Hi, Asynchronous Transfer Mode (ATM) offers services to three different types of

Question

Hi,

Asynchronous Transfer Mode (ATM) offers services to three different types of technologies: Constant Bit Rate (CBR), Unspecified Bit Rate (UBR) and Available Bit Rate (ABR). Each technology has its pros/cons when dealing with ATM.

in 400-500 word, submit a doc in which you examine UBR, CBR and ABR technologies. Choose one of these as the preferred method for transmitting data when bit-rate and minimum quality of service are top priorities and defend why this technology is the best of the three.

Explanation / Answer

What are Bit Rates?

Let's start with what a bit is. A bit is an acronym for "binary digit," the smallest possible unit of information in digital computing. It takes 8 bits to make a byte of information. Bit rate refers to the number of bits of data transferred in a file over a set length of time. It's the combination of video and audio streams in a file, and is commonly measured in number of bits per second.

Constant bit rate (CBR) and variable bit rate (VBR) are the main types of bit rate encoding. Scene complexity can vary significantly over several hours of recorded surveillance video, and the bit rate you select for recording will have an effect on image quality and bandwidth consumption.

A complex scene with moving action, such as traffic on a city street, or a scene with a lot of contrasting colors, will affect image quality and bandwidth consumption more than a less complex scene, such as an interior wall or hallway with very little action or movement.

Constant bitrate (CBR) is a term used in telecommunications, relating to the quality of service. Compare with variable bitrate.

With constant bit rate encoding, a fixed bit rate and bandwidth is used throughout the entire track or encoded video file. With a constant bit rate, image quality may fluctuate over the course of the video stream because some pieces of content are more difficult to compress than others.

In order for the bit rate to remain constant, the video may be encoded with fewer bits in some places or more bits in other places—resulting in inconsistent image quality. Since bandwidth consumption with constant bit rates does not vary, the file size is limited and more predictable than with variable bit rates.

Pre-planning your security video storage requirements is easier with constant bit rate because the amount of data being recorded never changes.

Pros ans Cons

When referring to codecs, constant bit rate encoding means that the rate at which a codec's output data should be consumed is constant. CBR is useful for streaming multimedia content on limited capacity channels since it is the maximum bit rate that matters, not the average, so CBR would be used to take advantage of all of the capacity. CBR would not be the optimal choice for storage as it would not allocate enough data for complex sections (resulting in degraded quality) while wasting data on simple sections.

The problem of not allocating enough data for complex sections could be solved by choosing a high bitrate to ensure that there will be enough bits for the entire encoding process, though the size of the file at the end would be proportionally larger.

Most coding schemes such as Huffman coding or run-length encoding produce variable-length codes, making perfect CBR difficult to achieve. This is partly solved by varying the quantization (quality), and fully solved by the use of padding. (However, CBR is implied in a simple scheme like reducing all 16-bit audio samples to 8 bits.)

In the case of streaming video as a CBR, the source could be under the CBR data rate target. So in order to complete the stream, it's necessary to add stuffing packets in the stream to reach the data rate wanted. These packets are totally neutral and don't affect the stream

The unspecified bit rate (UBR):

The unspecified bit rate (UBR) service category is one of five ATM service categories defined in the ATM Forum's Traffic Management Specification 4.0 .

The five service classes are:

            Constant bit rate (CBR)

            Variable bit rate non-real-time (VBR-nrt)

            Variable bit rate real-time (VBR-rt)

            Available bit rate (ABR)

            Unspecified bit rate (UBR) and UBR+

UBR is intended for non-real-time applications that do not require any maximum bound on the transfer delay or on the cell loss ratio.

The purpose of this document is to clarify the differences between a UBR permanent virtual circuit (PVC) and a variable bit rate, non-real time (VBR-nrt) PVC by illustrating that two such virtual circuits (VCs) with the same peak cell rate (PCR) experience very different bandwidth guarantees and scheduling priorities. These differences may affect the level of performance that users are provided on the connection.

Pros and Cons:

·         Allows for a high degree of statistical multiplexing by not reserving any minimum bandwidth per VC. The VCs use the bandwidth up to the configured PCR when available.

·         Models the best-effort service normally provided by the Internet. Suitable for applications tolerant to delay and not requiring real-time response. Examples include e-mail, fax transmission, file transfers, Telnet, LAN and remote office interconnections. Such applications are not sensitive to delay, but they are sensitive to cell loss. ATM switches, such as the Cisco Catalyst 8500 series, allocate larger maximum per-VC queue limits for UBR PVCs.

