COMPUTER NETWORKS
4TH Edition
BY Andrew s. Tanenbaum
Chapter5
WAN Transmission Media
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.
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.
WAN Transmission Media
• Public Switched Telephone Network
• High speed, High bandwidth dedicated
leased circuits
• High speed fiber optic cable
• Microwave transmission links
• Satellite links
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
WAN
Technology
Point-to-Point
Connection
Switched
Networks
Packet
Switching
Circuit
Switching
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
2- Switched Network
• Switching send data across different
routes
• Three types of switching
– Circuit switching
– Message switching
– Packet switching
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.
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
Circuit Switching
• Advantages
– guaranteed data rate
– once connected no channel access delay
• Disadvantages
– inefficient use of the transmission media (idle
time)
– long connection delays (first time)
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
Message Switching
• Route messages along varying paths for
more efficiency
• Switching devices are often PCs with
special software
Message Switching
• Advantages
– 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
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
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
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
Packet Switching
• Two methods of packet switching
– Datagram packet switching
– Virtual circuit packet switching
Datagram Packet Switching
• Message divided into a stream of packets
• Each packet has it’s own control
instructions
• Switching devices route each packet
independently
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