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Network Media| Category | | | Summary | | | Body | Layer 1- The Physical Layer –Network Media
Introduction:
The Physical layer of the osi network model deals with all Physical parts of the network, which include all physical media. Physical media includes copper cable, fibre optics etc.Since the physical layer defines the cable then new technology can progress at this layer without affecting any other layer.
The basic media types are as follows:
- Copper
- Optical fibre
- Wireless
Cabling standards:
Cabling standards use a format to describe the cable being used. Below is an example:
10 base T = This breaks down to 10Mps (Million instructions per second ) Base as in baseband /Broadband ,and the T stands for twisted pair wire.
Copper Wire:
Copper wire as been used for years in electronics and cables.It is relatively cheap and a very good conductor of electricity which makes this a good material for connecting computers together.
E.g – twisted pair. The cable is twisted to cut down interference from say mains electricity power sources etc.
Associated Standards:
The IEEE project 802 was bought in during the eighties for standards of LAN and MAN , mainly for the lower two layers of the osi seven layer model. IEEE together with Ethernet(standard for bus networks) make up the market share in LAN standards. Ethernet version 2 and IEEE 802.3 are also very compatible with each other.
Other standards as the apply to wiring are as follows:
- Interference:Wire which is used extensively in networking also as a problem with interference. If a wire is placed parallel with other wires pick up the electromagnetic pulse and can cause problems with the data. E.g 1s being interpreted as 0s. To minimize interference, networks use two basic wiring types,twisted pair or coaxial cable. Twisted pair cable is well used and is found in telephone networks. The twisted cable helps to cut down on interference.
- Twisted Pair wiring:Twisted pair cable is two copper wires with a coating of plastic on each wire to prevent the metal in the wire touching. The cables supply a signal path and a return path. The twist is to change the electrical properties of the cable which prevents electric currents in the wire from radiating energy thus cutting down on interference. Twisted pair comes as (UTP),unshielded twisted pair and (STP), shielded twisted pair.
- Unshielded twisted pair:Unshielded twisted pair is like STP apart from the shield and thus less protective to interference but is cheaper. UTP comes in five categories and can be used for fast transfer of data. Twisted pair is used with RJ45 connectors which can connect to Ethernet and is the standard connector for twisted pair.
Categories for twisted pair:
Type
Use
Category 1
Voice Only (Telephone Wire)
Category 2
Data to 4 Mbps (LocalTalk)
Category 3
Data to 10 Mbps (Ethernet)
Category 4
Data to 20 Mbps (16 Mbps Token Ring)
Category 5
Data to 100 Mbps (Fast Ethernet)
Shielded twisted pair:
The disadvantages of UTP is that it is susceptible to radio and electrical interference. STP is much suited for this problem, however the shield does make the cable bulkier but with it's advantages is suited to token ring topology.
Twisted pair cable – IBM Token Rings:
IBM promotes the use of STP for use in it's token ring network. Shielded twisted pair as discussed above fares better against interference, therefore you can run longer cable for a given data rate. Token rings tend to use one of three types of STP cable:
- Type 1 cable > Two pairs of insulated solid copper wire wrapped in a braided metal shield.
- Type 2 cable > Four pairs of unshielded wires and two pairs of shielded wires
- Type 9 cable > Quite as type one but as either a solid copper wire or braided copper in two shielded twisted pairs.
Twisted pair Categories:
As above we can see that data communication cable as been defined in five categories, all which offer different data rates:
Category
Maximum data rate
Usual application
CAT 1
Less than 1 Mbps
analog voice (POTS)Integrated Services Digital Network Basic Rate Interface in ISDNDoorbell wiring
CAT 2
4 Mbps
Mainly used in the IBM Cabling System for Token Ring networks
CAT 3
16 Mbps
Voice and data on 10BASE-T Ethernet
CAT 4
20 Mbps
Used in 16 Mbps Token RingOtherwise not used much
CAT 5
100 Mbps1000 Mbps (4 pair)
100 Mbps TPDDI155 Mbps ATMGigabit Ethernet
CAT 5E
100 Mbps
100 Mbps TPDDI155 Mbps ATM
CAT 6
200-250 MHz
Super-fast broadband applications
As you can see it ranges from CAT 1 to CAT 5E. Some of the uses are described on page 2.
