Chapter 18 Virtual-Circuit Networks

Chapter 18 of Data Communications and Networking by Behrouz A. Forouzan focuses on Virtual-Circuit Networks, specifically covering Frame Relay and Asynchronous Transfer Mode (ATM). Here’s a detailed breakdown:

18.1 Frame Relay

Frame Relay is a high-speed, packet-switching WAN protocol designed in the 1990s to address the limitations of its predecessor, X.25.

Key Aspects of Frame Relay:

1. High-Speed Data Rate: Frame Relay was developed to handle higher data rates than X.25, which was limited to 64 kbps.

2. Minimal Error and Flow Control: Unlike X.25, Frame Relay provides only basic error detection at the data link layer and does not incorporate flow control, which makes the transmission faster.

3. Permanent and Switched Virtual Circuits:

   • PVC (Permanent Virtual Circuit): A pre-established path used for long-term communication between two points.

   • SVC (Switched Virtual Circuit): A temporary connection established when needed and terminated afterward.

4. DLCI (Data Link Connection Identifier): Used to identify virtual circuits within the Frame Relay network.

5. Error Detection Only: There is no error correction; errors are handled by higher layers in the protocol stack.

Frame Relay also supports Voice Over Frame Relay (VOFR), a technology to carry voice traffic over a Frame Relay network, though with limitations in voice quality.

18.2 Asynchronous Transfer Mode (ATM)

ATM is another virtual-circuit-based technology that supports high-speed data transfer. It operates using fixed-size cells of 53 bytes, unlike variable-sized frames used by Frame Relay.

Design Goals of ATM:

1. Support for High-Speed Networks: ATM is designed to work with high-bandwidth optical fibers and supports very high data rates.

2. Connection-Oriented Communication: ATM ensures reliable and predictable communication paths, essential for real-time transmissions like video and voice.

3. Minimized Software Overhead: The system prioritizes hardware solutions to handle communication tasks efficiently.

ATM Architecture:

1. Cell Structure: ATM uses a fixed cell size of 53 bytes (5 bytes for the header and 48 bytes for the payload), which allows for low-latency, efficient packet-switching.

2. Layers in ATM:

   • Physical Layer: Responsible for transmitting the data.

   • ATM Layer: Handles the creation and management of the virtual paths and circuits.

   • Application Adaptation Layer (AAL): Converts higher-layer data (from applications) into ATM cells.

   • AAL Subdivisions: Different AAL types exist to handle various data types, such as constant bit-rate streams or packets requiring no error control.

3. Congestion Control and QoS: ATM includes features to ensure Quality of Service (QoS) for real-time data like voice and video. This includes prioritizing packets and managing traffic to avoid congestion.

Connection Setup:

ATM supports both Permanent Virtual Circuits (PVCs) and Switched Virtual Circuits (SVCs) similar to Frame Relay. However, ATM is much more scalable and adaptable for larger networks and real-time communication.

Summary of Concepts:

• Virtual Circuit: Both Frame Relay and ATM use virtual circuits to facilitate communication across networks. However, ATM uses fixed-length cells, whereas Frame Relay uses variable-length frames.

• Efficiency: ATM’s design is optimized for real-time applications, while Frame Relay is more cost-effective and simpler but less suited for real-time multimedia.

• Congestion Management: ATM provides robust mechanisms for ensuring high performance under varying network loads, which is critical for handling bursty and multimedia data.

This chapter highlights the strengths and limitations of two prominent technologies used in wide-area networks, particularly focusing on how they establish and maintain virtual circuits to manage data transfer efficiently.