64 Kbps service and is guaranteed 64 Kbps, resulting in a connection that operates like a 64 Kbps leased line.

Network1 and Network2 select a flexible connection in which the access rate exceeds the CIR. They pay for the 64 Kbps CIR and any extra used, depending on availability and terms. The extra bandwidth is shared. When all users send data, the minimum guaranteed bandwidth is 64 Kbps.

If no other users are transmitting, the full 128 Kbps bandwidth is available. In this case, R1 can utilize the additional 64 Kbps.

• If no other user is transferring data, 128 Kbps is available.
• If all other users are transferring data, only 64 Kbps is available.
• If some users are transferring data, bandwidth speed may range from 64 Kbps to 128 Kbps.

Any data transmission above the CIR is called a burst. There are two types of bursts: B_c (committed burst rate) and B_e (excessive burst rate). B_c is the maximum amount of data that can be sent during a specific time interval above the CIR under normal conditions. B_e is the additional data that can be sent if the network allows it. B_c and B_e are measured in bits and are controlled by the network provider.

B_c (committed burst rate)

A small amount of additional bandwidth is permitted to accommodate minor traffic bursts.

B_e (excessive burst rate)

The remaining bandwidth is B_e (excessive burst rate). If allowed by the Telco, you can set the bandwidth at this rate, but it's rarely used.

Oversubscription

Oversubscription occurs when the sum of all CIRs (CIR + B_c + B_e) exceeds the access rate. Oversubscription is usually not allowed. Any data in excess is dropped.

Frame Relay Congestion Control

Frame Relay uses three data frame bits to signal congestion: DE, FECN, and BECN.

Discard Eligibility (DE)

Packets sent beyond the Committed Information Rate (CIR) may be dropped if the network is congested. The Discard Eligibility (DE) bit marks such packets. When the network is congested, the Frame Relay switch drops packets with the DE bit set. If there is no congestion, these packets pass through. The DE bit signals which packets are safe to discard during congestion.

Forward Explicit Congestion Notification (FECN)

It is a bit in the data frame header. When the Frame Relay switch detects congestion, it sets this bit to 1. It alerts the destination device along the virtual circuit's path that congestion has occurred, so it can take necessary action.

Backward Explicit Congestion Notification (BECN)

When frames marked with the congestion notification bit arrive at the destination, the destination router sends a frame back to the source with the Backward Explicit Congestion Notification (BECN) bit set. This frame informs the source device that congestion has been detected, prompting it to reduce or slow data transmission on the virtual circuit.

With little congestion, Frame Relay offers fast, reliable, and affordable service. But ongoing congestion reduces service quality, leading to many dropped frames.

Frame Relay LMI (Local Management Interface) protocol

Before sending data, a DTE device checks if the remote endpoint is active by using status-check messages called Keepalives. Devices regularly send Keepalive messages to each other; if a device does not receive a Keepalive within a certain time, it assumes the remote device or path is down. In Frame Relay, these Keepalives are exchanged with the network switch (not directly with other routers) to confirm connectivity.

Frame Relay uses the Local Management Interface (LMI) protocol to exchange keepalive messages between the endpoint device (DTE, such as a router) and the network device (DCE, such as a Frame Relay switch). The DTE sends LMI status enquiry messages to the DCE to confirm the network is operational. The DCE responds with a status reply that includes the status of all virtual circuits connected to the DTE, including their operational state and configuration settings (CIR, B_c, B_e, and Data Link Connection Identifier (DLCI) numbers). If the DTE does not get a response, it assumes the link or switch is down.

LMI status inquiry

LMI status enquiry

A simple query asking a simple question "Are you there?". The response to this query is also simple: "Yes, I am here.".

LMI full status enquiry

A complete query seeking full information: "Tell me everything that is related to me". The response to this query contains all information that is related to DTE. "Here is all the information that is related to you".

There are three types of LMI: Cisco, ANSI, and Q.933A. Each type differs slightly from the others, resulting in incompatibility. The same LMI option must be configured on both ends of the connection. If the LMI type is not manually configured, routers utilize the autosense feature. This feature enables the router to automatically detect the LMI type used by the Frame Relay switch and configure itself accordingly.

DLCI (Data Link Connection Identifier)

Frame Relay enables the connection of multiple VCs through a single physical access link. For example, six VCs can be connected via a single serial interface by dividing it into six sub-interfaces, each assigned to a VC. Frame Relay must identify which sub-interface corresponds to each VC before transmitting data. DLCI (Data Link Connection Identifier) numbers are used to map interfaces to VCs.

Because a VC has two endpoints, it requires two DLCI numbers, one for each end. The telecommunications provider assigns the DLCI values. The same or different DLCI numbers may be assigned to each end. The DLCI number must be unique only between the Frame Relay switch and the DTE router. If different DLCI numbers are assigned at each end, Frame Relay converts the DLCI in transit.

Conclusion

Frame Relay provides a cost-effective, efficient solution for connecting multiple sites in a wide-area network. Using virtual circuits instead of dedicated leased lines significantly reduces hardware and operational costs while maintaining flexibility and scalability. Its key features (such as virtual circuits, dynamic bandwidth allocation, and congestion control mechanisms) make it suitable for a wide range of networking scenarios, particularly when cost and resource optimization are priorities. While newer technologies have emerged, understanding Frame Relay remains essential for grasping the evolution of WAN technologies and their foundational concepts.