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

andDatagram Operation

17

InternalVirtualCircuit

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