Try to compare three different modes of exchange

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Three kinds of exchange technology and its comparison 2010-09-22 17:17:57

Classification: System operation and Maintenance

A Circuit switching:

"Circuit switching" (circuit switching), also known as "line switching", is a connection-oriented service. Before the data circuit exchange between two computers through the communication subnet, the first is to establish an actual physical line connection in the communication subnet. The most common example of circuit switching is the telephone system. circuit switching is based on the principle of switch structure to achieve data exchange. The main task is to connect the input of the required communication with the output of the call, that is, the switch is responsible for establishing a physical path between the two. After the completion of the continuation of the task, the communication between the two sides of the content and format are not subject to the switch. The main feature of circuit switching is the requirement to establish an actual physical path between the two sides of the communication, and this path is exclusive throughout the communication process.

1 . Classification of circuit switching

Circuit switching is divided into time division switching (Timedivision switching,TDS) and air separation (space Division switching,SDS ) in two different ways.

Time Division Exchange is divided into a number of non-overlapping timeslot, by different time slots to establish a different sub-channel, through the time slot switching network to complete the time slot removal, so as to achieve the voice exchange between the line and outgoing Exchange Mode. The key to time-division switching is the switching of the slot position, which is controlled by the calling dial. In order to achieve timeslot switching, voice memory must be set. In the sampling period, n slots are deposited into N memory units, and the time slots are entered in order to deposit. If the output is read in a specific order, this can change the order of the time slots and realize the time slot exchange.

an empty sub-switch refers to the incoming line in the exchange process through the location of the space to select the outlet, and establish a continuation. After the communication is over, it is removed. For example, the artificial switch on one end of the plug rope attached to the plug hole, by the operator according to the call request to the other end of the plug rope connection is called the outlet plug hole, this is the most image of the air separation mode. In addition, electromechanical (Electromagnetic machinery or relay type), stepping, crossbar, semi-electronic, program-controlled analog user switches and broadband switches can use the principle of air separation to achieve the exchange requirements.

2 . Three stages of circuit switching

The entire circuit switching process consists of three stages of establishing a line, occupying a line, and transmitting and releasing a line. The following are presented separately.

( 1 ) Circuit Building

Just like a phone call to establish a path between the two sides of the call, the circuit exchange of data communication before the transmission of information before the call process to establish an end-to-end circuit. Its specific process is as follows.

① initiates a request to a terminal site (responder site) that transmits to the endpoint through an intermediary node.

② If an intermediate node has an idle physical line that can be used, receives the request, assigns the line, and transmits the request to the next intermediate node; The entire process continues until the end point. If the intermediate node does not have an idle physical line to use, the connection to the entire line will not be implemented. Access to the data transfer phase is allowed only after the physical route is established between the two sites of communication.

Once the ③ line is assigned, no other site will be available until it is released, even if there is no data transfer on the line at any one time.

( 2 ) Data transfer

After the circuit-switched connection is established, the data can be sent from the source node to the intermediate node, and then the intermediate node is exchanged to the end node. Of course, the endpoint can also send data to the source node via an intermediate node. This data transfer has the shortest propagation delay, and there is no blocking problem unless there is an unexpected line or node failure that causes the circuit to be interrupted. However, in the entire data transmission process, the established circuit must always remain connected, the information transmission delay of both parties depends only on the delay of the electromagnetic signal transmission along the media.

( 3 ) Circuit Removal

When the data transfer between sites is complete, the action of releasing the circuit is performed. This action can be initiated by any site, releasing the line request through the intermediary node sent to each other, releasing the line resources. When the removed channel is idle, it can be used by other communications.

2 . Characteristics and advantages and disadvantages of circuit switching

The characteristics of circuit switching are as follows.

exclusivity: The entire line still does not allow other sites to be shared, even if no data can be transmitted between the sites after the circuit is established and before the line is released. Just like on the phone, we always have to dial the number after the call to set up the connection, whether you speak or not, as long as you do not hang up the machine, this connection is designed for you, if there is no connection available, the user will hear a busy signal. Therefore, the utilization rate of the line is low and the congestion at the successive time is easily caused.

Good real-time: Once the circuit is established, all the resources of the communication Parties (including the line resources) are used for this communication, in addition to a small amount of transmission delay, there is no other delay, with good real-time performance.

Circuit-switched devices are simple, eliminating the need for any caching device.

