In Business, What is a Burn Rate?

A variable bit rate service (VBRS) is a coding service. This service does not use a fixed average rate to compress data. The average code rate of each data is variable, and it can be changed according to the content of the data. The bit rate of this service can change according to the network conditions. When the network is completely free of congestion, the preset maximum value can be reached throughout the permanent virtual circuit. Of course, this cannot guarantee the average data throughput in a specific period Negotiation can be performed between the ATM access device and the switch, and the maximum bit rate that can be guaranteed in a short time can also be negotiated ^[1] .

An ATM network is a network capable of transmitting many different types of communication services at the same time. The way in which each type of service traffic is handled within the network depends on the characteristics and application requirements of the service flow. The ATM Forum divides these services into five categories: Constant Bit Rate (CBR), Real Time Variable Bit Rate (rt-VBR), and Not Real Time Variable Bit Rate (nrt-VBR), Unspecified Bit Rate (UBR), and Available Bit Rate (ABR). The five types of broadband bearer services can be distinguished from two aspects. Differentiated from service characteristics, they describe the traffic characteristics of the service itself, also known as source traffic parameters, such as peak cell rate, minimum cell rate, maintainable cell rate, and maximum burst length; but other service characteristic parameters are not It does not represent the traffic of the service itself, but reflects the time requirements of the service, such as the cell jitter tolerance allowed by the service. Distinguish from the service quality of the ATM layer of the business, including peaks. Peak cell delay jitter, maximum cell transmission delay, severely erroneous cell block ratio, cell error rate, cell loss rate, cell error insertion rate, etc.

VBR is not a new coding method or coding algorithm, it is a concept corresponding to CBR. It is the abbreviation of VariableBitRate. It does not use a fixed average rate to compress data. The average bit rate of each data is variable. It can be changed according to the content of the data. The bit rate of this service can change according to the network conditions. When the network is completely free of congestion, the preset maximum value can be reached on the entire permanent virtual circuit. Of course, this cannot guarantee the average data throughput in a specific time period. Negotiation can be performed between the ATM access device and the switch, and the maximum bit rate that can be guaranteed in a short time can also be negotiated. ^[1]

It mainly has two types: real-time and non-real-time.

Real-time variable bit rate (rt-VBR)

It is mainly used to describe a variable data stream, which is a real-time service. It has strict requirements on delay and jitter. It is often used in delay-sensitive applications. The difference from CBR is rt. VBR does not require a constant bandwidth, and its transmission rate changes throughout the process. This type of service is suitable for sources with large burst characteristics, such as the transmission of compressed images. The characteristics of the rt-VBR service are connection-oriented, variable bit rate, and timing relationship between communication endpoints. The source cells send bursts, and the bandwidth occupied by the communication endpoint changes with the terminal information transmission rate at different times. It is considered the most effective way to send speech, video, and other delay and jitter sensitive communications over ATM networks.

Non real-time variable bit rate (nrt-VBR)

It is mainly used for communication occasions that are sent regularly and belongs to non-real-time services. To use such services, the parameters that need to be specified on the end system are: peak cell rate, sustained cell rate, and maximum burst size rate. The transmission rate of the nrt-VBR service also changes throughout the process. It can be used for data transmission that requires strict time response, such as banking, process monitoring, and email. The characteristics of the nrt-VBR service are connection-oriented, the bit rate is variable, and there is no timing relationship between the communication endpoints. The bandwidth occupied by a communication endpoint changes with the terminal information transmission rate at different times. The ATM network guarantees a continuous cell rate to the communication terminal and provides a low CLR. At the same time, the communication terminal is required to send information at a rate no greater than. It is mainly used for data transmission with less stringent requirements on delay and delay variation, which requires low cell loss rate ^[1]

The transmission bandwidth of the virtual circuit of the VBR service is variable.

The characteristics of the rt-VBR service are connection-oriented, the bit rate is variable, and there is a timing relationship between the communication endpoints. The source cell sends bursts, and the bandwidth occupied by the communication endpoint changes with the terminal information transmission rate at different times. The average rate of a VBR connection over a long period of time is SCR. The ATM network guarantees SCR to the communication terminal. At the same time, the communication terminal must not send information at a rate greater than the PCR. The cell delay is stricter and is used to support services that require high real-time performance, such as voice services with silence detection and compression-coded video image services.

The characteristics of the nrt-VBR service are connection-oriented, the bit rate is variable, and there is no timing relationship between the communication endpoints. The bandwidth occupied by a communication endpoint changes with the terminal information transmission rate at different times. The ATM network guarantees SCR to the communication terminal and provides a low CLR. At the same time, the communication terminal is required to send information at a rate no greater than the PCR. It is mainly used for data transmission, such as E-MAIL, FAX, etc., which requires low cell loss rate for delay and delay change. When applying for VBR, the user and the network need to agree on the transmission rate (such as PCR, SCR, etc.).

