What Is a Routing Transit Number?

The transit network refers to the network between the originating network and the terminating network. It is mainly used to forward calls and information received from the previous network to the next network.

Communication must be networked,

As shown in the figure on the right, (a) in the figure is

The information transfer method is also called the store-before-send method, which is different from the circuit transfer method. At this time, it is not necessary to establish a circuit before transmitting information. When transmitting a group of information, it does not necessarily occupy the same circuit; The information is sent to an endpoint and stored.

If the forward channel is available, it is forwarded to another node; if the forward channel is occupied, it is stored in that node, queued, and forwarded when it is available, until it is delivered to the destination. It can be known from the queuing theory that the network resource utilization rate in this way can be high for circuit switching; especially for short messages, there is no need to time to set up the circuit, and the benefits are even more obvious. In this way, the call loss when transmitting information generally no longer exists, and the price paid is information delay.

Of course, this is assuming that the storage capacity of each node is large enough, otherwise, when there are too many information packets, the storage device will overflow, causing the information to be lost in the network. Usually, the loss should not be too much, and the storage amount at each node should be large enough, which also shows that this method is suitable for short messages; routing control and flow control can also be used to avoid overflow of storage devices on the node and minimize Small information is lost.

It can be seen that call loss is no longer the main quality indicator of the information transfer network; the average delay of information has become a quality indicator of concern to people. Excessive delay is detrimental to any information; it is generally allowed when it does not exceed a certain value. This specific value is different for various types of information. For real-time information such as voice and TV, the allowed delay is small, so circuit transfer is often used; because information transfer often cannot guarantee such a small delay; for computer information or non-real-time information such as telegrams and telex Second, even graded delays are often allowed, and information transfer networks have great advantages. Therefore, the average delay is often the optimization target in the information transfer network. That is, under certain network resources, the minimum average delay is sought, or if the average delay is less than a certain threshold, the network resource investment is reduced. ^[2]

Modern communication networks are composed of many circuit connections and transfer point groups. In fact, the development of transfer technology has affected the composition of the network, and the diversity of the composition of the network has put forward different requirements for the transfer technology. Switching technology is one of the faster developing technologies in modern telecommunication systems. In particular, the widespread application of electronic computers has caused profound changes in switching technology.

Transfer methods can generally be divided into two categories. One type is called circuit switching. For example, a city mobile phone is a circuit switching. When it is necessary to exchange information in both directions in a timely manner like a telephone intercom, a circuit transfer must be used. The other type is called storage transfer, sometimes called message transfer, and can be used for one-way information transmission, like telegram transfer belongs to this category. Of course, the telegraph transfer can also be transferred through the circuit, and during the circuit transfer, there is often a problem of storage of control information, so it is actually difficult to completely separate the two. For convenience, these two transfer methods will be discussed separately below.

Switch network circuit switch

The main task of circuit switching is to connect the input line (called the calling line) that requires communication with the called output line. In addition to the output and input circuit equipment, it is mainly composed of two parts, the connection unit and the control unit. In the original manual transfer equipment, the output and input lines were respectively connected to many plug holes of the switchboard, and the connection equipment was a number of cords, that is, several connection lines. In addition to the monitoring facilities such as indicator lights, the control part is done manually, that is, the interphone method is used to ask the called line number of the calling line, and the plug of the cord is connected to the corresponding plug hole to connect the circuit. Or tell the calling line that the called line is busy. Please wait for a while.

The main disadvantage of manual transfer is that the transfer speed is low, especially in large communication networks, it often takes many transfers to connect the two end users. At the same time, during the connection process, the channel is also occupied, resulting in a decrease in the channel's actual time for conversation, which means that the channel utilization rate is low, the possibility of being busy is increased, and the connection speed is further reduced.

In modern telecommunication systems, manual transfers have gradually been replaced by automatic transfers. The city's automatic telephone is the earliest automatic communication local communication network. There is also great progress in automatic transfer in large networks. The connection unit has evolved from mechanical connection contacts such as stepping, rotating, and horizontal to using semiconductor components as connection points, and even non-contact connection technology such as time division;

Connection unit schematic

Extend to multi-level connections to adapt to the increase in the number of outputs and inputs; the control unit has evolved from signal control directly to the calling end to storage control and even computer control. One of the important goals in these developments is to speed up the connection. The latest automatic transfer technology in modern telecommunication systems will now be briefly introduced.

