COAXIAL
Describe and compare commonly used Local Area Network media types and their associated standards
On completion of this learning outcome the student should be able to:
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Assessment Criteria |
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Assessment Methods |
The assessment method for this learning outcome is: Short Answer Test |
Describe the characteristics of coaxial cables and associated standards
There are two types of coaxial cable commonly used in network installations.
Thin Wire (RG58)
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· Transmission Rate - 10 Mbps.
· Maximum Length - 180 metres/segment.
· Impedance - 50 ohm RG58, conductor diameter - 0.9 mm.
· BNC connections.
· 0.5 metre between consecutive connections.
· Up to 30 nodes per segment.
The standard used for ethernet over this type of cable is called 10BASE-2
Thick Wire
· Transmission Rate - 10 Mbps.
· Maximum Length - 500 metres/segment.
· Impedance - 50 ohm, conductor diameter - 2.17 mm.
· Uses transceivers and AUI (Attachment Unit Interface) cable.
· Up to 100 nodes per segment.
· Total Maximum Extended Length (by Repeaters) - 1500 metres.
The standard used for etehrnet over this type of cable is called 10BASE-5.
Describe the characteristics of twisted pair cables and associated standards
The most common type of twisted pair cabling in use today is Unshielded Twisted Pair cable or 10BASET cable. Attributes of such cable are:
· Transmission Rate - 10 Mbps for Category 3, up to 100Mbps for Category 5.
· Maximum Length - 100 metres/segment.
· Cable connectors used for this type of cabling are called RJ45
The standards used for transmission over this type of cable is 10BASE-T, 100BASE-TX, 100VG-AnyLAN
Describe the characteristics of optic fibre cables and associated standards
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FDDI
FDDI is a topology to transmit data on a fibre network.
FDDI uses a dual ring topology, which is to say that it is comprised of two counter-rotating rings. A dual-attached rooted station on the network is attached to both of these rings
A dual-attached station on the ring has at least two ports - an A port, where the primary ring comes in and the secondary ring goes out, and a B port where the secondary ring comes in, and the primary goes out. A station may also have a number of M ports, which are attachments for single-attached stations. Stations with at least one M port are called concentrators.
The sequence in which stations gain access to the medium is predetermined. A station generates a special signaling sequence called a Token that controls the right to transmit. This Token is continually passed around the network from one node to the next. When a station has something to send, it captures the Token, sends the information in well formatted FDDI frames, then releases the token. The header of these frames includes the address of the station(s) that will copy the frame. All nodes read the frame as it is passed around the ring to determine if they are the recipient of the frame. If they are, they extract the data, retransmitting the frame to the next station on the ring. When the frame returns to the originating station, the originating station strips the frame. The token-access control scheme thus allows all stations to share the network bandwidth in an orderly and efficient manner.
Key attributes of FDDI are:
· Transmission Rate - 100 Mbps
· Maximum Length - 10 kilometres
· Maximum number of stations - 500
The following excellent resources are available from the InterOperability Lab at the University of New Hampshire:
Cisco also provides a FDDI Overview.
Describe the characteristics of wireless networks and associated standards
Wireless LANs use one of three transmission techniques: spread spectrum, narrowband microwave, and infrared.
Spread Spectrum
Spread spectrum is currently the most widely used transmission technique for wireless LANs. It was initially developed by the military to avoid jamming and eavesdropping of the signals. This is done by spreading the signal over a range of frequencies, that consist of the industrial, scientific, and medical (ISM) bands of the electromagnetic spectrum. The ISM bands include the frequency ranges at 902 MHz to 928 MHZ and at 2.4 GHz to 2.484 GHz, which do not require an FCC license.
The first type of spread spectrum developed is known as frequency hopping spread spectrum. This technique broadcasts the signal over a seemingly random series of radio frequencies. A receiver, hopping between frequencies in synchronization with the transmitter, receives the message. The message can only be fully received if the series of frequencies is known. Because only the intended receiver knows the transmitter's hopping sequence, only that receiver can successfully receive all of the data. Most vendores develop their own hopping-sequence algorithms, which all but guarantees that two transmitters will not hop to the same frequency at the same time.
