1. Describe the three most common types of cabling media used in LANs.
Coaxial Cable. Coaxial cable comes in two versions: Thinnet and Thicknet. Thinnet looks like regular TV cable.* It is about 1/4 inch in diameter and is very flexible and easy to work with. In contrast, Thicknet is about 1/2 inch in diameter and not very flexible. Thicknet is older and not very common anymore except as a backbone within and between buildings. Coax transmits at 10 Mbps..
Twisted Pair. Twisted pair looks like telephone wire and consists of insulated strands of copper wire twisted together. There are two versions of twisted pair cable: Shielded Twisted Pair (STP) and Unshielded Twisted Pair (UTP). STP is commonly used in Token Ring networks and UTP in Ethernet networks where it is referred to as "10baseT." Transmission rates vary between 10-100 Mbps..
Fiber-Optic Cable. Fiber-optic cable consists of a thin cylinder of glass surrounded by glass cladding, encased in protective outer sheath. Fiber-optic cable is very fast (100 Mbps). It can transmit over long distances (2 km +) but is expensive.
*Don’t confuse Thinnet cable (RG 58) with cable TV cable (RG 59). They look alike but they are not interchangeable.
2. What are the recommended maximum segment lengths for each type of cable?
The type of cable plays a role in how fast a signal will degrade as it is transmitted. The following are the recommended maximum cable lengths:
Thinnet- maximum length of segment (terminator to terminator) is 185 meters (607 feet)
Thicknet -maximum length of segment (single run) is 500 meters (1,640 feet)
Unshielded Twisted Pair (UTP) - maximum length of cable between hub and computer is 100 meters (328 feet)
Fiber-Optic - maximum length of cable is 2 kilometers. (6,562 feet)
3. What is a backbone?
A backbone is a generic term used to describe media that interconnects a number of computers, segments or subnets.
In its most common form, a backbone is used to connect hubs. Each hub represents a segment on which individual workstations are connected via UTP cable. The hubs from different segments are then connected to each other with thinnet cable. In this case, the thinnet cable functions as the backbone that links the hubs.
In another example, a backbone may be a length of cable that serves as a trunk. Drop cables are attached from the backbone to connect individual workstations.
A backbone is often used to connect networks in separate buildings. Organizations typically use fiber-optic cable for this type of backbone.
4. What are the characteristics of coaxial cable?
· Transmission rate of about 10 Mbps
· Maximum cable length of 185 meters for Thinnet, 500 meters for Thicknet
· Good resistance to electrical interference
· Less expensive than fiber-optics but more expensive than twisted pair.
· Flexible and easy to work with (Thinnet)
· Wire type is 20 AWG for Thinnet (R-58) and 12 AWG for Thicknet.
Ethernet designation is 10base2 (Thinnet)
or 10base5 (Thicknet, also referred to
5. What are the different types of coaxial cable?
· RG-58 A/U – Thinnet, stranded wire core, 50 ohms
· RG-58 /U – Thinnet, solid wire core, 50 ohms
· RG-59 – Thicknet, cable television, broadband
· RG-62 – ArcNet, 75 ohms
A cable's designation is typically printed on its outer sheathing.
6. What connection hardware is used with thinnet coaxial cable?
Thinnet utilizes the following connection hardware referred to as ‘BNC’ components:
· Terminator – a resister used to absorb the signal once it reaches the end of the bus; connects to a T- or barrel connector; RG-58 requires a 50-ohm terminator; RG-62 requires a 75-ohm terminator.
· Cable Connector – the interface at the end of the cable that is used to connect to a barrel or T-connector
· T-Connector – used to connect to a NIC and another cable connector or a terminator (you cannot connect directly to a NIC with a cable connector; you must use a T-connector)
· Barrel Connector – used to splice to segments of cable together or attach a terminator at the end of the cable
7. What connection hardware is used with thicknet coaxial cable?
Thicknet utilizes an AUI (Attachment Unit Interface) connector to attach to a Network Interface Card (NIC). The AUI connector has 15 pins and is alternatively referred to as a DB-15 or DIX connector.
