Frame Relay Tutorial | Architecture, frame, header

Frame relay is a packet switching technology developed to send and receive data over high speed digital connection such as ISDN. Frame Relay is a standardized wide area network (WAN) protocol operates on layer-2 (i.e. data link layer) of the OSI stack. Its usage has declined in recent years with the rise of more advanced technologies such as MPLS and Ethernet. Understanding frame relay protocol architecture can provide insights into networking concepts.

ITU-T and ANSI have defined specifications for the frame relay connection between DTE and DCE. Frame relay network supports two types of virtual circuits i.e. SVC (Switched Virtual Circuit) and PVC (Permanent Virtual Circuit). Refer SVC vs PVC➤ for more information.

Frame Relay Features

Following are the features of Frame Relay:
• Developed for ISDN, but widely used in public and private non ISDN networks.
• Higher throughput compare to X.25 protocol.
• No hop to hop flow control or error control. Upper layer protocols should take care to detect and recover discarded frames.
• Multiplexing and switching of logical connections are taken care by layer 2 and not layer-3.
• Control signals are carried out by separate logical connection than user data.
• Supports multiple protocols such as NetBIOS, ATM, TCP/IP, voice etc.
• Each connection is identified by unique DLCI(Data link connection identifier).
Refer ATM vs Frame Relay➤ and Frame relay vs X.25➤ which mentions difference between X.25, Frame Relay and ATM virtual networks.

Frame Relay network architecture

There are several key components in a frame relay network. It includes virtual circuits (VCs), Data Link Connection Identifier (DLCI), Frame format, Access devices, Frame relay switches, frame forwarding , congestion management and performance monitoring.

Frame Relay Network

The figure-1 depicts architecture of frame relay network. It is packet switched network consisting of DTE (Data Terminal Equipment) and DCE (Data Circuit Terminating Equipment). Refer DTE versus DCE➤. Here frame relay switch is used to route the packets between two DTEs as explained below.

Frame Relay Topologies

Frame relay supports following WAN topologies as shown in the diagram.
• Peer (point-to-point)
• Star (hub and spoke), PVCs exist from main site to each remote site.
• Partial mesh
• Full mesh (Here each router has PVC to every router in the frame relay network)
The same have been depicted in the figure-4 above.

Frame relay provides user with multiple independent data links to one or more destination stations. Traffic directed on the data links are multiplexed to utilize access lines and resources efficiently. As multiplexing is done at data link layer, end to end delay is minimized.

Frame relay provides service similar to leased line. The frame relay network transport user traffic within frame regardless of its contents. Frame relay service is typically available at fractional and full T1/E1 rates. The service is also offered at T3 rates by some of the frame relay vendors.

Frame Relay Switch

The figure-5 depicts Operation of Frame Relay Switch. Ports are mapped with DLCI values. There are two tables used in frame relay configuration viz. frame relay map and frame relay switching table.

Frame relay map consists of two fields viz. IP address and DLCI values. Frame relay map is a table stored in RAM which defines remote interface(ip address) to which specific DLCI number is mapped. This table can be made automatically or manually depending upon frame relay topology.

Frame relay switching table consists of four fields viz. IN_port, IN_DLCI, OUT_port and OUT_DLCI.

FRAD-Frame Relay Assembler Disassembler

The figure-6 depicts Operation of FRAD (Frame Relay Assembler and Disassembler). It is a specialized device which is designed to provide connection between LAN and Frame Relay WAN (Wide Area Network).

Frame Relay Frame Format | Standard and LMI frame types

Standard Frame Relay Frame

The figure-2 depicts standard frame relay frame. The Data information to be transmitted over frame relay network is encapsulated in a frame which uses only 2-5 bytes of overhead as mentioned below.

Standard frame relay frame consists of start flag, header, data (variable length up to 16000 octets), FCS and end Flag. Start flag and end flag are used for frame delimiters or as frame synchronization. FCS is used as checksum for detecting errors. The fields of frame relay header have been described in table-1 below.

Standard Frame
Relay Header Field
DLCI (10 bits) (6+4) bits, It is the short form of Data Link Connection Identifier. Represents address of the frame which corresponds to PVC. DLCI value specified represents virtual connection between DTE and Frame Relay Switch. Each virtual connection which is multiplexed on single physical channel is represented by unique DLCI.
C/R 1 bit, designates whether frame is a command or a response.
EA 1 bit, Extended Address field, designates upto 2 additional bytes in frame relay header. It helps to expand number of possible addresses.
FECN 1 bit, Forward Explicit Congestion Notification, used in congestion control
BECN 1 bit, Backward Explicit Congestion Notification, used in congestion control
DE 1 bit, Discard Eligibility
EA 1 bit

LMI frame relay format

The figure-3 depicts LMI type of frame relay frame.
The fields of LMI frame are mentioned in the table-2 below.

LMI frame field Description
Flag It delimits the start and end of a frame.
LMI DLCI If this field is of value 1023, it identifies frame as LMI frame instead of basic standard frame relay frame.
Unnumbered Information Indicator Sets poll/final bit equal to zero
Protocol Discriminator contains value which indicates that frame is LMI frame
Call Reference contains zero, not used
Message Type labels frame either as "status inquiry message" or "status message"
Information Elements (IEs) consists of variable number of IEs. Each IE consists of IE identifier, IE length and Data fields.
Frame Check Sequence (FCS) It will ensure integrity of the data to be transmitted. It is used for error detection.

Advantages and Disadvantages of frame relay network

Advantages of frame relay :
• As there is no error detection incorporated in frame relay, greater speeds can be achieved.
• It can dynamically allocate bandwidth on need basis.
• Congestion control mechanism is implemented in frame relay. This reduces network overhead in the network. It implements two congestion notification mechanisms (FECN, BECN) as mentioned above. During congestion condition, these fields are set to '1'.

• It does not perform flow control and error control. This has to be taken care by upper layers.

Networking related links

RF and Wireless tutorials