Multimedia Over Atm Networks - MULTIMEDIA

Video Bitrates over ATM

The ATM Forum supports various types of video bit - rates:

  1. Constant Bitrate (CBR). For example, for uncompressed video or CBR - coded video. As mentioned before, if the allocated bitrate of CBR is too low, cell loss and distortion of the video content are inevitable.

  2. Variable Bit Rate (VBR). The most commonly used video bitrate for compressed video. It can be further divided into real - time Variable Bitrate (rt - VBR) suitable for compressed video, and non real - time Variable Bit Rate (nrt - VBR) for specified QoS.

  3. Available Bit Rate (ABR). As in IP - based service, data transmission can be backed off or buffered due to congestion. Cell loss rate and minimum cell data rate can sometimes be specified.

  4. Unspecified Bit Rate (UBR). Provides no guarantee onany quality parameter

Headers and trailers added at the CS and SAR sublayers

Headers and trailers added at the CS and SAR sublayers

ATM Adaptation Layer (AAL)

AAL converts various formats of user data into ATM datastreams and vice versa.The following lists five types of AAL protocols:

  1. AAL type 1 supports real - time, constant bitrate (CBR), connection - oriented data - streams.

  2. AAL type 2 was intended for variable bitrate (VBR) compressed video and audio. However, the protocol never really materialized and is now inactive.

  3. AAL types 3 and 4 were similar, and have since been combined into one type: AAL type 3 / 4. It supports variable bitrate (VBR) of either connection - oriented or connectionless general (non - real - time) data services.

  4. AAL type 5 was the new protocol introduced for multimedia data transmission. It promises to support all classes of data and video services (from CBR to UBR, from rt - VBR to nit - VBR). It is assumed that the layers above the AAL are connection - oriented and that the ATM layer beneath it has a low error rate.

As the above figure shows, headers and trailers are added to the original user data at the Convergence Sublayer (CS) and Segmentation And Reassembly (SAR) sublayer. They eventually form the 53 - byte ATM cells with the 5 - byte ATM header appended.

The existence of the five different types of AAL was due largely to history. In particular, all AAL types except AAL 5 were developed by the telecommunications industry and were generally unsuitable for interactive multimedia applications and services.

  • AAL 3 / 4 has an overhead of designating 4 bytes for each SAR cell, whereas AAL 5 has none at this sublayer. Considering the numerous SAR cells, this is a substantial saving for AAL 5. It is of course possible only with modem, relatively error - free fiber - optic technology.

Table Comparison of AAL types

Comparison of AAL types

  • As part of the SAR trailer, AAL 3 / 4 has a checksum field for error checking. To cut down the overhead, the checksum is only 10 bits long, which is unfortunately inadequate. AAL 5 does it at the CS and allocates 4 bytes for the checksum. Again, it is based on the assumption that bit - transmission error is rare. However, when AAL 5 does error checking, it has enough information from the long checksum.

By now, AAL 5 has superseded AAL 3/4. The ATM Forum agrees that beside CBR services that will use AAL 1, every other service will use AAL 5. For more details of the AALs, see Tanenbaum and Stallings.

MPEG - 2 Convergence to ATM

The ATM Forum has decided that MPEG - 2 will be transported over AAL5. By default, two MPEG - 2 packets (each 188 bytes) from the transport stream (TS) will be mapped into one AAL - 5 service data unit (SDU).

When establishing a virtual channel connection, the following QoS parameters must be specified:

  1. Maximum cell transfer delay (latency)
  2. Maximum cell delay jitter
  3. Cell loss ratio (CLR)
  4. Cell error ratio (CER)
  5. Severely errored cell block ratio (SECBR)

Table Support for digital video transmission

Support for digital video transmission

An audio - visual service - specific convergence sublayer (AVSSCS) is also proposed, to enable transmitting video over AAL5 using ABR services.

Multicast over ATM

Compared to IP multicast, which is a "best - effort" service provided on top of UDP, multicast in ATM networks had several challenges:

  1. ATM is connection - oriented; hence, ATM multicasting must set up all multipoint connections.
  2. QoS in ATM must be negotiated at connection setup time and be known to all switches.
  3. It is difficult to support multipoint - to - point or multipoint - to - multipoint connections in ATM, because AAL 5 does not keep track of multiplexer number or sequence number. It cannot reassemble the data correctly at the receiver side if cells from different senders are interleaved at their reception.

    Scalable and efficient ATM multicast (SEAM) and shared many - to - many ATM reservations (SMART) are two approaches to multicasting over ATM. The former uses a unique identifier and the latter a token scheme to avoid the ambiguity caused by cell interleaving.

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