The transparent transmission of user data requires the automatic rate matching between the sending and receiving parties.

The advantage of circuit switching is that the data transmission is reliable and rapid, and it is not lost and the original sequence is maintained. The disadvantage is that in some cases, the capacity of the channel is wasted when the circuit is idle , and the time taken to establish and dismantle the circuit is not worth the loss, as the duration of the data transmission phase is short. Therefore, it is suitable for the remote batch processing information transmission or the real-time between the system high demand for large data transmission situation. This method of communication is usually calculated according to the bandwidth, distance and time of the reservation.

Two Message exchange:

The so-called "storage Switching" refers to the data exchange, the buffer memory is first cached, and then processed by the queue.

" Storage Switching" is also divided into "message Exchange" ( Message Switching ) and "packet Switching" ( Packet Switching ) two kinds,

This section first describes the "message exchange".

The basic idea of message exchange is to store the user's message in the memory of the switch, and then send the message to the receiving switch or user terminal when the required output circuit is idle , so the message Exchange system is also called "Storage-forwarding" system. The message exchange is suitable for public telegram.

1 . Message exchange principle

The process of implementing message Exchange is as follows.

(1) If a user has a request to send a message, it is necessary to first send the message to the message header, including the destination address and source address information, and send the resulting message to the switch. When the communication controller in the switch detects that a subscriber line has a message input, it sends an interrupt request to the central processor and feeds the message to the memory device verbatim.

(2) after receiving the message, the central processor can process the message, such as analyzing the message header, identifying and determining the route, and then transferring the message to the external mass memory, waiting for an idle output line.

(3) Once the line is idle, the message is then transferred from the external memory into the internal memory, via the communication controller to the line sent out.

2 . Characteristics of message exchange

"Store-and-forward": The message Exchange method first is to store the entire message by the switch, and then do the necessary processing when there is a line idle.

1) non-exclusive lines, the data of multiple users can be stored and queued to share a line.

2) The process of no-line establishment, improve the utilization of the line.

3) Support multi-point transmission (one message to multiple users, simply add "address fields" in the messages, the intermediate nodes are copied and forwarded according to the Address field).

4) intermediate nodes can be converted into data format to facilitate the collection of receiving sites.

5) The error detection function is added to avoid the unnecessary transmission of error data.

3 . Advantages and disadvantages of message exchange Mode

The advantages of message exchange include the following aspects.

High Line utilization, channel can be shared for multiple messages;

No need to start transmitter and receiver to transmit data at the same time, the network can be staged;

When the traffic volume is large, the message can still be received, but the transmission delay will increase;

A message may be sent to multiple destinations;

The switching network can transform the speed and code of the message;

It can realize the function of error control and error correction of the message.

The shortcomings of the message Exchange method are as follows.

The middle node must have a large storage space;

The delay of data transmission is increased because of "storage-forwarding" and queuing;

The length of the message is not specified, the message can only be persisted on disk, the disk reads take up the extra time;

Any message must be queued: messages of different lengths require different lengths of processing and transmission times, even for very short messages (for example, session information in interactive communication);

When the channel error rate is high, frequent retransmission, the message exchange is difficult to support real-time communication and interactive communication requirements.

Three Packet switching

"Packet Switching" (Packet switching) is similar to the "packet switching" technique, but specifies the length of data (called grouping) that the switch processes and transmits, and the data groupings of different users can be interleaved across the physical links in the network. is currently the most widely used switching technology, which combines the advantages of line switching and message exchange, so that its performance is optimal. to understand the advantages of packet switching, first understand the difference between "message" and "Packet Grouping".

1 . Packet and message grouping

When data is transmitted through the communication subnet, there canbe two ways of message (message) and packet grouping (Packet). message transmission, regardless of the length of the transmitted data, it is sent as a logical unit, and the packet transmission mode is limited to the maximum length of the transmission data, if the transmission of data exceeds the specified maximum length, the sending node will be divided into several packets packet sent.

Because the packet length is short, it is easy and time-consuming to make errors in transmission error, and the maximum data length is limited, so the storage space utilization and transfer efficiency of storage and forwarding nodes can be improved. The common data network adopts the packet switching technology.

