Question

Write short notes on Multimedia over ATM networks. (Multimedia Computing)

Answer 1

Answer on Question #53260, Engineering / Other

Task:

Write short notes on Multimedia over ATM networks. (Multimedia Computing)

Answer:

> Video Bit-rates over ATM:

- CBR (Constant Bit Rate): if the allocated bit-rate of CBR is too low, then cell loss and distortion of the video content are inevitable.

- VBR (Variable Bit Rate): the most commonly used video bit-rate for compressed video, can be further divided into:

* rt-VBR (real-time Variable Bit Rate): for compressed video.

* nrt-VBR (non real-time Variable Bit Rate): for specified QoS.

- ABR (Available Bit Rate): data transmission can be backed off or buffered due to congestion. Cell loss rate and minimum cell data rate can sometimes be specified.

- UBR (Unspecified Bit Rate): no guarantee on any quality parameter.

ATM Adaptation Layer (AAL):

- Converts various formats of user data into ATM data streams and vice versa.

- Different types of protocols of (AAL):

- AAL Type 1: supports real-time, constant bit rate (CBR), connection-oriented data streams.

- AAL Type 2: intended for variable bit rate (VBR) compressed video and audio (inactive).

- AAL Types 3 and 4: have been combined into one type — AAL Type 3/4. It supports variable bit rate (VBR) of either connection-oriented or connectionless general (non real-time) data services.

- AAL Type 5: the new protocol introduced for multimedia data transmissions, promising to support all classes of data and video services (from CBR to UBR, from rt-VBR to nrt-VBR).

Fig. 1: Headers and Trailers added at the CS and SAR sublayers

- Headers and trailers are added to the original user data at the CS (Convergence Sublayer) and SAR (Segmentation And Reassembly sublayer)

eventually form the 53-byte ATM cells with the 5-byte ATM header appended.

Table 1: Comparison of AAL Types

- AAL 3/4 has an overhead of designating 4 bytes header/trailer for each SAR cell, whereas AAL 5 has none at this sublayer. Considering the numerous number of SAR cells, this is a substantial saving for AAL 5.

- As part of the SAR trailer, AAL 3/4 has a (short) 10-bit "Checksum" for error checking. AAL 5 does it at the CS and allocates 4 bytes for the Checksum.

Table 2: Support for Digital Video Transmission

$\succ$ MPEG-2 Convergence to ATM

- The ATM Forum has decided that MPEG-2 will be transported over AAL5:

- Two MPEG-2 packets (each 188 bytes) from the Transport Stream (TS) will be mapped into one AAL-5 SDU (Service Data Unit).

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

- Maximum cell transfer delay.

- Maximum cell delay.

- Cell Loss Ratio (CLR).

- Cell Error Ratio (CER).

- Severely Errored Cell Block Ratio (SECBR).

$\succ$ Multicast over ATM

- Multicast in ATM networks had several challenges:

- ATM is connection-oriented; hence ATM multicasting needs to set up all multipoint connections.

- QoS in ATM must be negotiated at the connection set-up time and be known to all switches.

- 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.