1. WAN Transmission Media
COMPUTER NETWORKS
4TH Edition
BY Andrew s. Tanenbaum
Chapter5
WAN Transmission Media

2. What is a WAN?
Wide area network or WAN is a computer network covering a wide geographical area, involving a vast array of computers.
The best examples is the internet.

3. How to build a WAN?
WANs are most often built using leased lines. A set of switches and routers are interconnected to form a Wide Area Network.

4. WAN Transmission Media
Public Switched Telephone Network
High speed, High bandwidth dedicated leased circuits
High speed fiber optic cable
Microwave transmission links
Satellite links

5. Services provided by PSTN
Voice – Plain Old Telephone Service (POTS)
Based on the Voice Channel (3600 Hz)
Data transmission services
consist of services such as :
switched : Switching send data across different routes
X.25: This set of protocols allows computers on different public networks to communicate through an intermediary computer at the network layer level.
Frame Relay: (FR). A high-speed packet-switched data communications service, similar to X.25.
ISDN: Integrated Services Digital Network (ISDN) is available from local phone providers and takes advantage of digital phone switching systems.
ATM: Asynchronous Transfer Mode

6. Technology
WAN
Point-to-Point
Connection
Networks
Switched
Switching
Packet
Circuit

7. 1- Point-to-Point Connection
Uses leased lines
It is a permanent full time connection, user has exclusive use.
Sending types:
Datagram transmission : Sending by packets
Data stream transmission : Sending by bytes
Advantage:
Line is always reserved for the user
Disadvantage:
Expensive

8. 2- Switched Network Switching send data across different routes
Three types of switching
Circuit switching
Message switching
Packet switching

9. Switching
Circuit Switching: A connection (electrical, optical, radio) is established from the caller phone to the callee phone. This happens BEFORE any data is sent.
Message Switching: The connection is determined only when there is actual data (a message) ready to be sent. The whole message is re-collected at each switch and then forwarded on to the next switch. This method is called store-and-forward. This method may tie up routers for long periods of time - not good for interactive traffic.
Packet Switching: Divides the message up into blocks (packets). Therefore packets use the transmission lines for only a short time period - allows for interactive traffic.

10. Circuit Switching
Connects the sender and receiver by a single physical path for the duration of the session
PSTN uses circuit switching
Before transmission a dedicated circuit must be established

11. Circuit Switching Advantages Disadvantages guaranteed data rate
once connected no channel access delay
Disadvantages
inefficient use of the transmission media (idle time)
long connection delays (first time)

12. Message Switching Each message is treated as an independent unit
has its own source and destination address
Each is transmitted from device to device
Each intermediate device stores the message until the next device is ready
store and forward

13. Message Switching
Route messages along varying paths for more efficiency
Switching devices are often PCs with special software

14. Message Switching Advantages Disadvantages
efficient traffic management
reduces network congestion
efficient use of network media
messages can be sent when receiver down
Disadvantages
delay of storing and forwarding
costly intermediate storage

15. Packet Switching Packet switching breaks messages into packets
Packets travel different routes (independent routing)
Each packet has its own header information
Packets small enough to be stored in RAM thus quicker than message switching

16. Packet Switching Advantages
improves the use of bandwidth over circuit switching
shorter transmission delays than message switching (stored in RAM)
less disk space
smaller packets to retransmit

17. Packet Switching Disadvantages More RAM More complex protocols
more processing power for switching device
Greater number of packets greater chance for packet loss or damage

18. Packet Switching Two methods of packet switching
Datagram packet switching
Virtual circuit packet switching

19. Datagram Packet Switching
Message divided into a stream of packets
Each packet has it’s own control instructions
Switching devices route each packet independently

20. Virtual Circuit Packet Switching
Similar to circuit switching
Before transmission of the sending and receiving device agree on:
maximum message size
network path
establish a logical connection (virtual circuit)
All packets travel on the same virtual path