X.25, Frame Relay & ATM

X.25, Frame Relay & ATM

Switched Network • Stations are not connected together necessarily by a single link • Stations are typically far apart • Messages are not broadcast to every station

Three Types of Switched Communication Network • Circuit-switched • Message-switched • Packet-switched

Circuit-Switched Network • Before any data can be sent, an end-to-end circuit must be established • This circuit is maintained for the duration of the transfer of all the data • The data can be digital or analog and the signal can be either type as well • Connection is usually full-duplex • Is inefficient – channel capacity is dedicated for the duration of the connection • Example – Pubic telephone system

Circuit-Switched Network – Cont. • Bits are transmitted as fast as they are received – no storage of data at the intermediate nodes • Disadvantages • Both stations must be available at the same time for data exchange • Resources in the network are dedicated for the duration of the transmission

Message-Switched Network • It is not necessary to establish a dedicated path between the two stations • The sending station appends a destination address to the message • The message is passed through the network from node to node • At each node the entire message is received, stored briefly, and then transmitted to the next node

Advantages of Message-Switched Network • Line efficiency is greater • Sender and receiver do not have to be available at the same time • Duplicate copies of message can be sent to different destinations • Message priorities can be established • Error control and recovery can be built into the network

Disadvantage of Message-Switched Network • Not suited to real-time traffic • Delay through network is relatively long and varies considerably

Packet-Switched Network • Very much like message switching • Principal external difference is that the length of the message found internally has a maximum length • A typical maximum length is several thousand bits • Messages above the maximum length are divided up into smaller units and sent out one at a time • These smaller units are called packets • Packets, unlike messages, are typically not filed at the intermediate nodes

Packet-Switched Network • The simple rule of limiting the maximum size of a data unit has a dramatic effect on performance • There are two different ways the network can handle the stream of packets that make up the message: • Datagram • Virtual circuit

Datagram Approach to Packet-Switched Network • Each packet is treated independently • The packets may take different paths to the destination • The packets might arrive in a different sequence from the order in which they were sent • The packets may have to be reordered at the destination

Virtual Circuit Approach to Packet-Switched Network • A logical connection is established before any packets are sent • All packets follow the same path through the network • This does not mean that there is a dedicated path, as in circuit switching

Advantages of the Datagram Approach • Call setup phase is avoided • This is important if a station wished to send only one or a few packets • More flexible – incoming packets can be routed away from congestion when it develops • Datagram delivery is more reliable – if a node fails, packets can be sent on an alternate route

Three Examples of Packet-Switched Protocols • X.25 – Virtual Circuit • Frame Relay – Virtual Circuit • ATM – Virtual Circuit

X.25 • Based upon existing analog copper lines that experience a high number of errors • Uses the virtual circuit approach • A set of international protocols approved in 1976 • Provides a way to send packets across a packet-switched public data network • The redundant error checking is done at each node.

X.25 Devices • Data Terminal Equipment (DTE) • Terminals, personal computers, and network hosts • Located on premises of subscriber • Data Circuit-terminating Equipment (DCE) • Modems and packet switches • Usually located at carrier facility • Packet Switching Exchange (PSE) • Switches that make up the carrier network

Sample X.25 Network

Packet Assembler/Disassembler (PAD) • Used for DTE devices that are too simple to implement X.25 (such as character-mode terminals) • Acts as intermediary device between DTE and DCE • Performs three functions • Buffering to store data until a device is ready to process it • Packet Assembly • Packet Disassembly

PAD in Action

X.25 Physical Layer • Several well-known standards are used for X.25 networks • X.21bis – supports up to 2 Mbps • 15-pin connector • RS-232 (EIA/TIA-232) – supports up to 19.2 Kbps • 25-pin connector • RS-449 (EIA/TIA-449) – supports up to 64 Kbps • 37-pin connector • V.35 – supports up to 2 Mbps • 34-pin connector • Uses serial communications in either asynchronous or synchronous modes

X.25 Data Link Layer • Link Access Procedure, Balanced (LAPB) is the protocol used for this layer • LAPB is a version of HDLC • HDLC in Asynchronous Balanced Mode (ABM) • DTE and DCE are peers and can both perform all functions • LAPB manages communication and packet framing between DTE and DCE devices • Makes sure that frames are delivered in sequence and error-free • Uses sliding window of 8 or 128 frames

Frame Relay • No longer need the overhead associated with X.25 and analog copper wires • Similar to X.25, but does not have the added framing and processing overhead to provide guaranteed data transfer • Link-to-link reliability is not provided – if a frame is corrupted, it is silently discarded • Upper-level protocols such as TCP must detect and recover discarded frames • See Figure 2-9 for Frame Relay encapsulation of IP datagrams

Frame Relay History and Overview • Frame Relay was originally designed for use on Integrated Services Digital Network (ISDN) • Usually considered a replacement for X.25 using more advanced digital and fiber optic connections • Does not perform error correction at intermediate nodes making it faster than X.25 • When an error is detected (FCS) the frame is discarded and correction is left up to higher layer protocols • Original standard proposed in 1984 but widespread acceptance did not occur until the late 1980’s • Service Description Standard (ITU-T I.233) • Overall service description and specifications, Connection Management • Core Aspects (ITU-T Q.922) • Frame Format, Field Functions, Congestion Control • Signaling (ITU-T Q.933) • Establishing and Releasing switched connections and status of permanent connections

Frame Relay Devices • Data Terminal Equipment (DTE) • Terminals, Personal Computers, routers, and bridges typically at the customer location • Data Circuit-terminating Equipment (DCE) • Typically packet switches owned by the carrier that transmit data through the WAN

Sample Frame Relay Network

X.25 ATM PPP X.25 ATM PPP Frame Relay FRAD FRAD Frame Relay Assembler/Disassembler (FRAD) • To handle frames from other protocols a FRAD is used to provide conversion to Frame Relay packets • A FRAD can either be a separate device or part of a router/switch

Frame Relay mapping to OSI Model Application Other Services Presentation Session Transport Network Data Link LAPF Frame Relay Protocol Physical Any Standard

Frame Relay Physical Layer • No specific protocol is defined • Any protocol recognized by ANSI can be implemented

Frame Relay Data Link Layer • Link Access Protocol for Frame Modes Services (LAPF) is the protocol defined for Frame Relay Layer 2 services • LAPF is a version of HDLC • Does not provide flow or error control • Uses Address field for DLCI (addressing) as well as for congestion control

ATM • Destined to replace most existing WAN technologies • Improves on performance of Frame Relay • Based upon 53-byte cells of fixed size • 48 bytes of application information together with a 5-byte ATM header • The standard-sized cells allow switching mechanisms to achieve faster switching rates • Rates of 155 – 622 Mbps are achieved with theoretical rates up to 1.2 Gbps • Compatible with twisted-pair, coax, and fiber

X.25, Frame Relay & ATM