Page 74 - M. Abrahim Thesis

Chapter (3) Networking Media and Data Link Layers Protocols

As a rough estimate, 8 inches (20.3 cm) per nanosecond often is used for calculating
propagation delay down a UTP cable. For 100m of UTP, this means that it takes just under 5
bittimes for a 10BASE-T signal to travel the length of a 100m cable (about 4.92 bit-times). Simply
moving the decimal point over results in 49.2 bit-times at 100 Mbps, and 492 bit-times at 1000
Mbps.

For CSMA/CD Ethernet to operate, the sending station must become aware of a collision
before it has completed transmission of a minimum-size frame. At 100 Mbps, the system timing
is barely capable of accommodating 100m cables. At 1000 Mbps, special (very inefficient)
adjustments were required because nearly an entire minimum-size frame has been transmitted
before the first bit reaches the end of the first 100 meters of UTP cable. It is easy to see why half
duplex was not permitted in 10-Gb Ethernet.
https://www.youtube.com/watch?v=HLziLmaYsO0&t=43s

3.8 Collision Domains and Broadcast Domains

It is important to identify the medium as a shared environment because this shared
environment causes collisions. A similar situation can occur with an automobile on a highway. If
there is only one car, there is nothing to collide with. However, if more than one automobile is
trying to use the same section of road at the same time, as shown in Figure 3-20, a collision
occurs. The same is true for networks.
If more than one computer tries to transmit data on the same network segment at the
same time, a collision occurs.
A collision is a situation that can occur when 2 bits propagate at the same time on the
same network.

A small, slow network could work out a system that allows only two computers to send
messages, with both agreeing to take turns.
The problem is that many computers are connected to large networks, with each one
wanting to communicate millions of bits every section.

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