Question

The CS-PDU for AAL 5 contains up to 47 bytes of padding, while the AAL3/4 CSPDU only contains up to 3 bytes of padding. Explain why the effective bandwidth of AAL 5 is always the same as, or higher than, that of AAL.3/4, given a PDU of a particular size.

Short answer

AAL 5 achieves higher effective bandwidth than AAL 3/4 due to its larger PDU support and decreasing padding significance with larger PDUs.

Step-by-step solution

Understand the padding requirements

The AAL 5 protocol can include up to 47 bytes of padding and AAL3/4 includes up to 3 bytes of padding. The padding is used to ensure that the entire PDU fits into the standard cell structure used by ATM networks.

Calculate the overhead for each AAL type

For AAL 5, the overhead includes the 47 bytes of padding plus an 8-byte trailer, resulting in a maximum total overhead of 55 bytes. For AAL3/4, the overhead includes the 3 bytes of padding plus a 4-byte header and a 4-byte trailer, resulting in a total overhead of 11 bytes.

Analyze the impact on effective bandwidth

Since the total overhead reduces the number of bytes available for actual data within each ATM cell, the higher the padding and overhead, the less efficient the protocol is. Comparing the total overhead, AAL5 is significantly higher than AAL3/4.

Explain the impact of padding

The influence of padding becomes significant with smaller PDU sizes. AAL 5's larger padding reduces its effective data payload more than AAL 3/4. For larger PDUs, the fixed overhead becomes a smaller percentage of the total payload, rendering AAL 5's efficiency comparable to or better than AAL 3/4.

Effective bandwidth comparison

Given that both padding and fixed overhead decrease the effective bandwidth, larger PDUs make the padding less significant. With AAL 5 having larger but less frequently required padding, it achieves similar or even better bandwidth efficiency compared to AAL 3/4 with larger PDUs.

Key concepts

ATM Networks

Asynchronous Transfer Mode (ATM) networks are a type of telecommunication that encapsulates data into fixed-size cells, making it suitable for different types of services like voice, video, and data. Unlike other networking technologies that use variable-length packets, ATM networks use fixed-size cells of 53 bytes each. This uniform size helps ATM achieve efficient and predictable performance, especially beneficial for real-time services.

AAL Protocols

ATM Adaptation Layer (AAL) protocols handle segmentation and reassembly of data to fit the fixed-size ATM cell structure. Different AAL types meet various application needs. AAL5 and AAL3/4 are especially crucial for comparing performance metrics like bandwidth efficiency.

PDU Padding

Padding is additional data added to a Protocol Data Unit (PDU) to align it with the required cell structure or packet size in a network. For ATM networks using AAL protocols, padding ensures the data fits snugly within the 48-byte payload space.

Bandwidth Efficiency

Bandwidth efficiency measures how effectively a network utilizes its available transmission capacity. Overhead and padding are critical factors that influence this efficiency in ATM networks.