Twisted pair standards (below are but a few):
Specification
Cable Type
Maximum length
10BaseT
Unshielded Twisted Pair
100 meters
10Base2
Thin Coaxial
185 meters
10Base5
Thick Coaxial
500 meters
10BaseF
Fiber Optic
2000 meters
100BaseT
Unshielded Twisted Pair
100 meters
100BaseTX
Unshielded Twisted Pair
220 meters
Coaxial Cable:
Coaxial cable as been around for a long time, it is used for TV Ariel cable and computer network cable etc. Coaxial comprises of a single copper wire which is the conductor at the centre. Then there is a plastic layer and the braided metal shield. The plastic layer is to insulate the conductor and shield. The shield is used to protect against interference from motors computers etc.
Coaxial cable standards:
There are two cable types of cable used in networking they are:
- 10 base 2 (Thin Ethernet) > This uses a BNC connector which goes into the Ethernet port, and is a 50ohm coaxial cable. The maximum length for 10base2 is 185 metres and is more flexible than thicknet. The system thinnet is made for is baseband transmission operating at 10mps.
- 10Base 5(Thick Ethernet) > This also uses a BNC connector to connect to our Ethernet. It is thicker as it's name suggests and thus not a flexible as thinnet. As with the terms 10base5 we can break this down to 10mps, baseband and 500mtrs maximum length for the cable. Although it is thicker it does have the advantage of a further range.
Extended Bus LANS:
As you can see from the information above, a single segment LAN using 10BASE 2 cable as it limitations.For example, 10BASE 2 as a maximum range of 185mtrs and can support only 30 computers. It is however possible to extend this by using what is called a repeater. There is a limit to this though and is best remembered by the 5,4,3 rule:
- Five is the maximum of segments allowed
- Four is the maximum of repeaters allowed
- Three is the maximum active segments allowed
Connection Taps:
The connectors used for coaxial cable are achieved via two methods:
- Intrusive taps (BNC connector)
- Non intrusive tap, or Vampire Tap
Optical Fibre:
Optical fibre is thinner than a humans hair and converts electrical signals to infra red or light and then gets converted back into electrical signals. Optical fibre is made up of glass or plastic.
Advantages:
Fibre optics offer a very high bandwidth almost unlimited.
Unlike the other cables, interference doesn't affect fibre optics, as it does not conduct electricity
Because the cable is made of glass it can be put underground in the soil, as it will not corrode.
It is lighter and thinner than other materials so doesn't require much duct space.
Disadvantages of fibre optics:
Fibre optics cables main disadvantage is it's cost and special equipment required to install the cable.
Construction:
Fibre optics consists of the following:
- Core: Thin glass centre of the fibre where light travels.
- Cladding: Outer optical media that surrounding the core that reflects light back into the core.
- Buffer coating: Plastic coating that protects the fibre from damage and moisture.
Fibre optics are constructed in different ways.Some examples are as follows:
As you can see we have the following:
(A) Step index Fibre
(B) Graded index fibre
(C) Single mode fibre
Step Index:
In step index fibre, the refractive index of the core is uniform and undergoes an abrupt change at the core cladding boundary. Step index gets it's name from the abrupt change called the step change in refractive index.
Graded index fibre:
In graded index fibre the refractive index of the core varies gradually as a function of radial distance from the fibre center.
Single Mode fibre:
Single mode fibre have a step index refractive index profile. This is the most expensive optical cable to produce and install, but can give a typical data rate of 50mps,with a maximum distance of 30km without repeaters.
Wireless:
This is another way of transmitting data without using wires via radio waves. The beauty of this is that it does not require cables.However it does have a restricted distance it can travel.
Satellite:
Satellite is the ultimate way of transmitting signals. As long as the line of sight to the satellite is their, you can transmit to anyone within the world.
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This article was imported from the CyberArmy University site. (original author: condor1)
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