BASIC OF WANs

Basics of WANs

A WAN typically consists of two components, which are:

• Transmission lines: These lines move bits of data between the nodes. Transmission lines could be circuits, channels, or trunks.
  
• Switching elements: Switching elements are specialized nodes to connect two or more transmission lines. The elements direct the information from an incoming transmission line to an outgoing transmission line. The switching element or the node has specialized software installed on it to determine the appropriate outgoing transmission line. The switching elements are also known as packet switching nodes.

Transmission Lines

Transmission lines used for WAN connectivity are normally leased from the service provider, such as a telephone company, and are known as leased lines. After being leased by an organization, the leased line functions as a dedicated transmission line that caters to the network traffic of the specific organization.

Switching

Switching is the technique of dynamically selecting a path from multiple paths that carry data to its desired location. WAN uses the following three types of switching techniques:

1. Circuit switching
2. Message switching
3. Packet switching

Circuit Switching
In circuit switching, a dedicated path or communication channel is set up for use between two communicating nodes. Until the connection lasts, only the two nodes can use the channel. After the communication between the two nodes is over, the channel becomes available to another set of communicating nodes. A telephone conversation is an example of circuit switching.

The biggest disadvantage of the circuit switching technology is that if the specified path fails, the communication between the two nodes is disrupted completely. Circuit switching also results in wasting the bandwidth of a communication channel. This is because, during a connection, the channel is not used for any other purpose.

Message Switching
In message switching, there is no direct communication channel between the sender and the receiver. The message is stored temporarily at the switching element and forwarded later. The message is always stored and forwarded in entirety. The message is forwarded from the switching element one hop at a time. A typical example of message switching is e-mail and fax.

NB: A message might consist of a number of data packets.

Message switching makes full use of the bandwidth of the communication channel because multiple messages can be sent using one channel. Message switching transmits the messages in their entirety so there is no need to assemble or disassemble the packets in the messages. In addition, message switching does not require the sender and the receiver to be present at the same time. However, the disadvantage of message switching is that the communication between the sender and the receiver is not interactive.

Packet Switching

Packet switching segments a message into small packets, which are then transmitted over a communication channel. Due to the breaking up of message into small packets, a channel can be used by many users at the same time.

The main advantage of packet switching is that due to fixed packet sizes, the data packets of a single user do not block the transmission channel. In addition, packet switching does not statically reserve the bandwidth. Instead, packet switching acquires and releases the bandwidth, as required. However, the disadvantage of packet switching is that the sizes of data packets are restricted.

The following figure shows WAN connectivity between two LANs at Florida and Texas, over the Internet by using the services of a service provider. This service provider could be a telephone company. T1 and E1 are dedicated phone lines.

WAN Connectivity