Start with the link unit. When there are fewer output and input circuits, in principle, the connection method can be as shown in the figure on the right. If there are m input lines, there are mn intersections in each output line. Each of these crossing points is provided with a contact or a switching element. To connect any input line to any output line, it is necessary to connect the contacts at the corresponding intersections. This structure is called an n × m matrix switch and is a non-blocking way. Because as long as the called line is not busy, no matter how many circuits have been connected before, you can always connect the crosspoint and connect an input line to it. However, when the number of input and output circuits is large, this method is not suitable. On the one hand, the number of contacts is too large and uneconomical, on the other hand, the mechanism for controlling these many contacts is also complicated.

At this time, a multi-level matrix is generally used to implement the transfer. The figure on the right is an example of a three-level matrix. At this time

Three-level matrix structure

The input and output lines are divided into several groups. Each group has a matrix switch. The outputs of these matrix switches are connected through one or more matrix switches. In the figure (d) on the right, each group of input or output wires connected to a matrix switch (a block is equivalent to the set of contacts in the figure above) has only one output or input. As long as a matrix switch in the middle can input and output The line is connected.

It can be seen that this arrangement can greatly reduce the number of contacts, but blocking phenomenon often occurs, because as long as one of the input lines of the same group is communicating, occupying the output of a 20 × 1 matrix switch, the other cannot be connected. In order to reduce blocking, 10 intermediate matrix switches are used instead in (b) on the right. Blocking still occurs at this time.

There is also a pseudo-blocking phenomenon. Take the figure (b) on the right as an example for illustration. Let 1 be connected to 1 '. beforehand

There is already only one empty line a 'on the left side of the cable. At this time, the a line is not occupied and could be connected. However, if the control system does not know the full wiring situation beforehand, 1 must be connected.

Go, because the b 'line is already occupied at this time, it cannot be connected

Receive 1 'again. If the call is considered blocked, the calling line is required to wait. This is a false blocking phenomenon. To avoid this, the control system needs to know the full picture.

Blocking will reduce the transfer speed, so it should be avoided as much as possible. However, in order to save equipment, a certain limit of the blocking rate is allowed. Just as when designing the number of channels, a certain limit of the blocking rate is also allowed. The blocking of the switching device is generally related to the number of stages of the matrix switch. The more the number of stages, the smaller the number of contacts can be when the blocking rate is constant. The resistance introduced by the point will increase. The increase of serial-sequence contacts also reduces the reliability of the device, because if one contact fails, the circuit is interrupted. Therefore, we must find an optimal solution according to the actual situation. For example, the arrangement of connection units should be selected according to the total number of output and input lines, and the frequency of calls.

The connection and disconnection of these contacts are controlled by the control unit. This was previously done using electromagnetic machinery or relays. In recent electronic switches, electronic circuits are often used to accomplish this. There are also major changes in the control method. In the past, it was directly controlled by the calling user, that is, dialing a number of numbers to form a series of pulses. These pulses were used to rotate the switch circuit. In this way, the transfer speed is limited by dialing, and it is easy to cause the above-mentioned false blocking. Especially when the input and output lines are the connection between the transfer centers, it is not ideal to be directly controlled by the dial signal. For example, when using a circuitous route, the routing selection must be grasped by the end user. In fact, this is not possible; this also makes the number of dialing very large, because at that time there are many transfer points, each transfer point Has its number, and all of these transfer points have to be dialed by the user.

Transfer network information transfer

After the circuit transfer is automated, it can greatly improve the connection rate and channel utilization. But usually, communication services are uneven, sometimes busy, and sometimes empty. This poses difficult problems for arranging the channel capacity and the capacity of the switching equipment. If the set capacity is determined according to the blocking rate at the busiest time, the utilization rate at the shortest time is very low; on the contrary, if it is determined at the busiest time, the blocking rate at the busiest time is so high that it is connected. It is very slow, and the repeated circulation of control signals takes up many channels. It is better to have a way to average out the busy and empty states, so the information transfer method is proposed.