One of the unique characteristics of Thicknet is the use of an external transceiver. (The transceiver is used to convert signals from parallel to serial for transmission on the network.) The Thicknet cable attaches to the transceiver via a clamp or vampire tap. The NIC connects to the transceiver using a drop cable with AUI connectors.
8. What is Plenum cable?
Plenum refers to the space in buildings between the ceiling and the next floor above it. Because of the potential fire hazard, building codes are very specific about what type of wiring can be placed in this area. refers to the space in buildings between the ceiling and the next floor above it. Because of the potential fire hazard, building codes are very specific about what type of wiring can be placed in this area.
Plenum cable refers to coaxial cabling that meets the minimum standards to allow it to be strung in the plenum area without having to use special conduit. The insulation and jacket on plenum cabling must be fire resistant and not give off toxic fumes when burned. refers to coaxial cabling that meets the minimum standards to allow it to be strung in the plenum area without having to use special conduit. The insulation and jacket on plenum cabling must be fire resistant and not give off toxic fumes when burned.
Not all coaxial cable is plenum cable. The most common type of coaxial cable is PVC (polyvinyl chloride) which is more flexible and easier to work with than plenum but does not have the same fire resistance features. PVC cable can give off toxic fumes when burned.
9. What are the key characteristics of UTP (unshielded twisted pair)?
· Transmission rate of 10-100 Mbps
· Maximum cable segment of 100 meters
Most susceptible to electrical interference or ‘crosstalk’ (although
shielding may lessen
Less expensive than coax or fiber-optic. In some cases, preinstalled
· Very flexible and easy to work with
· Wire type is 22-26AWG
· Uses an RJ-45 connector
· Ethernet designation is 10baseT
10. What are the characteristics of shielded twisted pair (STP)?
Shielded twisted pair (STP) is similar to UTP except it contains a copper braid jacket to ‘shield’ the wires from electrical interference. It can support transmissions over greater distances than UTP.
11. What are the 5 categories of UTP and what is the minimum acceptable category for 10baseT?
The following categories of unshielded twisted pair (UTP) cable were established by the EIA/TIA* to support the networks indicated:
· Category 1 - Traditional telephone cable; supports voice only, not data
· Category 2 - Data transmissions up to 4 Mbps (but not token ring)
· Category 3 - 10 Mbps Ethernet
· Category 4 - 16 Mbps token-ring
· Category 5 - 100 Mbps; supports ATM
The minimum acceptable cable for 10baseT Ethernet is Category 3.
*EIA/TIA 568 is the standard developed by the Electronic Industries Association/Telecommunications Industry Association applies to all UTP that works with networks.
12. Are there other cable specifications?
Yes. There are several different specifications used to classify cable. One of the oldest is the AWG (American Wire Gauge) rating. This rating measures the thickness or gauge of the wire with the size being inverse to the rating. For example, a 22 AWG cable is thicker than a 24 AWG cable. 22 AWG wire is typically used in telephone wire and UTP.
IBM uses its own system of cable classification whereby cables are categorized as ‘types.’ For example, Type 3 wire is basically equivalent to the Category 3 wire discussed earlier. However, not all of the types used by IBM coincide exactly with a particular category. In the non-IBM world, UTP cable is typically referred to by its category classification and coax by its RG designation.
13. What connectors are used with UTP?
UTP uses a connection called the RJ-45 connector. It looks similar to a common telephone connector (RJ-11) except it is slightly larger. The RJ-45 has 8 pins while the RJ-11 has only 4.
UTP cable typically runs from a computer’s NIC and plugs directly into a wall plate much like a regular telephone. It is not uncommon to find wall plates in newer buildings labeled as ‘Voice’ (for telephones) or ‘Data’ (for UTP). The cable running from the wall jack is rarely attached directly into a hub. Instead the individual cables are collected and organized with patch panels located in a ‘wiring closet.’
14. Do I have to have a hub to use UTP cable?
No. If you have two computers and their NICs have RJ-45 connections, then you can simply connect them with UTP cable. However, you will need a special type of UTP cable called a ‘crossover cable.’ This cable is a normal UTP cable with the pin connections switched on one end so that the NICs can talk to each other. You can make a crossover cable by removing the RJ-45 connector at one end and switching (or ‘crossing’) the wire pairs. The downside to using a crossover cable is that it only allows you to connect two computers. If you need to connect more than two computers, you have to buy a hub or use coax cable.