·         The only attributes specified as part of UBR are the PCR and the cell delay variation tolerance (CDVT). The PCR only provides an indication of a physical bandwidth limitation within a VC.

·         Note: A relatively new variant of UBR, called UBR+, allows an ATM end-system to signal a minimum cell rate to an ATM switch in a connection request, and the ATM network attempts to maintain this minimum as an end-to-end guarantee. Refer to the document Understanding the UBR+ Service Category for ATM VCs.

·         VCs of other ATM service categories have a higher priority as viewed by the ATM interface segmentation and reassembly (SAR) scheduler. When competition for a cell timeslot arises, the scheduler gives the timeslot to a VC of service classes with a higher priority.

·         It does not place any bounds with respect to the cell loss ratio (CLR) or to the cell transfer delay (CTD). The end-system is expected to handle and adjust for any cell loss or delay.

·         It does not guarantee cell delivery. Retransmission occurs at higher layers.

Available Bit Rate (ABR):

The ATM Forum publishes multi-vendor recommendations to further the use of ATM technology. The Traffic Management Specification  Version 4.0 defines five ATM service categories that describe both the traffic transmitted by users onto a network as well as the Quality of Service (QoS) that a network needs to provide for that traffic. The five service categories are listed here:

·         Constant bit rate (CBR)

·         Variable bit rate non-real-time (VBR-nrt)

·         Variable bit rate real-time (VBR-rt)

·         available bit rate (ABR)

·         unspecified bit rate (UBR) and UBR+

     What is ABR?

·         When you assign an ATM virtual circuit to the ABR service category it configures a router to transmit at a rate that varies with the amount of bandwidth available in the network or along the end-to-end transmission path. When the network is congested and other source devices are transmitting, there is little available or leftover bandwidth. However, when the network is not congested, bandwidth is available for use by other active devices. ABR allows end-system devices like routers to take advantage of this extra bandwidth and increase their transmission rates. Therefore, ABR uses mechanisms that allow ABR VCs to make use of any bandwidth available in the network at any point in time.

·         An ABR VC binds a source router to a contract with the ATM switch network. As part of this contract, a source router agrees to examine information that indicates whether or not the network is congested and, in turn, adapt the source transmission rate if required. In return, the ATM switch network agrees to drop no more than a maximum number of cells when congestion occurs. The ratio of dropped cells to transmitted cells is known as the cell loss ratio (CLR).

·         In addition, an ABR VC uses a closed-loop model. With a closed loop, a source router sends data cells or special cells (called forward resource management [RM] cells) into the ATM network. The switches in the ATM network mark or set bits in these cells as they flow along the end-to-end path. The destination router turns these cells around as backward RM cells. By setting certain bits or fields, the ATM network and destination router provide feedback used to control the source rate in response to bandwidth changes in the network or at the destination.

·         The ABR service category is designed for VCs that carry file transfers and other bursty, non-real-time traffic that requires some minimum amount of bandwidth (specified via a minimum cell rate) to be available while the VC is configured and active. With ABR, the delay or variation in delay from source to destination router can vary and can be a large value. This makes ABR unsuitable for real-time applications. The CBR and VBR service categories address applications that require tight boundaries on throughput and delay.

Abr vs Cbr vs Vbr:

An MP3 file, for example, that has an average bit rate of 128 kbit/s transfers, on average, 128,000 bits every second. ... At a given bitrate, VBR is usually higher quality than ABR, which is higher quality than CBR (constant bitrate).

·         In terms of quality, generally VBR > ABR > CBR at the same approximate size and bitrate.

·         CBR only has a limited "bit reservoir" to allow limited borrowing of bits from easier passages to assist with difficult ones.

·         VBR uses the acoustic model and quality level to determine the amount of bits used and their distribution. Encoders with sub-optimum acoustic models procuced miserable VBR results, clouding the early reputation of VBR.

·         ABR is a compromise, using a VBR-style distribution of bitrate, but subject to an overrall target bitrate, rater tha an overall target quality irrespective of bitrate.

·         As soon as you depart from CBR, you lose the exact correlation beteween data and time, usually resulting in problem with capacity estaimates at the very least.

·         The popular preset modes of LAME encoder, alt-preset standard and alt-preset extreme (actually, they changed to just "preset", and finally were absorbed into the VBR modes some time ago), are example of well tuned VBR with quality considerably better than a capacity-equivalent CBR.

·         Basically, for maximum quality in the space avaialble, or minimum space for whatever quality you consider acceptable - VBR.
If your player or software has any issues with VBR , USE CBR

·         There are few good applications for ABR... anything incompatible with VBR is likely to have problems with ABR, and if targeting a particular size, selecting & testing VBR is better.

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