The main principle of information transfer is to store the information to be transmitted, and send it out when the channel is free. As long as the storage time is long enough, such as 24 hours, the busy and empty states of the channel can be evened out within one day, and the necessary channel capacity and switching equipment capacity are greatly compressed. Obviously, this method is not allowed for communication information that requires real-time transmission, but it can be used for data signals such as teletypewriters. Of course, the storage time is too long, such as 24 hours, which is not suitable. Sometimes the priority level can be used to send the information that requires fast transmission in time, and the channel capacity can be appropriately increased to reduce the waiting time.

In fact, in the manual transfer, the telegram information is often transferred by information, that is, the received telegram is recorded in the transfer center, and then sent in a certain order in the corresponding doubling channel according to the address. Here Information hangs in the hands, written, on paper. In the later semi-automatic system, the telegrams were stored on the paper strips in the form of automatic punching. The staff sorted them according to the address and transmitted them to the corresponding transmission lines by using a transmission belt. The typewriter sent it out. If another transfer is needed, the next transfer center will perform this procedure again, and the information is stored on the punched note.

The modern information transfer system also has the same principle, except that the automation program is very high and is usually operated by small electronic computers, which almost replaces all the regular work usually done by the staff. Only in special cases, the computer can not alarm. It is handled manually.

A simplified block diagram of this system is shown on the right.

Simplified block diagram

The input device is connected to each input line, and the other end (other transfer center) of each input line is contacted to test the condition of the line and convert the input data into a form suitable for storage. It is usually necessary to be able to identify the beginning and end of each header and message (there are start and end characters in the input signal) in order to separate the various messages and store them in appropriate memory respectively; it must also be able to identify a certain The serial number of a telegram from a line to determine whether they are continuous and prevent loss.

The output device interfaces with each output line, contacts with the other end of each output line, and tests the circuit. It should also judge the end of the header and the message in order to take a pending telegram from the memory. Sometimes the information processor only sends a unified, shorter turtle terminating character, and the counterpart of the output line often requires a longer terminating signal. For example, in a semi-automatic device that pulls a paper tape, it requires an output device to supplement it. There may also be other functions related to a specific output line that require this output device to complete.

Both input and output have storage devices. In addition to the start and end characters, a telegram can be divided into headers and messages; the header contains the addresses of the original sending station and the last receiving station, and various official signs such as priority, serial number, transfer Jing address, date, number of reports, etc. This header is often different in the input signal and the output signal. The latter should at least include the serial number of this station, the logo transferred by this station, and so on. Therefore, when the information processor transfers input storage to output storage, appropriate changes should be made. However, the message is generally unchanged and can be directly transferred from input storage to output storage. In order to save the storage equipment, sometimes the output storage is omitted, and the information processor directly connects the input storage message part to the corresponding output device, which is just like circuit switching. But the output and input are stored, the flexibility can be greater, and the processor can be simpler.

The storage device can not only make the information waiting channel idle as mentioned earlier, but also can switch channels of different speeds. For example, the input signal of 50 baud can be connected to the output circuit of 75 baud or higher after being stored.

The information processor is the core of the information transfer device. Its task depends on the requirements and may be complicated. In modern large-scale transfer centers, electronic computers are often used to complete information processing tasks, and in those centers, there are often devices for circuit transfer and information transfer, and computers are even more necessary. Generally speaking, the main task of an information processor is to consult the input storage, identify their destination addresses from the header, edit and record, select the appropriate output line according to the routing program, and transfer the input storage to the output storage. Go and change the header appropriately. Sometimes it is necessary to identify the priority of the report and so on, and arrange them on the output line in order of priority. Information transfer is generally not routed like circuit transfer. Route selection is specified in advance, but it may also be disabled for a certain period of time due to a certain line failure or other reasons, then the route must be changed. At this time, it is necessary to change the routing procedure so that the information processing machine performs according to the new procedure. This usually involves manual participation. In addition, the output and input timing signals are also supplied by the information processor.

Information transfer is to handle one-way information transmission, which is different from general circuit transfer to handle two-way conversation. After the sender sends the message, it is necessary to wait until the receiver sends back a positive signal before knowing that the message has been sent to the other party as needed. If information was lost during transmission or transfer, it was unknown at the time. In order to correct this shortcoming, in addition to the information processing machine of each transfer center to record, generally known as the flow log, write down the necessary part of all the telegram headers that have been processed for future reference, there must be a flow Audit equipment to facilitate timely discovery, so as not to require the sender to resend later. ^[3]