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UNIT 2

Transmission Media - Guided

There are 2 basic categories of Transmission Media:

Guided andUnguided.

Guided Transmission Media uses a "cabling" system that guides the data signals along aspecific path. The data signals are bound by the "cabling" system. Guided Media is alsoknown as Bound Media. Cabling is meant in a generic sense in the previous sentencesand is not meant to be interpreted as copper wire cabling only.

Unguided Transmission Media consists of a means for the data signals to travel but

nothing to guide them along a specific path. The data signals are not bound to a cablingmedia and as such are often called Unbound Media.

There3 basic types of Guided Media:

Twisted PairCoaxial CableOptical Fibre

Twisted Pair

The wires in Twisted Pair cabling are twisted together in pairs. Each pair would consist ofa wire used for the +ve data signal and a wire used for the -ve data signal. Any noise thatappears on 1 wire of the pair would occur on the other wire. Because the wires areopposite polarities, they are 180 degrees out of phase (180 degrees - phasor definition ofopposite polarity). When the noise appears on both wires, it cancels or nulls itself out atthe receiving end. Twisted Pair cables are most effectively used in systems that use abalanced line method of transmission: polar line coding (Manchester Encoding) asopposed to unipolar line coding (TTL logic).

The degree of reduction in noise interference is determined specifically by the number ofturns per foot. Increasing the number of turns per foot reduces the noise interference. Tofurther improve noise rejection, a foil or wire braid shield is woven around the twistedpairs. This "shield" can be woven around individual pairs or around a multi-pairconductor (several pairs).

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Cables with a shield are called Shielded Twisted Pair and commonly abbreviated STP.Cables without a shield are called Unshielded Twisted Pair or UTP. Twisting the wirestogether results in a characteristic impedance for the cable. A typical impedance for UTPis 100 ohm for Ethernet 10BaseT cable.

UTP or Unshielded Twisted Pair cable is used on Ethernet 10BaseT and can also be usedwith Token Ring. It uses the RJ line of connectors (RJ45, RJ11, etc..)

STP or Shielded Twisted Pair is used with the traditional Token Ring cabling or ICS -IBM Cabling System. It requires a custom connector. IBM STP (Shielded Twisted Pair)has a characteristic impedance of 150 ohms.

Coaxial Cable

Coaxial Cable consists of 2 conductors. The inner conductor is held inside an insulatorwith the other conductor woven around it providing a shield. An insulating protectivecoating called a jacket covers the outer conductor.

The outer shield protects the inner conductor from outside electrical signals. The distancebetween the outer conductor (shield) and inner conductor plus the type of material usedfor insulating the inner conductor determine the cable properties or impedance. Typicalimpedances for coaxial cables are 75 ohms for Cable TV, 50 ohms for Ethernet Thinnetand Thicknet. The excellent control of the impedance characteristics of the cable allowhigher data rates to be transferred than Twisted Pair cable.

Optical Fibre

Optical Fibre consists of thin glass fibres that can carry information at frequencies in thevisible light spectrum and beyond. The typical optical fibre consists of a very narrow

strand of glass called the Core. Around the Core is a concentric layer of glass called theCladding. A typical Core diameter is 62.5 microns (1 micron = 10-6 meters). TypicallyCladding has a diameter of 125 microns. Coating the cladding is a protective coatingconsisting of plastic, it is called the Jacket.

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An important characteristic of Fibre Optics is Refraction. Refraction is the characteristicof a material to either pass or reflect light. When light passes through a medium, it"bends" as it passes from one medium to the other. An example of this is when we lookinto a pond of water.

(See image 1 below)

If the angle of incidence is small, the light rays are reflected and do not pass into thewater. If the angle of incident is great, light passes through the media but is bent orrefracted.

(See image 2 below)

Optical Fibres work on the principle that the core refracts the light and the claddingreflects the light. The core refracts the light and guides the light along its path. The

cladding reflects any light back into the core and stops light from escaping through it - itbounds the media!

Optical Transmission Modes

There are 3 primary types of transmission modes using optical fibre.

They are

a) Step Index

b) Grade Indexc) Single Mode

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Step Index has a large core the light rays tend to bounce around, reflecting off thecladding, inside the core. This causes some rays to take a longer or shorted path throughthe core. Some take the direct path with hardly any reflections while others bounce backand forth taking a longer path. The result is that the light rays arrive at the receiver atdifferent times. The signal becomes longer than the original signal. LED light sources areused. Typical Core: 62.5 microns.

Step Index Mode

Grade Index has a gradual change in the Core's Refractive Index. This causes the lightrays to be gradually bent back into the core path. This is represented by a curvedreflective path in the attached drawing. The result is a better receive signal than StepIndex. LED light sources are used. Typical Core: 62.5 microns.

Grade Index Mode

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Note: Both Step Index and Graded Index allow more than one light source to be used(different colours simultaneously!). Multiple channels of data can be run simultaneously!

