Packet Switching and X.25 Protocol

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Information about Packet Switching and X.25 Protocol

Published on March 13, 2014

Author: milesgalario


Packet Switching (X.25 Protocol) Prepared by: Miles Kevin B. Galario

Packets • Network packet is a formatted unit of data carried by a packet-switched network. • Consist of 2 types of data: – Control information – User data

Packet Format

Asynchronous data transmission

Packet Switching

Packet Switching • Began around early 1960’s • First developed by Paul Baran • Is a digital networking communications method that groups all transmitted data - regardless of content, type, or structure – into suitably-sized blocks, called packets.

Advantages of Packet Switching • Line efficiency • Data rate conversion • Packets are accepted even when network is busy • High data transmission quality • When it comes to billing: – Fee is just the duration of connectivity

Disadvantages of Packet Switching • More complex • Data Transmission Delays • Packets may be lost on their route (Datagram)

Packet Switching Datagram Approach

Datagram Packet Switching • 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

Datagram Approach

Packet Switching Virtual Circuit Approach

Virtual Circuit Packet Switching • 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 • There is a call set up before the exchange of data (handshake).

Virtual Circuit Approach

SVC vs. PVC • Switched Virtual Circuit (SVC) • Permanent Virtual Circuit (PVC)

16 External Virtual Circuit and Datagram Operation

17 Internal Virtual Circuit and Datagram Operation

Datagram vs. Virtual Circuits Datagram Virtual Circuits  No call setup phase  More flexible  Better if number of packets are not very large  Network can provide sequencing and error control  Packets are forwarded more quickly  Less reliable

Packet Switch vs. Circuit Switching • circuit switching was designed for voice • packet switching was designed for data • transmitted in small packets • packets contains user data and control info – user data may be part of a larger message – control info includes routing (addressing) info • packets are received, stored briefly (buffered) and past on to the next node

Packet Switch vs. Circuit Switching

Packet Switching Service Protocols • There are five protocols in use for packet- switched services: – X.25 – Asynchronous Transfer Mode (ATM) – Frame Relay – Switched Multimegabit Data Service (SMDS) – Ethernet/IP packet networks

X.25 Protocol • ITU-T standard for interface between host and packet switched network • Oldest packet switched service. • Almost universal on packet switched networks and packet switching in ISDN • X.25 offers switched virtual circuit and permanent virtual circuit services.

X.25 Protocol • X.25 is a reliable protocol, meaning it performs error control and retransmits bad packets. • Although widely used in Europe, X.25 is not in widespread use in North America. The primary reason is the low transmission speed, now 2.048 Mbps (up from 64 Kbps) .

PAD • Packet Assembler/Disassembler –is a communications device which provides multiple asynchronous terminal connectivity to an X.25 (packet- switching) network or host computer.

X.25 - Physical • Interface between station node link • Two ends are distinct – Data Terminal Equipment DTE (user equipment) – Data Circuit-terminating Equipment DCE (node) • Physical layer specification is X.21 • Can substitute alternative such as EIA-232

X.25 - Link • Link Access Protocol Balanced (LAPB) • Provides reliable transfer of data over link • Sending as a sequence of frames

X.25 - Packet • Provides a logical connections (virtual circuit) between subscribers • All data in this connection form a single stream between the end stations • Established on demand • Termed external virtual circuits

X.25 Use of Virtual Circuits

Asynchronous Transfer Mode (ATM) • Asynchronous transfer mode (ATM) is a newer technology than X. 25. • ATM for the MAN/WAN environment operates in a similar way to its operation over backbone networks discussed in the last chapter.

X.25 vs. ATM • 4 differences between ATM and X.25 are: –ATM performs encapsulation of packets, so they are delivered unchanged across the network. –ATM is unreliable; i.e., it provides no error control, so error control must be handled at another layer (typically by TCP).

X.25 vs. ATM • ATM provides quality of service information enabling priority setting for different transmissions types (e.g., high for voice, lower for e-mail). • ATM is scalable, since basic ATM circuits are easily multiplexed onto much faster ones.

Issues with X.25 • key features include: – call control packets, in band signaling – multiplexing of virtual circuits at layer 3 – layers 2 and 3 include flow and error control • hence have considerable overhead • not appropriate for modern digital systems with high reliability

User Data and X.25 ProtocolUser Data and X.25 Protocol Control InformationControl Information

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