Broadcast Domains & CSMA/CA

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Broadcast Domains & CSMA/CA

In networking, Layer-2 hardware, including switches, Access points & bridges, are programmed to forward Broadcast Message so as to reach all Hosts on the LAN.

The area over which a Broadcast message reaches is known as the Broadcast Domains, which is synonymous with the size of the Local Area Network.

Because the Broadcast Domain represents the interconnected topology of layer-2 equipment, more Switches, Access Points, and Bridges equates to a larger Broadcast Domain.

By comparison, however, Routers do not forward Broadcast Messages, and therefore mark the boundaries of the Broadcast Domain, that is, the LAN.

When the Broadcast Domain becomes too large or populous, Broadcast Messages lead to Broadcast Storms, where normal Host traffic causes LAN performance to suffer tremendously, noting increased latency and reduced throughput.

A good rule of thumb is to limit the size of a LAN to no greater than a couple hundred Network Devices, before building another Network Segment and Range.

Building a new LAN is as simple as adding another Router, with new LAN interfaces.

However, a more cost-effective, practical way to add additional Network Segments and Ranges is to implement Virtual LANs on existing Network Devices.

As more Local Networking continues to trend toward mobile computing, Wireless Design & planning for Networks becomes increasingly more important.

Like Switches, Access Points expand the Broadcast Domain, but face a unique challenge, since Wireless Stations in the same proximity, by design, compete for shared access to the Wireless Channel over which they transmit.

Whenever two Wireless Stations attempt to transmit over top of each other, a collision can result at nearby Receiver Radios, necessitating a retransmission.

In order to reduce the chance of collisions, the 802.11 Wireless protocol relies on Carrier Sense Multiple Access / Collision Avoidance, or CSMA/CA for short, which basically requires Wireless Stations “Listen before Talk”.

If the Wireless Channel is occupied, wait, then listen again before transmitting.

Despite reducing collisions, CSMA/CA is an imperfect access method, as seen in scenarios involving Hidden Nodes, where a receiver hears two transmitters, but the transmitters cannot hear each other, whether due to proximity or physical obstructions.

And although mechanisms like Request-to-Send, Clear-to-Send, mitigate the effect of Hidden Nodes, CSMA/CA remains an imperfect access method, with upper limits on the density and volume of stations in a wireless coverage area.

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