Single Mode has separate distinct Refractive Indexes for the cladding and core. The lightray passes through the core with relatively few reflections off the cladding. Single Modeis used for a single source of light (one colour) operation. It requires a laser and the coreis very small: 9 microns.

Single Mode

Wireless Transmission Media - Unguided

Unguided Transmission Media is data signals that flow through the air. They are notguided or bound to a channel to follow. They are classified by the type of wavepropagation.

Radio waves

Radio waves have the longest wavelengths in the electromagnetic spectrum. These wavescan be longer than a football field or as short as a football. Radio waves do more than justbring music to your radio. They also carry signals for your television and cellular phones.

The antennae on your television set receive the signal, in the form of electromagneticwaves, that is broadcasted from the television station. It is displayed on your televisionscreen.

Cable companies have antennae or dishes which receive waves broadcasted from yourlocal TV stations. The signal is then sent through a cable to your house.

Micro waves

Microwaves have wavelengths that can be measured in centimeters! The longermicrowaves, those closer to a foot in length, are the waves which heat our food in a

microwave oven.

Microwaves are good for transmitting information from one place to another becausemicrowave energy can penetrate haze, light rain and snow, clouds, and smoke.

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Local Loop uses Twisted Pair, Analog signalingTrunks : Uses fiber-optics or Microwave , mostly digital

Toll office: each end office has number of outgoing lines to one or more nearby switchingcenters.End office or Tandem office: Nearest Telephone Office.Modems are used to convert digital to analog vice versa.

When a computer sends digital data over telephone line. The data must first be convertedto analog form by a modem for transmission over the Local loop, then converted todigital for transmission over long-haul trunks, then back to analog over the local loop atthe receiving end and finally back to digital by another modem.

For leased lines it is possible to go digital from start to finish, but expensive.

Long-haul trunks uses multiplexing.

Cellular Radio

Radio waves are used for communication .Notebook computers , mobiles pagers, cordless telephone are all belong to cellular radiotechnology.These devices are now merged and produced portable computers capable of sending andreceiving phone calls, faxes, email, as well as looking up information in remotedatabases.

Pager

A small device can be used to receive short messages.

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Communication satellites

Communications satellites allow radio, television, and telephone transmissions to be sentlive anywhere in the world. Before satellites, transmissions were difficult or impossible atlong distances. The signals, which travel in straight lines, could not bend around theround Earth to reach a destination far away. Because satellites are in orbit, the signals canbe sent instantaneously into space and then redirected to another satellite or directly totheir destination.

The satellite can have a passive role in communications like bouncing signals from theEarth back to another location on the Earth; on the other hand, some satellites carryelectronic devices called transponders for receiving, amplifying, and re-broadcastingsignals to the Earth.

Communications satellites are often in geostationary orbit. At the high orbital altitude of35,800 kilometers, a geostationary satellite orbits the Earth in the same amount of time ittakes the Earth to revolve once. From Earth, therefore, the satellite appears to bestationary, always above the same area of the Earth. The area to which it can transmit iscalled a satellite's footprint. For example, many Canadian communications satellites have

a footprint which covers most of Canada.

Communications satellties can also be in highly elliptical orbits. This type of orbit isroughly egg-shaped, with the Earth near the top of the egg. In a highly elliptical orbit, thesatellite's velocity changes depending on where it is in its orbital path. When the satelliteis in the part of its orbit that's close to the Earth, it moves faster because the Earth'sgravitational pull is stronger. This means that a communications satellite can be over theregion of the Earth that it is communicating with for the long part of its orbit. It will onlybe out of contact with that region when it quickly zips close by the Earth.

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Data Link Layer

The Data-Link layer is the protocol layer in a program that handles the moving of data in

and out across a physical link in a network. The Data-Link layer is layer 2 in the OpenSystems Interconnect (OSI) model for a set of telecommunication protocols.

The Data-Link layer contains two sublayers that are described in the IEEE-802 LANstandards:

Media Access Control (MAC)

Logical Link Control (LLC)

The Data-Link layer ensures that an initial connection has been set up, divides outputdata into data frames, and handles the acknowledgements from a receiver that the dataarrived successfully. It also ensures that incoming data has been received successfully byanalyzing bit patterns at special places in the frames.

The Media Access Control (MAC) data communication protocol sub-layer, also knownas the Medium Access Control, is a sublayer of the data link layer specified in the seven-layer OSI model (layer 2). It provides addressing and channel access control mechanismsthat make it possible for several terminals or network nodes to communicate within amultipoint network, typically a local area network (LAN) or metropolitan area network(MAN).

The MAC layer addressing mechanism is called physical address or MAC address. Thisis a unique serial number assigned to each network adapter, making it possible to deliverdata packets to a destination within a subnetwork, i.e. a physical network consisting ofone or several network segments interconnected by repeaters, hubs, bridges and switches,but not by routers. An example of a physical network is an Ethernet network.

