Switching in Computer Network

Switching

 Whenever we have multiple devices, we have the problem of how to connect them to make one-to-one communication possible. One solution is to establish point-to-point connection between each pair of devices using mesh topology. However, mesh topology is impractical for large number of devices, because the number of links increases exponentially with number of systems. That means we need n(n-1)/2 connections to establish a mesh networks with n systems. A better alternative is the use of switched network. In this model, the network consists of a set of interconnected nodes among which information are transmitted from source node to destination node through different routes, which is controlled by the switching mechanism. A switched network consists of a series of interlinked nodes, called switches. There exist three methods of switching techniques: circuit switching, packet switching, and message switching.

Taxonomy of switched Networks

Circuit Switching

Circuit switching creates a direct physical connection between two devices, such as computers or phones. A circuit-switched network is made of a set of switches connected by physical links, in which each link is divided into n channels. The circuit switching process involves three main activities,

1.     circuit establishment

2.     Data transfer

3.     Circuit disconnect

Thus, the actual physical connection between the source and destination host must be established before the data is transmitted. The connection ones established remains exclusively and continue for the entire duration of data exchange and the circuit becomes disconnected only when sender wants so.

The three phases are also known as setup phase, data transfer phase and teardown phase respectively. Circuit switched networks are not as efficient as the other two types of networks because resources are allocated during the entire duration of the connection. This may cause resource wastage. The delay in this type of network is minimal. Traditional telephone systems are use circuit switching in physical layer.

Space Division Switching

In space division switching, the paths in the circuit are separated with each other spatially that is different ongoing connections at a same instant uses different switching paths, which are separated spatially. Crossbar switches and multistage space division switches are example of space division switching.

 Time division switching

Time division switches use time division multiplexing to achieve switching. There two methods used in time division switching: time slot interchange and TDM bus.

Time slot interchange (TSI)

We are using the following example to discuss TSI and TDM,

Assume that we have four nodes at sending side and four nodes at receiving. The nodes are labeled  as shown below,

Time division multiplexing

The above figure shows the results of ordinary time-division multiplexing. Data are received at the same order as they are send, data from 1 go to 1, data from 2 to go to 2, from 3 to go to 3 and from 4 go to 4. If we want to send the data in the following order, it is not possible with time division multiplexing,

1 to 3

2 to 4

3 to 1

4 to 2

In this type of situations, we use time slot interchange techniques 

Time slot interchange

Here we insert a device called a time slote interchange into the link. TSI changes the ordering of the slots based on the desired connection. 

Message Switching

In message switching instead of establishing a dedicated physical connection between source and destination, the message is sent to the nearest directly connected switching node. This node stores the message, checks for errors, select best available route and forwards the message to the next intermediate node. The line becomes free again for other messages, while the process is being continues in some other nodes. Due to the mode of action, this method is also known as store and forward technology.

Where the message hope from node to node to its final destination . each node stores the full message, checks for errors and forward it.

Basic Idea

·        Each node receives and store the message

·        Determine the next leg of the route

·        Queues the messages to go out on that line

Advantages

·        Data rate conversion possible

·        Message priorities can be used, to satisfy the requirements if any

·        Line efficiency is greater.

·        Even under heavy traffic, packets are accepted possibly with a greater delay in delivery.

Packet switching

Circuit switching is less suitable for data and other non-voice transmission.  In the case of non-voice transmission, the line may often idle and its facilities are wasted. Circuit switching is inflexible once circuit has been established that circuits is the path taken by all parts of the transmission whether or not it remains the most efficient available.

In packet switched network, data are transmitted in discrete units pf variable block length called packets. The maximum length of the packet depends on the network. Each packet contains one data part and a header with control information’s. The packets are sent over the network; machine to machine, the packet is stored briefly then routed according to the control information in header. In a packet-switched network, there is no resource reservation; resources are allocated on demand.

Two approaches are used to packet switching that is datagram and virtual circuit approach

Data gram networks

In datagram approach to packet switching, each packet is transmitted independently from all other. This approach can cause the datagram’s of a transmission to arrive at their destination out of the order. The datagram networks are sometimes referred to as connectionless networks. A switch in a datagram network uses a routing table that is based on the destination address. Switching in the Internet is done by using the datagram approach to packet switching at the network layer.

Virtual circuit Approach

In the virtual circuit approach to packet switching, the relationship between all packets belonging to a message is preserved. A single route is selected between source and destination at the beginning of the session. When data are sent, all packets of the transmission travel one after another along the route. The packets may arrive at the destination with different delays if resource allocation is on demand. There exist two types of virtual circuit switching techniques switched virtual circuit (SVC) and Permanent Virtual Circuit (PVC).

Switched virtual circuit (SVC)

in this approach a virtual circuit is created and exists only for the duration of the current specific exchange.

Permanent Virtual Circuit (PVC)

In PVC the same virtual circuit is provided between two systems and because it is always in place, it can be used without connection establishment and connection termination.

Public Switched Telephone Network (PSTN)

PSTN is an example of circuit switched network. The hierarchy of PSTN is shown below, 

PSTN hierarchy

1. Regional offices (class 1)

2. Sectional offices (class 2)

3. Primary offices (class 3)

4. Toll offices (class 4)

5. End offices (class 5)

Local loops

The local loop, a twisted-pair cable that connects the subscriber telephone to the nearest end office or local central office when used for voice, has a bandwidth of 4000 Hz.

Trunks

Trunks are transmission media that handle the communication between offices. Transmission is usually through satellite links or optical fibers.