The CBR service can actually be regarded as a special VBR service where the source sends cells at a uniform rate of probability 1. Its characteristics are connection-oriented, have a fixed bit rate, and there is a timing relationship between the communication endpoints. The PCR can be transmitted continuously during the entire connection process, and the communication terminal can transmit at a cell rate equal to or less than the PCR at any time and period. The ATM network will ensure the reliable transmission of cells within the agreed fixed bandwidth. The CBR service requires a short network transmission delay and low delay jitter, and can be used to support real-time fixed-rate image and voice services.

Variable bit rate service voice service

The voice service may be the most easily defined service in the broadband network and the service most familiar to people, but it may be a service that is difficult to achieve well on the ATM network. The voice service is characterized by a relatively low rate, which is generally 64 Kbit / s. If voice compression technology is used, it may reach 16 Kbit / s or less. On the other hand, the voice service requires high delay and delay jitter. Due to the low rate, it takes a long time to assemble an ATM cell, that is, for a voice of 64Kbit / s rate, it takes 5.875ms to assemble a 53-byte cell. When the time delay between the echo and the sound it emits exceeds 25ms, the human ear can distinguish it. According to the Japanese NEC test, echo suppression should be added when the time delay is greater than 18ms, which means that for 64Kbit / s speech With no echo canceller, the maximum transmission distance is 2425km, and longer distance calls have to be added with echo suppression. The problem is even worse for lower rate voices. In order not to add an echo suppressor, the only solution is to try to reduce the delay in assembling the cells. There are two main methods: one is to multiplex multiple voices into one AAL1 (ATM Adaptation Layer type1, ATM adaptation layer Type 1), so that for a single channel of speech, the delay in assembling cells is small, at the cost of complex control, because there must be additional information for the receiving end to recover each channel of speech from the cell; the second is Voice data is sent before it is full of cells. At this time, the cost of reducing delay is a waste of bandwidth.

Another issue that is still under study is to improve the efficiency of the network by using the mute cancellation technology of voice, which is the same as the starting point of the previous voice interpolation technology. In voice communication, only 40% of the time is really voiced, and the remaining 60% of the time is the silent period. If the mute cancellation technology is properly used, 2.5 times of users can be accessed on the same bandwidth path. Therefore, although the voice service was previously regarded as a typical representative of the constant-rate circuit-switched service, it is actually a VBR service. This is the main business form of communication at present. The ATM network must support circuit switching services well. The ATM Forum has specially formulated circuit simulation recommendations for this purpose. Currently, existing equipment has implemented circuit simulation.

Variable bit rate service video service

In broadband networks, video image users and traffic will dominate. Due to the introduction of compression coding technology, the video transmission rate is greatly reduced. At the same time, due to the different information redundancy of each frame of image, the video service with constant image quality has a high burstiness. If VBR video is transmitted at the peak constant rate, it will cause a great waste of network resources, and users will bear higher costs.

ATM networks can economically support bursty variable-bit-rate services. However, people's understanding of VBR video characteristics is far less familiar than voice and data communication characteristics. ATM brings new opportunities to image communication, but also brings new problems. This is mainly the impact of cell loss, delay, and delay jitter on image quality.

In the past image communication, the influence of the channel is mainly the bit error rate. And this kind of error is relatively scattered, due to permutation and other reasons in the ATM network, the entire cell will be lost. When using AAL1 adaptation, for a 2M, 25 frames per second, CIF (Common Intermediate Fomat) format source, losing one cell is equivalent to losing nearly two lines of data. More importantly, it is very likely that the lost cells contain information such as quantization tables and motion vectors, which will affect the image quality of up to several frames, so the cell loss must be handled. One method is to use layered coding to ensure a low cell loss rate for important information. The second method is to use the interleaving method to spread the influence of cell loss into each image coding frame.

The delay is caused by the following factors: image codec processing of the sender and receiver, segment reassembly, transmission delay, queuing delay, exchange delay, etc. The values of the first three terms are the same for all cells and can be called absolute delays, while the values of the last two terms are related to the state of the network at the time. They are different for each cell and can be called relative delays. Therefore, the impact of delay is also twofold. One is the absolute delay. For interactive services, the delay is specified. The excessive delay reflects the slow response speed of the other party. Experiments show that the one-way delay of 400ms is unbearable; for broadcast services such as television, there is no requirement for absolute delay. The other is the relative delay between cells. When this relative delay exceeds a certain value, its effect is the same as cell loss. For example, for a layer-encoded image, when the cell on the high-quality connection has arrived and the outline of the image has appeared, if the cell on the low-quality connection has not arrived, the decoder must think that the cell is lost When the cell reaches the receiver, the information it carries is meaningless.

Another important impact of ATM networks on image communication is delay jitter, which is the relative delay between different cells on the same connection. The basic method of equalizing delay jitter is to use a buffer at the receiving end. When the delay jitter exceeds a certain value, the buffer may overflow and the arriving cells will be lost. The large-capacity buffer can equalize larger delay jitter and reduce cell loss due to delay jitter, at the cost of an increase in the absolute value of the delay. In practical applications, it should be studied which delay and cell loss have more influence according to the specific application, and a compromise should be made between them. ^[2]