The MAC sub-layer acts as an interface between the Logical Link Control sublayer andthe network's physical layer. The MAC layer emulates a full-duplex logicalcommunication channel in a multipoint network. This channel may provide unicast,multicast or broadcast communication service.

Error Control in networks

http://en.wikipedia.org/wiki/Subnetworkhttp://en.wikipedia.org/wiki/Unicasthttp://en.wikipedia.org/wiki/Subnetworkhttp://en.wikipedia.org/wiki/Unicast

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Common Data Link Protocols

The most common data link level protocols are listed here with a short description. Notethat most of these data link protocol are used for WAN and Modem Connections. LLC isa LAN data link protocol.

SDLC (Synchronous Data Link Protocol) This protocol was originallydeveloped by IBM as part of IBM's SNA (Systems Network Architecture). It was

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used to connect remote devices to mainframe computers at central locations ineither point-to-point (one-to-one) or point-to-multipoint (one-to-many)connections.

HDLC (High-level Data Link Control) This protocol is based on SDLC and

provides both a best-effort unreliable service and a reliable service. It is used withvarious serial interface protocols defined in the physical layer, such as EIA/TIA-232, V.24, V.35, and others.

SLIP (Serial Line Interface Protocol) SLIP is a data link control facility fortransmitting IP packets, usually between an ISP (Internet service provider) and ahome user over a dial-up link. SLIP has some limitations, including a lack of anyerror-detection and correction mechanisms. It is up to higher-layer protocols toperform these checks. Used over much of the same serial interfaces as HDLC.

PPP (Point-to-Point Protocol) PPP provides the same functionality as SLIP(i.e., it is commonly used for Internet connections over dial-up lines); but it is a

more robust protocol that can transport not only IP, but also other types ofpackets. Frames contain a field that identifies the type of protocol being carried(IP, IPX, and so on). It is used over much of the same serial interfaces as HDLC.

LAP (Link Access Procedure) LAP has reliability service features and comesin three varieties. LAPB (LAP Balanced) is a protocol that provides point-to-pointconnections on X.25 packet-switched networks. LAPD (LAP D-Channel)provides the data link control over the D channel of an ISDN (Integrated ServicesDigital Network) connection. LAPF (LAP Frame-Mode Bearer Services) providesthe data link for frame relay networks.

Frame Relay LAP used with X.25 is highly reliable, but it also has highoverhead. Frame relay does away with the reliability services (i.e., error-correction mechanisms are removed) to improve throughput.

LAN Data Link Controls

The IEEE (Institute of Electrical and Electronics Engineers) has defined a number ofLAN technologies in the data link layer, including Ethernet, Fast Ethernet, GigabitEthernet, and token ring. You can refer to the related entries page for more information.

The actual data link layer is split into two sublayers, called the MAC (Medium Access

Control) sublayer and the LLC (Logical Link Control) sublayer, The lower MAC layerdefines the media access method, which can be CSMA/CD (carrier sense multipleaccess/collision detection), token ring, or other IEEE physical interface. The LLCsublayer provides a way for the network layer to communicate with one of theseprotocols.

Standard Ethernet networks use CSMA/CD to physically monitor the traffic on the lineat participating stations. If no transmission is taking place at the time, the particularstation can transmit. If two stations attempt to transmit simultaneously, this causes a

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collision, which is detected by all participating stations. After a random time interval, thestations that collided attempt to transmit again. If another collision occurs, the timeintervals from which the random waiting time is selected are increased step by step. Thisis known as exponential back off.

CSMA/CD is a type of contention protocol. Networks using the CSMA/CD procedureare simple to implement but do not have deterministic transmission characteristics. TheCSMA/CD method is internationally standardized in IEEE 802.3 and ISO 8802.3.

A Token Ring network is a local area network (LAN) in which all computers areconnected in a ring or star topology and a bit- or token-passing scheme is used in order toprevent the collision of data between two computers that want to send messages at thesame time. The Token Ring protocol is the second most widely-used protocol on localarea networks after Ethernet. The IBM Token Ring protocol led to a standard version,specified as IEEE 802.5. Both protocols are used and are very similar. The IEEE 802.5Token Ring technology provides for data transfer rates of either 4 or 16 megabits persecond. Very briefly, here is how it works:

1. Empty information frames are continuously circulated on the ring.

2. When a computer has a message to send, it inserts a token in an empty frame (thismay consist of simply changing a 0 to a 1 in the token bit part of the frame) andinserts a message and a destination identifier in the frame.

3. The frame is then examined by each successive workstation. If the workstationsees that it is the destination for the message, it copies the message from the frameand changes the token back to 0.

4. When the frame gets back to the originator, it sees that the token has beenchanged to 0 and that the message has been copied and received. It removes themessage from the frame.

5. The frame continues to circulate as an "empty" frame, ready to be taken by aworkstation when it has a message to send.

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