4. Switching method classification. Channel, message and packet switching. Switching method comparison.

Switching - it is a process connection, disconnection, and informational exchange over electrical circuit. In Telecommunication network by means of switching realize connection between end subscriber terminal for transmitting (receiving) of information.

Switching methods

There are three basic switching methods. They are:

• Circuit switching;

• Message switching;

• Packet switching

Circuit switching (channel switching) establishes sequential connection of physical lines between two subscribers. This connection is called a voice transmission path. All the connections are duplex.

Every such connection includes three steps:

• Connection;

• Information exchange;

• Disconnection.

Message switching is associated with message mode of information transmission.

It is possible to improve the efficiency of channel usage by giving a subscriber only a part of the channel for a short period of time instead of giving him the whole channel. It will not be a real time connection, though. Because subscriber’s information is received and buffered before transmission.

If a telecommunication line has several transit nodes then every node receives and buffers subscriber’s messages. If there is an available transmission route then the node sends subscriber’s messages further according to their addresses. Every message has a header and information part. The header contains the destination address. Thus information delay is inevitable. This is why the message mode of information transfer can not be used for voice transmission.

Packet switching is associated with packet mode of information transfer

The packet mode of information transfer and its options (frame and cell modes) are capable of overcoming the mentioned drawbacks. All messages (usually the long ones) are divided into small equal parts. These parts are called the packets. Every packet has its unique serial number while its header contents the destination address. The packets can travel a switched network through different free routes in any order. However the packets are received and assembled in the initial order to recover the sent message. Delay is possible here. But it will be far less compared to the message mode because duration of retransmission for short packets is shorter compared to duration of retransmission for the whole message.

The real packet switches use the following modes. The datagram mode of virtual channel and virtual connection. The datagram mode means that every packet of a message goes regardless of other packets (in front or behind) of the same message. The mode of virtual channel requires a logical connection between subscribers before sending packets. It is called the virtual channel. Establishing the virtual channel, the virtual connection mode reserves buffer memory of all nodes along the route of packets. It guarantees no delays.

Circuit switching (channel switching) establishes sequential connection of physical lines between two subscribers. This connection is called a voice transmission path. All the connections are duplex.

Every such connection includes three steps:

• Connection;

• Information exchange;

• Disconnection.

Drawbacks of channel switching are listed below.

– Probability of lacking a channel in the required direction, which results in call loss.

– It is quite slow.

– Synchronization requirements.

– Low efficiency of channel usage. It is true because after establishing connection the channel can not be accessed by other connections even if the channel is idle.

– Rate of information transmission is constant (64 kbit/s). It limits subscribers in usage of the terminals with different productivity

Advantages of circuit switching are listed below.

– It provides real time information exchange, which means no delays.

– Controllers of switch nodes are involved in signaling process only.

Message switching is associated with message mode of information transmission.

It is possible to improve the efficiency of channel usage by giving a subscriber only a part of the channel for a short period of time instead of giving him the whole channel. It will not be a real time connection, though. Because subscriber’s information is received and buffered before transmission. If a telecommunication line has several transit nodes then every node receives and buffers subscriber’s messages. If there is an available transmission route then the node sends subscriber’s messages further according to their addresses. Every message has a header and information part. The header contains the destination address. Thus information delay is inevitable. This is why the message mode of information transfer can not be used for voice transmission.

Packet switching is associated with packet mode of information transfer

The packet mode of information transfer and its options (frame and cell modes) are capable of overcoming the mentioned drawbacks. All messages (usually the long ones) are divided into small equal parts. These parts are called the packets. Every packet has its unique serial number while its header contents the destination address. The packets can travel a switched network through different free routes in any order. However the packets are received and assembled in the initial order to recover the sent message. Delay is possible here. But it will be far less compared to the message mode because duration of retransmission for short packets is shorter compared to duration of retransmission for the whole message.

The packet switching has the following advantages compared to the channel switching.

– Channels operate effectively. It is possible to share channel resources dynamically with the help of priority system. The packets of high priority are transmitted faster (less delayed).

– The possibility to convert data transmission rate. It provides data exchange between the sources connected to channels with different throughput.

– No call losses even if the network is busy. Only information delay possible in this situation.

The real packet switches use the following modes. The datagram mode of virtual channel and virtual connection. The datagram mode means that every packet of a message goes regardless of other packets (in front or behind) of the same message. The mode of virtual channel requires a logical connection between subscribers before sending packets. It is called the virtual channel. Establishing the virtual channel, the virtual connection mode reserves buffer memory of all nodes along the route of packets. It guarantees no delays.

The drawbacks of packet switching are listed below.

– It requires some additional procedures of packet forming.

– It is necessary to receive all the packets to assemble a message.

– It is necessary to analyze all headers of all packets.