6.3 Multiple Access links and protocols

Summary

This video explores multiple access protocols in communication networks, discussing three broad classes: channel partitioning, random access, and taking turns protocols. It covers the strengths and weaknesses of each approach, emphasizing how they manage shared communication channels. Channel partitioning is efficient at high loads, random access excels at low loads, and taking turns protocols aim to combine the best of both worlds. The video also looks at practical implementations, such as the cable access network (DOCSIS), which uses a combination of FDMA, TDMA, and random access for upstream communication. The study wraps up with a look at switched local area networks.

Takeaways

• 😀 Taking turns protocols aim to combine the benefits of channel partitioning and random access protocols, ensuring fair channel allocation without collisions.
• 😀 Polling protocols involve a centralized controller that allocates time slots to nodes, but can create control overhead and represent a single point of failure.
• 😀 Token passing protocols work by passing a control token between nodes in a predetermined order, where only the node holding the token can transmit.
• 😀 Channel partitioning protocols are efficient at high loads but inefficient at low loads due to delays in accessing the channel.
• 😀 Random access protocols are highly efficient at low loads, allowing a single node to fully utilize the channel but suffer from collisions at high loads.
• 😀 Polling protocols ensure no collisions, but if nodes have nothing to send, they can remain idle, and there is a short polling delay for the next node to transmit.
• 😀 Token passing protocols reduce collisions, but introduce latency and can be disrupted if the token is lost or corrupted.
• 😀 The DOCSIS standard used in cable access networks divides the frequency band into FDMA sub-channels and uses TDMA slots for upstream transmission, along with random access protocols.
• 😀 Cable networks combine FDMA, TDMA, and random access protocols to handle both high and low traffic loads efficiently.
• 😀 The CMTS in a cable network sends a downstream map frame to allocate some upstream TDMA slots to specific homes, while other homes access unassigned slots using random access.
• 😀 The discussion of multiple access protocols in cable networks highlights the importance of hybrid approaches that leverage different protocols for different network conditions.

Q & A

What are the three broad classes of multiple access protocols discussed in the script?

The three broad classes of multiple access protocols are channel partitioning protocols, random access protocols, and taking turns protocols.

What is the main advantage of channel partitioning protocols?

Channel partitioning protocols can share the channel efficiently and fairly at high loads, allowing for 100% utilization of the channel.

What is a significant disadvantage of channel partitioning protocols?

A key disadvantage is inefficiency at low loads, as the channel may remain underutilized, and there is delay in accessing the channel if only one node has data to send.

How do random access protocols differ from channel partitioning protocols in terms of efficiency at different loads?

Random access protocols are efficient at low loads because there are few collisions, and a single node can fully utilize the channel. However, they suffer from collisions and overhead at high loads.

What is the primary goal of taking turns protocols?

The goal of taking turns protocols is to combine the strengths of both channel partitioning and random access protocols: no collisions and explicit channel allocation, while avoiding holding the channel unnecessarily when there’s no data to send.

Can you explain how polling protocols work in multiple access systems?

In polling protocols, a centralized controller coordinates access by polling nodes in sequence. Nodes transmit if they have data, otherwise they send a 'no data' message. The controller then polls the next node in line.

What is a key disadvantage of polling protocols?

A disadvantage of polling protocols is the control overhead involved, as well as the single point of failure introduced by the centralized controller.

What is the difference between polling and token passing protocols?

In polling, a centralized controller sequentially polls nodes to manage access. In token passing, nodes are arranged in a ring, and a token is passed from node to node. The node holding the token has the right to transmit data.

What are some of the disadvantages of token passing protocols?

Token passing protocols share some disadvantages with polling, such as control overhead, access latency, and the single point of failure represented by the token.

How does the cable access network utilize multiple access protocols in its design?

The cable access network uses a combination of FDM (Frequency Division Multiplexing) to divide frequency channels, TDMA (Time Division Multiple Access) for upstream channels, and random access using binary backoff for some upstream slots, all to manage multiple access in the network.