2 . Principle of packet switching

The packet switching principle is similar to the message exchange, but it specifies the length of data (called grouping) that the switching device processes and transmits. It can divide the long message into several small packets, and the data grouping of different sites can be interleaved on the same line, which improves the utilization of the line. The length of the grouping can be fixed, the system can use the cache technology to hold the packet, improve the speed of forwarding. the packet switching method is X. both packet switching networks and Ethernet are typical applications. in the X. packet switching network, the packet length is 131 bytes , including the user data of bytes and 3 bytes of control information, while in Ethernet, the packet length is 1500 bytes (good line quality and high transfer rate, the length of the packet can be increased slightly).

The key to the packet switching implementation is the selection of the packet length. The smaller the grouping, the greater the amount of redundancy (control information in the group) and the larger the proportion of the entire group, which will ultimately affect the efficiency of user data transmission; the larger the packet, the greater the probability of error in data transmission, the more the number of retransmission, and also the efficiency of the user data transmission.

How do you manage these packet streams? ? There are two ways to do this: datagrams and virtual circuits.

in a datagram, each packet is processed independently, as if each packet is handled independently in a message exchange, and each node chooses a path for each packet according to a routing algorithm, making them the same destination.

In a virtual circuit, before the data is transmitted, both the sending and receiving parties establish a logical connection in the network, but it is not a dedicated physical path, as in the circuit exchange, where each node has a buffer device, subject to the arrangement of this logical line, That is, according to the direction of the logical connection and the order of receiving output queueing and forwarding, so that each node does not need to make a path selection for each packet to judge, as if the two sides have a dedicated channel.

3 . Characteristics of packet Switching

The disadvantage of the message exchange is caused by the message too long, so the idea of packet switching is to limit the length of information sent and forwarded, to divide a large message into a certain length of information unit, called Grouping, and to store and forward packets in groups, and then reassemble the packets into a complete message at the receiving end. packet switching attempts to have the advantages of both message exchange and line switching, while minimizing the disadvantages of both. Packet switching and message exchange work in the same way, the main difference in form is: Packet switching network to limit the length of the transmitted data units.

Four Comparison of three kinds of data exchange technologies

For the sake of understanding and difference, this sectionto compares the above three kinds of exchange methods. First, the comparison between the "circuit switching" and "storage Switching" is made from the large classification.

1 . The main difference between "storage switching" mode and "circuit switching" mode

In the storage Exchange Mode, the sent data and the destination address, the source address and the control information form a data unit (packet or packet) into the communication subnet in a certain format. The node in the communication subnet is the communication control processor, which is responsible for the receiving, error checking, storing, Lu Xian and forwarding functions of the data unit, which are not available in the circuit switching mode.

Store-and-forward relative circuit switching has the following advantages.

Because the communication control processor in the communication subnet can store the packet, multiple packets can share the communication channel and the line utilization is high.

The communication control processor in the communication subnet has the function of road selection, and it can select the best path of packet packet through the communication subnet dynamically.

It can smooth traffic and improve system efficiency.

In each communication control processor in the communication subnet, error checking and error correcting processing are carried out, so the transmission error can be reduced and the system reliability is improved.

The communication control processor can transform the lines of different communication rates, and the different data code formats can be transformed.

2 . Comparison of circuit switching and packet switching

(1) from the way of allocating communication resources (mainly lines)

The circuit switching method statically allocates the line in advance , which causes the waste of the line resource and leads to the difficulty of the connection, while the packet switching mode can allocate the line dynamically (sequentially) , improve the utilization of the line, because memory resource is exhausted because of the use of memory to register the packet. And the middle node has to discard the phenomenon of grouping.

(2) from the perspective of the user's flexibility

the information transmission of circuit exchange is completely transparent , the user can define the content, rate, volume and format of transmitting information, and can transmit voice, data and image at the same time; the transmission of packet-switched information is translucent, The user must use the basic parameters as required by the grouping device.

(3) from the perspective of fees

The charge of circuit-switched networks is limited to the distance of communication and the time of use; the charges for packet-switched networks take into account the number of bytes (or groupings) transferred and the time of the connection.

3 . The above three kinds of data exchange technologies are summarized as follows.

Circuit switching: A physical path needs to be established before the data is transmitted, and the path will be fully occupied by a pair of users until the line is released.

Message exchange: The transmission of messages from the sender to the receiver in the way of storage and forwarding.

Packet switching: This method is similar to the message exchange, but the message is divided into groups and the maximum length of the packet is specified, and the packet is assembled into a packet after arriving at the destination.

Try to compare three different modes of exchange