Networking And Switching

For transmission of data beyond a local area, communication is typically achieved by transmitting data from source to destination through a network of intermediate switching nodes; the purpose of the switching nodes is to provide a facility that will move the data from node to node until the data reaches the destination. The figure below shows a simple switching network, where the end-stations may be computers, telephones, etc.

Note that each station attaches to a node, and the collection of nodes is known as a communications network - or switched communications networks, because data entering the network from a station are routed to the destination by being switched from node to node. (The data is a generic term here, meaning voice, video, image as well as ordinary data - numerical text.)

Note the following observations:

(1). Some nodes (such as node 5 and node 7) connect only to other nodes and not to end stations. Their sole task is the internal switching of data.

(2). Other nodes (such as 1 and 4) have one or more end stations attached; in addition to their switching functions, they also accept data from the end stations and deliver data to those end stations.

(3). Node-to-node links are usually multiplexed, either using FDM or TDM.

(4). To enhance the reliability of the network, it is always desirable to have more than one possible path between a pair of end stations. However, the network is not fully meshed (that is, there is not a direct link between every possible pair of nodes - it would be too expensive).

Two different technologies are used in wide-area switched networks: circuit switching and packet switching. Another approach that have evolved from packet-switching is Asynchronous Transfer Mode (ATM or cell relay).

Circuit Switching

This switching technique has the following features:

 There is a dedicated communication path between two end stations

 That path is a connected sequence of links between network nodes.

 On each physical link - within the network core - a logical channel is dedicated to the connection.

Communication via circuit switching involves three phases:

 Circuit Establishment

 Before any signals can be transmitted, an end-to-end (station-to-station) circuit must be established. In completing the connection from one station to another, a test is made to determine if the destination station is busy or is prepared to accept the connection, a process known as signaling.

 Data Transfer

 After circuit establishment, information can now be transferred from source to destination station (subject to destination station being prepared to accept the connection). The data may be analog or digital, and the connection is normally full-duplex (data transfer can occur in both directions simultaneously).

 Circuit Disconnect

 After some period of data transfer, the connection is terminated, usually by the action of one of the two stations. Signals must be propagated to the core network nodes to de-allocate the dedicated resources.

Channel capacity must be reserved between each pair of nodes in the path between source and destination stations. Each switching node must also have the internal switching capacity to handle the requested connection. The switches must have the intelligence to make these allocations and device a route through the network.

Channel capacity is dedicated for the duration of a connection between two end-stations - even if no data are being transferred; thus circuit switching is inefficient. But once the call establishment is made, there is no delay in information transfer, for information is transferred at a fixed data rate, and the delay at each node is negligible.

The best-known example of circuit switching is the public switched telephone network (PSTN), a collection of national networks interconnected to form the international service. Although originally designed and implemented to service analog telephone subscribers, the network handles substantial data traffic through modems and is gradually being converted to a digital network.

Circuit switching technology has been driven by those applications that handle voice traffic. Why? Because one of the requirements for voice traffic is that there must be virtually no transmission delay, and no variation in delay (known as jitter). These requirements are necessary to allow normal human conversation, which is interactive or real-time. This means that a constant transmission rate must be maintained, as transmission and reception occur at the same signal rate.

Circuit switching achieved its widespread, dominant position because it is well suited to the analog transmission of voice signals; in today's digital world, it is inefficient.

Review Questions:

1. Briefly describe the various multiplexing techniques.

2. Identify the phases involved in circuit switching.

3. Describe the four components of the PSTN

4. Identify the two essential requirements of voice traffic. Why is circuit switching so well suited to analog voice communications?

Packet Switching

Long distance circuit-switching telecommunications network was originally designed to carry voice traffic - the majority of the traffic