5. THE PHYSICAL LAYER OF THE OSI MODEL

Description of the physical layer :

In general the physical layer is comprised of any network component that you can touch.

When the physical layer receives data from the upper layers it encodes that data into signals that can be transmitted over whatever transmission medium the network is using. This process is known as signal encoding. If a network is using cable (bounded media) the ones and zeros of the data are translated into electrical pulses or "ons and offs".

The physical layer also specifies how much of the media is used. If a network signal uses the entire bandwidth it is said to be base band. Most LAN technologies, such as Ethernet, use baseband signaling. If a signal uses only one frequency (or part of the bandwidth) it is called broadband.

Finally, the physical layer specifies the layout of the transmission media or its physical topology.

Physical layer devices:
There are several common devices that operate, primarily, at the physical layer of the OSI model they are:
  1. Network Interface Cards (NICs)
  2. Transceivers
  3. Repeaters
  4. Hubs
  5. Multi-Station Access Units (MAUs)
Each one of these devices are responsible for such things as voltages, signal direction and signal strength.

 

Network Interface Card (NIC):

A NIC is probably the most common physical device on a network. It is a card that connects the computers internal bus to the network media. There are many different types of NICs and they vary depending upon the type of bus connection they employ and their media connection parts.

 

Transceiver:

A Transceiver is any part of the network interface that transmits and receives network signals. Every network interface contains a transceiver. Its appearance and function varies with the type of network cable and topology in use.

 

Repeater:

Repeaters are used to extend the length of a network segment. It does this by amplifying the signal it receives on one port and repeating it on another. The main drawback of a repeater is that it repeats everything, including line noise. This has the ultimate effect of limiting the number of repeaters that can be used on a network due to signal degradation.

The 5-4-3 rule dictates how many repeaters can be used on a network and where they are placed. This rule states that a single network can have 5 network segments connected by 4 repeaters, with 3 of the segments being populate (have computers on them). If the 5-4-3 rule is violated one station may not "see" the rest of the network.

Hub:
 A hub (a.k.a. concentrator) serves as a central connection point for several network devices. Put simply, a hub is a repeater with multiple ports. It repeats what it receives on one port to all other ports. For this reason a hub is also subject to the 5-4-3 rule. The two most important classifications of hubs are:
  1. Active: This type of hub is powered and cleans up the data signal as it amplifies it. This effectively doubles the effective segment's distance for the specific topology in use.
  2. Passive: A passive hub is not powered and only provides physical and electrical connections. Using this type of hub usually shortens the segment distance of a topology because it takes some power away from the signal.
Multi-Station Access Unit (MAU):
 This device is unique to Token Ring networks. Token Ring networks use a physical star topology but, a logical ring topology. On an Ethernet star topology a hub is the central device but, on a Token Ring network a MAU is the central device. A MAU is similar to a hub with one exception; it provides a data path that creates a logical ring on a token ring network. The data can travel in an endless loop between stations and you can have up to 33 MAUs chained together.


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