RFFrame Relay Tutorial: Architecture, Frame, and Header
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Frame Relay is a packet switching technology designed for sending and receiving data over high-speed digital connections, like ISDN. It’s a standardized wide area network (WAN) protocol that operates on Layer 2 (the data link layer) of the OSI model. While its use has declined with the rise of MPLS and Ethernet, understanding Frame Relay architecture provides valuable networking insights. ITU-T and ANSI have defined specifications for Frame Relay connections between DTE and DCE.
Frame Relay networks support two types of virtual circuits: SVC (Switched Virtual Circuit) and PVC (Permanent Virtual Circuit).
Frame Relay Features
Here’s a rundown of key Frame Relay features:
- Developed for ISDN but widely used in public and private non-ISDN networks.
- Offers higher throughput compared to the X.25 protocol.
- Doesn’t implement hop-to-hop flow control or error control. Upper-layer protocols are responsible for detecting and recovering discarded frames.
- Multiplexing and switching of logical connections are handled by Layer 2, not Layer 3.
- Control signals are carried on separate logical connections from user data.
- Supports multiple protocols, including NetBIOS, ATM, TCP/IP, and voice.
- Each connection is identified by a unique DLCI (Data Link Connection Identifier).
Frame Relay Network Architecture
A Frame Relay network comprises several key components: virtual circuits (VCs), Data Link Connection Identifiers (DLCIs), frame format, access devices, Frame Relay switches, frame forwarding, congestion management, and performance monitoring.
Figure 1: Frame Relay Network Architecture
Figure 1 illustrates the architecture of a Frame Relay network. It’s a packet-switched network made up of DTE (Data Terminal Equipment) and DCE (Data Circuit Terminating Equipment). A Frame Relay switch routes packets between DTEs.
Figure 2: Frame Relay Topologies
Frame Relay supports the following WAN topologies:
- Peer (Point-to-Point)
- Star (Hub and Spoke): PVCs exist from the main site to each remote site.
- Partial Mesh
- Full Mesh: Each router has a PVC to every other router in the network.
Frame Relay gives users multiple independent data links to one or more destination stations. Traffic is multiplexed to efficiently utilize access lines and resources. Multiplexing at the data link layer minimizes end-to-end delay. Frame Relay offers a service similar to a leased line. The network transports user traffic within frames, regardless of content.
Frame Relay service is often available at fractional and full T1/E1 rates and sometimes at T3 rates, depending on the vendor.
Figure 3: Frame Relay Switch Operation
Figure 3 shows the operation of a Frame Relay switch. Ports are mapped with DLCI values. Two tables are crucial for configuration: the Frame Relay map and the Frame Relay switching table.
- Frame Relay Map: Contains IP address and DLCI values. This table, stored in RAM, defines the remote interface (IP address) to which a specific DLCI number is mapped. It can be created automatically or manually, depending on the topology.
- Frame Relay Switching Table: Consists of IN_port, IN_DLCI, OUT_port, and OUT_DLCI fields.
Figure 4: FRAD (Frame Relay Assembler and Disassembler) Operation
Figure 4 illustrates the operation of a FRAD, a device that connects a LAN to a Frame Relay WAN.
Frame Relay Frame Format: Standard and LMI Frame Types
Figure 5: Standard Frame Relay Frame
Figure 5 depicts a standard Frame Relay frame. Data for transmission is encapsulated in a frame with minimal overhead (2-5 bytes). The standard frame includes a start flag, header, data (variable length up to 16000 octets), FCS, and an end flag. Start and end flags serve as frame delimiters or for synchronization. FCS provides a checksum for error detection.
The fields of the Frame Relay header are described in the table below:
| Field | Description |
|---|---|
| DLCI (10 bits) | Data Link Connection Identifier, representing the address of the frame, which corresponds to the PVC. It represents the virtual connection between the DTE and the Frame Relay switch. |
| C/R (1 bit) | Designates whether the frame is a command or a response. |
| EA (1 bit) | Extended Address field, allowing for up to 2 additional bytes in the header, expanding the number of possible addresses. |
| FECN (1 bit) | Forward Explicit Congestion Notification, used for congestion control. |
| BECN (1 bit) | Backward Explicit Congestion Notification, used for congestion control. |
| DE (1 bit) | Discard Eligibility. |
| EA (1 bit) | Extended Address. |
Figure 6: LMI Frame Relay Format
Figure 6 depicts the LMI type of Frame Relay frame. The fields of the LMI frame are mentioned in the table below.
| LMI Frame Field | Description |
|---|---|
| Flag | Delimits the start and end of a frame. |
| LMI DLCI | A value of 1023 identifies the frame as an LMI frame instead of a basic standard frame relay frame. |
| Unnumbered Information Indicator | Sets the poll/final bit equal to zero. |
| Protocol Discriminator | Indicates that the frame is an LMI frame. |
| Call Reference | Contains zero and is not used. |
| Message Type | Labels the frame as either a “status inquiry message” or a “status message.” |
| Information Elements (IEs) | Consists of a variable number of IEs, each with an IE identifier, IE length, and data fields. |
| Frame Check Sequence (FCS) | Ensures the integrity of the data to be transmitted and is used for error detection. |
Advantages and Disadvantages of Frame Relay Networks
Advantages:
- Higher speeds can be achieved due to the absence of error detection.
- Bandwidth can be dynamically allocated as needed.
- A congestion control mechanism is implemented, reducing network overhead. Frame Relay uses FECN and BECN congestion notification mechanisms. During congestion, these fields are set to ‘1’.
Disadvantages:
- It doesn’t perform flow control or error control, which must be handled by upper layers.
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