Even though the FCC has made some rules for frequency hopping spread spectrum technologies. The FCC dictates that the transmitters must not spend more than 0.4 seconds on any one channel every 20 seconds in the 902 MHz band and every 30 seconds in the 2.4-GHz band. Also, the transmitters must hop through at least 50 channels in the 902-MHz band and 75 channels in the 2.4-GHz band--a channel consists of a frequency width which is determined by the FCC. The IEEE 802.11 committee has drafted a standard that limits frequency hopping spread spectrum transmitter to the 2.4-GHz band.
The other type of spread spectrum communication is called direct sequence spread spectrum, or pseudonoise. This method seems to be the one that most wireless spread-spctrum LANs use. direct sequence transmitter spread their transmissions by adding redundant data bits called "chips" to them. Direct sequence spread spectrum adds at least ten chips to each data bit. Like a frequency hopping receiver, a direct sequence receiver must know a transmitter's spreading code to decipher data. This spreading code is what allows multiple direct sequence transmitters to operate in the same area without interference. Once the receiver has all of the data signal, it uses a correlator to remove the chips and collapse the signal to its original length.
As with frequency hopping spread spectrum, the FCC has also set rules for direct sequence transmitters. Each signal must have ten or more chips. This rule limits the practical raw data throughput of direct sequence transmitters to 2 Mbps in the 902-MHz band and 8Mbps in the 2.4-GHz band. Unfortunately, the number of chips is directly related to a signal's immunity to interference. In an area with lots of radio interference, you'll have to give up throughput to avoid interference. The IEEE 802.11 committee has drafted a standard of 11 chips for direct sequence spread spectrum.
Frequency hopping radios currently use less power than direct sequence radios and generally cost less. While direct sequence radios have a practical raw data rate of 8 Mbps and frequency hopping radios have a practical limit of 2 Mbps. So if high performance is key and interference is not a problem, go with direct sequencing. But if a small, inexpensive portable wireless adapter for a notebook or PDA is needed a the frequency hopping method should be good enough. With either method of spread spectrum the end result is a system that is extremely difficult to detect, does not interfere with other services, and still carries a large bandwidth of data.
Narrowband Microwave
Microwave technology is not really a LAN technology. It's main use is to interconnect LANs between buildings. This requires microwave dishes on both ends of the link. The dishes must be in line-of-sight to transmit and collect the microwave signals. Microwave is used to bypass the telephone company when connecting Lans between buildings.
One major drawback to the use of microwave technology is that the frequency band used requires licensing by the FCC. Once a license is granted for a particular location, that frequency band cannot be licensed to anyone else, for any purpose, within a 17.5 mile radius.
Infrared
Infrared LANs use infrared signals to transmit data. This is the same technology used in products like remote controls for televisions and VCRs. These LANs can be setup using either a point-to-point configuration or a sun-and-moon configuration where the signals are diffused by reflecting them off of some type of surface.
The major advantage of infrared is its ability to carry a high bandwidth, but its major disadvantage is that they can easily be obstructed, since light cannot pass through solid objects.
Wireless LAN Transmission Techniques
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Spread Spectrum |
Narrowband Microwave |
Infrared |
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Frequency |
902MHz to 928 MHz ; 2.4 GHz to 2.4385 GHz ; 5.725 GHz to 5.825 GHz |
18.825 GHz to 19.205 GHz |
3 x 10^14 Hz |
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Maximum coverage |
105 to 800 feet, or up to 50,000 square feet |
40 to 130 feet, or up to 5000 square feet |
30 to 80 feet |
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Line of sight required |
No |
No |
Yes |
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Transmit power |
Less than 1 W |
25 mW |
N/A |
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License required |
No |
Yes |
No |
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Interbuilding use |
Possible with antenna |
No |
Possible |
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Rated speed (% of 10 Mbps wire) |
20% to 50% |
33% |
50% to 100% |
The following references are useful when researching Wireless Technology
· Working Group for Wireless Local Area Networks
· Wireless Data Networking (By Nathan J. Muller)
· What is a Wireless LAN? (By Joel B. Wood)
· Selecting a Wireless LAN Technology (By Proxim)
· WLANA: The Wireless LAN Allia
Explain the advantages and disadvantages of various media types
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Media |
Advantages |
Disadvantages |
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Thin coax |
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Thick coax |
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Unshielded Twisted Pair |
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Fibre Optic |
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Wireless |
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Describe methods of interconnection between different media types
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Transceivers are often used to establish networks across different types of cable media.
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Pictured on the left is a transceiver that connects UTP to an AUI connection, whilst on the right is a transceiver which converts UTP to RG58 cable.