In this post, we are going to brief switching basics. Switching is the process, which is required for transfer information in the establishment on demand of an individual from the desired inlet to the desired outlet within a set of inlets and outlets. Telecommunication networks are designed to carry information between users. This information is provided by a user through the appropriate terminal unit. When two users want to exchange information the connection between them is established to exchange the data between them. But when there are more than two users in the network and for example, any two wants to communicate between them needs to be connected to each other. This becomes more complicated as the number of users increased. This is shown in fig. 1.
To simplify this switching node are introduced in telecom networks. This is shown in fig. 2.
In its simplest form, this switching node can provide a connection path between a pair of users on request. Users are connected to the node by the transmission system, which is called a subscriber loop. This connecting path may be provided for a duration of a connection in circuit switching and only when information is really transmitted for packet switching. A practical telecommunication network contains many switching nodes connected between themselves in hierarchical order.
Thus the basic function of the switching system is to establish an electrical path between a given pair of input and output. The set of input and output circuits are known as inlets and outlets respectively. To do this function there is need of hardware to establish such connection and is called as Switching Matrix or Switching Networks. Fig 3 shows a basic model of switching network.
As shown in fig. 3 there are N Inlets and M Outlets. When,
Number of Inlets (N) = Number of Outlets (M)
Then this type of switching network is known as Symmetrical Network.
The inlets and outlets may be connected to local subscriber lines or trunks. Fig. 4 shows such inlets and outlets connection.
As shown in fig there are four types of connection may be established
- Local call connection: It is between incoming subscribers and outgoing subscribers
- The incoming call connection is between incoming trunk and a subscriber.
- The outgoing call connection is between the subscriber and outgoing trunk.
- Transit call connection between incoming trunk and outgoing trunk.
Switching techniques for data transmission
Telephone networks are basically designed to carry voice traffic but can be used to carry data traffic.
- Voice traffic is continuous in nature. It is real time and requires low bandwidth.
- Whereas data traffic is non-real time or near real time. It is bursty in nature and requires high bandwidth for short duration.
Most large internetworks consist of multiple physical paths between sender and receiver. Intermediate routers are used for moving packets across these diverse links. Switching makes it possible. There are three types of switching.
- Circuit switching
- Message Switching
- Packet Switching
Circuit switching connects the sender and receiver by a single path for the duration of a conversation. This path exists as a physical circuit. Telephone switching equipment, for example, uses address numbers to establish a path that connects the sender telephone to the receiver telephone. Once a connection is established a dedicated path exists between both ends until the connection is terminated.
Circuit switching in computer networks operates much same way. A complete path (end-to-end) must exist before communication takes place. The computer that is initiating data transfer establishes the dedicated path and ensures reliable packet delivery.
On the upside, Circuit switching provides a dedicated transmission channel with guaranteed data rate. Also, there is no virtually no channel access delay after the circuit has been established.
On the downside, this type of dedicated connectivity is an inefficient use of the communications media. Dedicated channels are relatively expensive if you consider that no one else can use the channel while it’s occupied. This is the reason due to which circuit switching is subject to long connection delays. Note that ATM is an exception because of advanced internal technology.
Message switching does not establish a dedicated path between the two stations. Instead, conversations are divided into messages. Each message is packaged with its own destination address and then transmitted from switching equipment to switching equipment through the internetwork. Each router (node processor) receives the message, stores it briefly and then transmits it to the next device. This type of network sometimes called a store-and-forward Switching.
As shown in fig. each message switching equipment stores portions of the complete packet (message) as they traverse through the network. For data transmission routers are used. Each router must have sufficient storage capacity to handle many gigabytes of temporary transmissions. Message switching has some drawbacks like it does not support real-time connectivity.
Packet switching combines the advantages of both circuit and message switching. Packet switching breaks datagrams into small parts called packets. Each packet is constructed with source and destination addresses that allow it to work its way through the internetwork and find its destination. Because packets have strictly defined maximum lengths they can be stored in RAM instead of on disk, which makes store and forwarding faster and easier. In general, packet switching uses one of two strategies :
Datagram Packet Switching
Virtual Circuit Packet Switching
Datagram packet switching is similar to message switching in that each datagram (message) has various paths to choose from. Datagram floods the network channels and finds their way to the receiver using various paths. Incidentally, these paths are determined using intelligent routing mechanisms. Once datagram arrives, they are reordered according to the sequence number.
Virtual circuit packet switching relies on the logical connection between the sender and receiver. The connection is formed when the devices exchange messages at the outset of a conversation. These messages allow sender and receiver to agree on the conversational parameters, including message size, a path to be taken, and conversation length. This strategy is almost like circuit switching. The difference is that virtual circuits can be temporary, that is, lasting only as long as the sender and receiver think they should. If, for example, the two devices find a more efficient path, they can switch in mid-conversation. Virtual circuits appear to the user as point-to-point links, but in reality, they are temporary logical paths for packet switching.
Elements of Switching System
A switching system consists of a number of elements that switching, control, and signaling functions. Several elements of a switching system and their interconnection is shown in fig.
Trunk Interface circuits and Subscriber Line Interface Circuits are used for terminating the subscriber lines and trunks.
Switching Network provides the switching path. Basically, it is a hardware that is used for establishing a connection between a given inlet-outlet pair.
Junctors are used in the switching system and it provides a folded connection for the local subscribers and the service circuits. Service Lines are used for maintenance and testing purposes.
Line Scanning Units sense and obtain signaling information from the respective lines. Distributor Unit sends out signaling information on the respective lines.
The Control Subsystem sends out signaling information to the subscriber and other exchanges connected to the outgoing trunks. If the control subsystem is an integral part of switching network itself, systems are known as Direct Control switching systems. If the control subsystem is outside the switching network then it is known as Common Control switching systems. Common Control is also known as Indirect Control or Register Control.
Operator Console provides interconnection with the switching system for maintenance and administrative purposes
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