FRAME RELAY

What is Frame Relay?• high-performance WAN protocol• operates at the physical and data link layers• Originally designed for use across ISDN interfaces• An example of packet-switched technology• described as a streamlined version of X.25

Introduction• Frame Relay (FR) is a high-performance WAN

protocol that operates at the physical and data link layers of the OSI reference model.

• FR originally was designed for use across Integrated Service Digital Network (ISDN) interfaces.

• Today, it is used over a variety of other network interfaces as well.

• FR is an example of a packet-switched technology.• Packet-switched networks enable end stations to

dynamically share the network medium and the available bandwidth. 3

Frame Relay Devices• Devices attached to a Frame Relay WAN fall into the

following two general categories:• Data terminal equipment (DTE)

• DTEs generally are considered to be terminating equipment for a specific network and typically are located on the premises of a customer.

• Example of DTE devices are terminals, personal computers, routers, and bridges.

• Data circuit-terminating equipment (DCE)• DCEs are carrier-owned internetworking devices.• The purpose of DCE equipments is to provide clocking and

switching services in a network, which are the devices that actually transmit data through the WAN.

4

Frame Relay Devices (cont.)

5

Frame Relay vs. X.25• Frame Relay is a Layer 2 protocol suite, X.25 provides services

at Layer 3• Frame Relay offers higher performance and greater

transmission efficiency than X.25

Frame Relay Devices

Figure 1 Frame Relay Devices

Circuit-Switching• Long-haul telecom network designed for voice• Network resources dedicated to one call• Shortcomings when used for data:

• Inefficient (high idle time)• Constant data rate

8

Packet-Switching• Data transmitted in short blocks, or packets• Packet length < 1000 octets• Each packet contains user data plus control info (routing)• Store and forward

9

Advantages with compared to Circuit-Switching• Greater line efficiency (many packets can go over shared link)• Data rate conversions• Non-blocking under heavy traffic (but increased delays). When

traffic becomes heavy on a circuit-switching network, some calls are blocked.

• Priorities can be used.

10

Disadvantages relative to Circuit-Switching• Packets incur additional delay with every node they pass

through• Jitter: variation in packet delay• Data overhead in every packet for routing information, etc• Processing overhead for every packet at every node traversed

11

Switching Technique• Large messages broken up into smaller packets• Datagram

• Each packet sent independently of the others• No call setup• More reliable (can route around failed nodes or

congestion)• Virtual circuit

• Fixed route established before any packets sent• No need for routing decision for each packet at each

node

12

Frame Relay Virtual Circuits• provides connection-oriented data link layer communication• a logical connection between two data terminal equipment

across a Frame Relay packet-switched network• provide a bi-directional communications path from one DTE

device to another

Frame Relay Virtual Circuits• Switched virtual circuits (SVCs)

• temporary connections requires sporadic data transfer between DTE devices across the Frame Relay network

• Call Setup• Data Transfer• Idle• Call Termination

Frame Relay Virtual Circuits• Permanent Virtual Circuits (PVCs)

• used for frequent and consistent data transfers between DTE devices across the Frame Relay network

• Data Transfer• Idle

Congestion Control Mechanism• Forward-explicit congestion notification (FECN)• Backward-explicit congestion notification (BECN)

Forward-explicit congestion notification (FECN)

• initiated when a DTE device sends Frame Relay frames into the network

• When the frames reach the destination DTE device, the frame experienced congestion in the path from source to destination

• flow-control may be initiated, or the indication may be ignored

Backward-explicit congestion notification (BECN)• DCE devices set the value of the BECN bit to 1 in frames

traveling in the opposite direction, informs the receiving DTE device that a particular path through the network is congested

• flow-control may be initiated, or the indication may be ignored

Frame Relay Discard Eligibility (DE)• (DE) bit is used to indicate that a frame has lower importance

than other frames• When the network becomes congested, DCE devices will

discard frames with the DE bit

Frame Relay Error Checking• common error-checking mechanism known as the cyclic

redundancy check (CRC)• CRC compares two calculated values to determine whether

errors occurred during the transmission

Frame Relay Network Implementation• consists of a number of DTE devices connected to remote

ports on multiplexer equipment via traditional point-to-point services

Public Carrier-Provided Networks

• Frame Relay switching equipment is located in the central offices of a telecommunications carrier

• The DCE equipment also is owned by the telecommunications provider

• The majority of today’s Frame Relay networks are public carrier-provided networks

Private Enterprise Networks• the administration and maintenance of the network are the

responsibilities of the enterprise• All the equipment, including the switching equipment, is

owned by the customer

Frame Relay Frames

Figure 3 Frame Relay Frame

Frame Relay Frames• Flags indicate the beginning and end of the frame• Three primary components make up the Frame Relay frame

• the header and address area• the user-data portion• the frame-check sequence (FCS)

Frame Relay Frames• The address area (2 bytes)

• 10 bits represents the actual circuit identifier• 6 bits of fields related to congestion management

Frame Relay Frame Formats• Standard Frame Relay Frame• LMI Frame Format

Standard Frame Relay Frame• Flags

• Delimits the beginning and end of the frame• The value of this field is always the same (7E or 01111110)

Standard Frame Relay Frame• Address – contains the following information

• DLCI: The 10-bit DLCI is the essence of the Frame Relay header, values have local significance only, devices at opposite ends can use different DLCI values for the same virtual connection

Standard Frame Relay Frame• Address

• Extended Address (EA): used to indicate whether the byte in which the EA value is 1 is the last addressing field, the eighth bit of each byte of the Address field is used to indicate the EA

Standard Frame Relay Frame• Address

• Congestion Control: consists of the three bits; FECN, BECN, and DE bits

Standard Frame Relay Frame• Data – Contains encapsulated upper-layer data

• serves to transport the higher-layer protocol packet (PDU) through a Frame Relay network

Standard Frame Relay Frame• Frame Check Sequence

• Ensures the integrity of transmitted data

LMI Frame Format

Figure 4 Nine fields comprise the Frame Relay that conforms to the LMI format

LMI Frame Format• Flag - Delimits the beginning and end of the frame• LMI DLCI - Identifies the frame as an LMI frame instead of a

basic Frame Relay frame• Unnumbered Information Indicator - Sets the poll/final bit to

zero

LMI Frame Format• Protocol Discriminator - Always contains a value

indicating that the frame is an LMI frame• Call Reference - Always contains zeros. This field

currently is not used for any purpose• Message Type

• Status-inquiry message: Allows a user device to inquire about the status of the network

• Status message: Responds to status-inquiry messages. Status messages include keep-alives and PVC status messages

LMI Frame Format

• Information Elements—Contains a variable number of individual information elements (IEs)• IE Identifier: Uniquely identifies the IE• IE Length: Indicates the length of the IE• Data: Consists of one or more bytes containing

encapsulated upper-layer data

• Frame Check Sequence (FCS) - Ensures the integrity of transmitted data

Frame Relay Inverse ARP and LMI Operation (cont.)

Hello, I am 172.168.5.7.4

5Frame Relay Map

172.168.5.5 DLCI 400 Active

Frame RelayCloudDLCI=100 DLCI=400

Frame Relay Map

172.168.5.7 DLCI 100 Active5

172.168.5.5 172.168.5.7