15. What is fiber-optic cable and how does it work?
Fiber-optic cable uses optical rather than electrical pulses to transmit signals.
Fiber-optic cable consists of pure silicon glass cylinders or strands surrounded by cladding. Each strand can pass a signal in only one direction so fiber-optic cable on a network typically consists of at least two strands: one for sending and one for receiving. Electronic signals generated by the computer are converted to optical signals in the form of photons which are transmitted (flashed) down the cable by a laser or light-emitting diode. A photo-detector on the other end collects the optical signals and they are converted back to electrical signals.
Unlike copper cable, the signals on fiber-optic cable are not subject to the problems of attenuation, capacitance, or crosstalk. This greatly increases the potential transmission distance. In addition fiber-optic cable is more secure than copper cable. It does not generate electromagnetic signals and any external tap is easily detected by a reduction in signal strength. Fiber-optic cable is generally more expense than copper cable.
16. What are the key characteristics of fiber-optic cable?
· Transmission rate of 100 Mbps
· Cable length of 2 kilometers or more
· Not affected by electrical interference
· Supports voice, video, and data
· Provides the most secure media
· Most expensive cable
· Not very flexible; difficult to work with
· Commonly used in backbones between buildings and Token Ring networks
· Specifications for fiber include the IEEE’s 10BaseFL (Ethernet) and ANSI’s FDDI or Fiber Distributed Data Interface (Token Ring).
17. List the pros and cons of the basic cable types.
· PRO: Flexible and easy to install; relatively good resistance to electronic interference; electronic support components are relatively inexpensive
· CON: Short cable length; more expensive than UTP; unsecure; hard to change configuration; thinnet generally not good for use between buildings
· PRO: Most flexible; cheapest cable (but requires expensive support components); easy to install; easy to add users; may be able to use existing phone cable if data grade
· CON: Shortest usable cable length; susceptible to electrical interference; unsecure; generally not good for use between buildings
· PRO: Fastest transmission rate; not susceptible to electrical interference; secure; good for use between buildings
· CON: Most expensive; relatively difficult to work with
The following items are a potential problem for copper wire cabling:
Attenuation. Attenuation refers to the degradation of signal strength (amplitude) that occurs in transmissions over long distances. Shortening the transmission distance or using repeaters can help solve this problem.
Impedance. Impedance is resistance and it affects a signal made up of various frequencies. The resistance changes at different frequencies, resulting in distortion of the signal. Shortening the transmission distance or lowering the frequency can help solve this problem.
Capacitance. Capacitance is the measure of stored electrical charge in a cable. This charge can distort transmissions by changing the shape of the signal (as opposed to amplitude). Thick or bundled (closely adjoining) cables contribute to capacitance. Unbundled, thin cable over shorter distances will reduce capacitance.
Crosstalk. Background noise is a form of electrical interference that is generated by external sources. If the external source of interference is an adjacent cable, it is referred to as crosstalk. Crosstalk is more common in UTP than in coaxial cable. Ambient noise is a form of background noise generated by shop equipment, fluorescent lights, etc.
Unlike copper wire cable, fiber-optic cable is generally not susceptible to the problems described above.
19. What factors should I considered when choosing cable?
The following factors should be considered when choosing the type of cable for your network:
· Size - How many nodes (computers) and what are the total distances between them?
· Cost - What is the budget and how much can be spent on cabling?
· Reliability - How dependent are your organization’s operations on the network?
· Speed - How many concurrent users are there be and how critical is response time?
· Security - How important is it to protect data from possible interception?
· Growth - What are the organization’s plans for growth?
· Administration - How will the network be administered?
· Electrical Interference - What is the physical environment in which the network will operate?
· Existing Cable - Are there conduits or cabling already in place that might be useable (e.g., data grade phone lines for UTP)?
20. What are the alternatives to wire or fiber-optic cable?
There are several alternatives to wire or fiber-optic cable as the transmission media. However, these ‘wireless’ options have not met with widespread acceptance primarily because 1) the technologies have not been fully developed and 2) they are expensive.
Some examples of wireless transmission media include: