1. WAN – Packet and Cell-switched Networks
X.25, Frame Relay
ATM
Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

2. Introduction
Starting from the 1980’s, the emphasis in wide area networking has been shifting from dedicated point-to-point connections (circuit-switched and leased lines) to switched alternatives (packet and cell-switched networks) because:
A full mesh network requires a large number of point-to-point connections while switched networks provide any-to-any connectivity.
Point-to-Point links are engineered to meet peak traffics. During non-peak hours, transmission capacity is wasted.
Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

3. Agenda Packet-switched Network
Connectionless and Connection-oriented Services
Datagram and Virtual Circuit
Permanent and Switched Virtual Circuits (PVC and SVC)
X.25 and Frame Relay Packet-switched Networks
Cell-switched Network – ATM
Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

4. Packet-Switched Network
In packet-switched network, messages are broken up into small packets before they are sent.
Packets from multiple sources travel through links and routers in an interleaved (STDM) fashion (i.e. the physical links are shared).
Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

5. Packets from multiple sources traveling through a packet-switch network
Source: Forouzan, Data Comm. and Networking, McGraw-Hill
Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

6. Advantages of Packet-Switched Network
Use network bandwidth more efficiently – as the communication links are shared by many users (instead of being dedicated to only one user as in the case of circuit-switched network)
Ideal for organizations that need to be on line all the time – because the service is usually paid by a fixed monthly connect fee plus usage charges
Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

7. Connectionless Transmission Service
Also known as Datagram service
Packets are forwarded on-the-fly by routers, based on the current best path to a destination.
Packets may travel through different routes and hence arrive the destination out of orders.
Normally work without error recovery procedures  unreliable
used to transfer short messages
e.g. UDP protocol
Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

8. Connectionless (i.e. Datagram) Service
Source: Forouzan, Data Comm. and Networking, McGraw-Hill
Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

9. Connection-oriented Transmission Services
A path (virtual circuit) must be established between source and destination across the network before packets can start to flow.
All packets go through the same path so they always arrive the destination in the right order.
Normally work with error recovery procedures  reliable
used to transfer long messages
e.g. X.25, Frame Relay networks, TCP protocol
Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

10. Virtual Circuits: PVC and SVC
A. Permanent Virtual Circuit (PVC)
similar to leased lines
established by user subscription and is always available to the DTE as a permanent resource
the major type of circuits available from packet-switched and cell-switched network providers
B. Switched Virtual Circuit (SVC)
like telephone calls
a connection is established first, then data are transferred and finally the connection is released.
Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

11. Permanent Virtual Circuit (PVC)
Source: Forouzan, Data Comm. and Networking, McGraw-Hill
Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

12. Switched Virtual Circuit (SVC)
Source: Forouzan, Data Comm. and Networking, McGraw-Hill
Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

13. X.25 Packet-switched Network
Has been around since the mid 1970's when most WAN circuits were still analog and thus susceptible to errors.
Provides very robust error checking to guarantees data integrity at the cost of large network delays
Speed ≤ 64 kbps
Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

14. Frame Relay Packet-switched Network
Became popular since the mid 1980’s when modern digital and optical networks have advanced to the extent that node-to-node error correction throughout the network is no longer necessary.
Leaves it up to the end stations (i.e. transport layer) to handle any errors.
Fast packet switching with minimal network delay.
Speed: up to T1 or even T3
Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

15. Frame Relay Packet-switched Network
Committed Information Rate (CIR)
defined as the rate at which the network agrees to accept data from a client over a specific virtual circuit
chosen at the time the connection is ordered from the communications provider
data sent in excess of the CIR may be discarded and not delivered when the frame relay network is congested
some inexpensive Frame Relay services are based on a CIR of zero
Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

16. Asynchronous Transfer Mode (ATM)
International standard for high-speed (up to 2.5 Gbps, usually run on optical fibers), connection-oriented, cell-switched technology.
Foundation of B-ISDN (Broadband ISDN) which supports both constant and variable bit rates.
No error recovery – just like Frame Relay network.
Unlike packet-switched networks where the packet size varies, ATM transmits fixed-length (53 bytes) data units called “cells.
because the cell size is so small and the throughput of the circuit is high, delay-sensitive traffic can be carried along with bursty types of data successfully. Voice and video work without glitches and data customers get bandwidth-on-demand.
Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

17. Asynchronous Transfer Mode (ATM)
Makes optimal use of circuit capacity – by statistically multiplexing cells onto the network according to the Quality-of-Service information contained in the cell header (i.e. like STDM)
QoS Classes:
Class A: constant bit rate (CBR), for uncompressed voice and video transmission
Class B: real-time variable bit rate (RT-VBR), for applications that can tolerate slight delays, e.g. compressed voice and video transmission
Class C: non-real-time variable bit rate (NRT-VBR), for bursty, non-critical applications, e.g. LAN-to-LAN data traffic.
Class D: two types - available bit rate (ABR) and unspecified bit rate (UBR), for file transfer and datagram transmission
Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

18. Source: WestNet Learning Technologies
Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

19. Where to Get More Information
WestNet Learning Technologies
Data Communications and Networking, by Behrouz A. Forouzan, Publisher: McGraw-Hill
Chapter 17, 18, 19
Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks