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NETWORKING CONCEPTS
Network A network is basically an interconnection of various points to make a connected system e.g Road, Telephone, Social Data Network A computer network therefore refers to an interconnection of computers and other peripherals by use of a communication link Peer to Peer Networks In a peer-to-peer network, all computers are considered equal; they all have the same abilities to use the resources available on the network. Peer-to-peer networks are designed primarily for small to medium local area networks. Strength (Setup Cost is low, simple to manage) Weaknesses (Decentralized, Low security) Client/Server Networks Client/server network operating systems allow the network to centralize functions and applications in one or more dedicated servers. Advantages (Centralized, Scalability, Flexibility, Interoperability, Accessibility) Disadvantages (Cost, maintenance, Dependence one server) Server Types A sever in a network is dedicated to perform some specific task in support of other computers on the network. One server may perform all these tasks, or a separate server may be dedicated to each task. Common server types
1. File server 2. Print server 3. Application server 4. Message server 5. Database server File Server It offers services that allow network users to share files with network .File servers , users can exchange, read, write and manage shared files and the data contained in them. They are designed specifically to support the file services for a network. The file servers include: a) File transfer b) File storage c) File update synchronization d) File archiving File Transfer Network offers file transfer services by typically transferring files between clients and servers. This calls for file security. Every network operating systems has its own level of file security. Higher level use passwords to control system access, File attributes to limit file usage and encryption schemes to prevent data from being obtained by unauthorized individuals. File Update Synchronization It involves keeping all the files up to date. It ensures that changes made to a file are organized in chronological order in which they actually took place and that files are properly updated. File synchronization is usually an added option or an upgrade in most network operating systems File Archiving This is the process of backing up files on offline storage devices such as tapes or optical disks. Print Server A print server on a network will offer the following advantages: Allow user to share printers
Allow users to place the printer where convenient not just near individual computer Achieve better performance by using high -speed network data transfer and print queues. Allow user to share network fax services. Cuts cost by allowing shared access to printing devices. Network operating systems offers print queues, which are special storage, are where print jobs are stored and then sent to the printer in an organized fashion. The print jobs are stored in the queue and then forwarded to the printer when the printer has finished the jobs scheduled ahead of it. At times jobs may be printed in the order of priority according top other criteria Objectives of Networking Sharing Backup Communication Security Network Components Clients Server NIC Media Repeater Hubs Bridges Switches Routers BRouters Gateway Software Users Geographical Classification of networks LAN MAN WAN Characteristics of LAN, MAN, WAN LAN (Cheap, fast, radius about 3KM) MAN (Relatively Cheap, fairly fast, radius about 10KM) WAN (Costly, Slow, No limits, not secures) Classification of Network by Processing Modes Distributed networking In a distributed networking each machine is capable of operating independently. The network is used for the easier sharing of data. Centralized Networking Centralized networks are used in mainframe configurations where the network client devices are actually dump terminals where all function are done on the server Collaborative networking This is where in a busy network having several servers, the server can share their processing capabilities (if situation demands due to one/more servers being overwhelmed) Advantages of Networks Cost Security Collaboration Speed Resource sharing Challenges of setting networks Initial costs administration/Personnel Breakdowns
Supported by several hardware and software venders. Disadvantages of a Tree Topology Overall length of each segment is limited by the type of cabling used. If the backbone line breaks, the entire segment goes down. More difficult to configure and wire than other topologies. Considerations When Choosing a Topology: Money. A linear bus network may be the least expensive way to install a network; you do not have to 1purchase concentrators. Length of cable needed. The linear bus network uses shorter lengths of cable. Future growth. With a star topology, expanding a network is easily done by adding another concentrator. Cable type. The most common cable in schools is unshielded twisted pair, which is most often used with star topologies. In Summary Physical Topology Common Cable Common Protocol Linear Bus Twisted Pair Coaxial Fiber Ethernet LocalTalk Star Twisted Pair Fiber Ethernet LocalTalk Star-Wired Ring Twisted Pair Token Ring Tree Twisted Pair Coaxial Fiber Ethernet DATA TRANSMISSION AND ELECTRIC SIGNALS Voltage Voltage, sometimes referred to as electromotive force (EMF), is an electrical force, or pressure, that occurs when electrons and protons are separated. The force that is created pushes toward the opposite charge and away from the like charge. Voltage is represented by the letter "V", and sometimes by the letter "E", for electromotive force. The unit of measurement for voltage is volt (V), and is defined as the amount of work, per unit charge, needed to separate the charges. Current Electrical current, or current, is the flow of charges that is created when electrons move. In electrical circuits, current is caused by a flow of free electrons. When voltage (electrical pressure) is applied, and there is a path for the current, electrons move from the negative terminal (which repels them), along the path, to the positive terminal (which attracts them). Current is represented by the letter "I". The unit of measurement for current is Ampere (Amp), and is defined as the number of charges per second that pass by a point along a path. Resistance Materials through which current flows, offer varying amounts of opposition, or resistance, to the movement of the electrons. Materials that offer very little, or no, resistance, are called conductors. Those that do not allow the current to flow, or severely restrict its flow, are called insulators. The amount of resistance depends on the chemical composition of the materials. Resistance is represented by the letter "R". The unit of measurement for resistance is the ohm (Ω). The symbol comes from the Greek capital letter "Ω" - omega. Alternating Current (AC) This is one of the two ways in which current flows. Alternating current (AC) and voltages vary with time, by changing their polarity, or direction. AC flows in one direction, then reverses its direction, and repeats the process. AC voltage is positive at one terminal, and negative at the other, then it reverses its polarity, so that the positive terminal becomes negative, and the negative terminal becomes positive. This process repeats itself continuously.
Direct Current (DC) This is the other way in which current flows. Direct current (DC) always flows in the same direction, and DC voltages always have the same polarity. One terminal is always positive, and the other is always negative. They do not change or reverse. Impedance Impedance is the measure of the combined opposition to the flow of both AC and DC current flow. The term resistance is generally used when referring to DC voltages. Impedance is the general term, and is the measure of how the flow of electrons is resisted, or impeded. Impedance is represented by the letter "Z". Its unit of measurement, like that for resistance, is the ohm (Ω). Voltage, Current, Resistance Relationship The mathematical relationship known as Ohm's Law relates to the amount of current that will flow through a piece of conductor when a voltage is applied to it. If the resistance of the piece of conductor is R Ohms, then the current that will flow is I = V/R. A multimeter measures voltage, current, and resistance The word 'signal' refers to information that can be transmitted by varying, in some way, a measurable quantity, such as electrical voltage, light, or radio waves. All of these can carry networking data. Signal types One type of signal is analog. An analog signal has the following characteristics: Is wavy Has a continuously varying voltage versus time graph Is typical of things in nature Has been widely used in telecommunications for over 100 years Another type of signal is digital. A digital signal has the following characteristics: Has discrete, or jumpy, voltage versus time graphs Is typical of technology, rather than nature Note: Digital signals are cheaper and less susceptible to noise but have greater attenuation Data networks have become increasingly dependent on digital (binary or two-state) systems. The basic building block of information is one binary digit, known as the bit or pulse. One bit, on an electrical medium, is the electrical signal corresponding to binary 0 or binary 1. This may be as simple as 0 (zero) volts for binary 0 and + volts for binary 1, or a more complex encoding. Signal reference ground is an important concept relating to all networking media that use voltages to carry messages. With optical signals, binary 0 would be encoded as a low-light, or no-light intensity (darkness). Binary 1 would be encoded as higher-light intensity (brightness), or other more complex patterns. With wireless signals, binary 0 might be a short burst of waves; binary 1 might be a longer burst of waves, or another more complex pattern. Six things could happen to this single bit: propagation delay attenuation reflection noise timing problem collisions Propagation means travel. The propagation speed depends on the actual material used in the medium, the geometry (structure) of the medium, and the frequency of the pulses. The time it takes the bit to travel from one end of the medium and back again is referred to as the round trip time, (RTT). If the propagation time is too long, you should re-evaluate how the rest of the network will deal with this delay. If the propagation delay is too short, one may have to slow down the bits, or save them temporarily (known as buffering ), so that the rest of the networking equipment can catch up with the bit. Attenuation causes signals propagating through the medium (cable, optical fiber) to reduce in strength. This can be minimized by the wavelength, or color, of the light that is chosen.
If the resulting electrical noise reaches a high enough level, it can become difficult for NICs to discriminate the noise from the data signal. This is particularly a problem because most LANs use frequencies in the 1- megahertz (MHz) frequency region, which happens to be where FM Radio signals, TV signals, and lots of appliances have their operating frequencies as well. There are a number of ways to limit EMI and RFI. One way is to increase the size of the conductor wires. Another way is to improve the type of insulating material used. However, such changes increase the size and cost of the cable faster than they improve its quality. Therefore, it is more typical for network designers to specify a cable of good quality, and to provide specifications for the maximum recommended cable length between nodes. Two techniques that cable designers have used successfully in dealing with EMI and RFI are shielding and cancellation. In cable that employs shielding, a metal braid or foil surrounds each wire pair or group of wire pairs. This shielding acts as a barrier to any interfering signals. When two wires in an electrical circuit are placed close together, their magnetic fields are the exact opposite of each other. Thus, the two magnetic fields will cancel each other out. They also will cancel out any outside magnetic fields as well. Twisting the wires can enhance this cancellation effect. By using cancellation in combination with the twisting of wires, cable designers can provide an effective method of providing self- shielding for wire pairs within the network media. Dispersion, jitter, and latency are actually three different things that can happen to a bit. They are grouped together because each affects the timing of a bit. Since millions and billions of bits travel on a medium in one second, timing is extremely important. Dispersion is when the signal broadens in time. It is caused by the type of media involved. If serious enough, one bit can start to interfere with the next bit and confuse it with the bits before and after it. The signal must not be allowed to spread out, since billions of bits per second may be sent on the network. Dispersion can be fixed by proper cable design, limiting cable lengths, and finding the proper impedance. In optical fibers, using laser light of a very specific wavelength can control dispersion. For wireless communications, dispersion can be minimized by the frequencies used to transmit. All digital systems are clocked, meaning it is the clock pulses that cause everything to happen. Clock pulses cause the CPU to calculate, the data to be stored in memory, and the NIC to send bits. If the clock on the source host is not synchronized with the destination, which is quite likely, timing jitter will occur. This means that bits will arrive a little earlier and later than expected. Jitter can be fixed by a series of complicated clock synchronizations, including hardware and software, or protocol synchronizations. Latency , is signal delay and is caused by two factors: Nothing can travel faster than the speed of light in a vacuum (3.0 x 10^8 meters/second) If the bit goes through any devices, the transistors and electronics introduce more latency. The solution to the problem of latency is the careful use of internetworking devices, different encoding strategies, and various layer protocols. A collision occurs when two bits from two different communicating computers are on a shared medium at the same time There are many ways to deal with collisions. One way is to detect them and simply have a set of rules for dealing with them when they occur, as in Ethernet. Another way is to try to prevent collisions by only allowing one computer on a shared media environment to transmit at a time. This requires that a computer have a special bit pattern called a token to transmit, as in Token Ring and FDDI. After a bit reaches a medium, it propagates. It may experience attenuation, reflection, noise, dispersion, or collision. A network needs to transmit far more than one bit. In fact, you want to transmit billions of bits in one second. All of the effects described thus far that can occur to one bit also apply to the various protocol data units (PDUs) of the OSI model. Eight bits equal one byte. Multiple bytes equal one frame. Frames contain packets. Packets carry the messages you wish to communicate. Networking professionals often talk about attenuated, reflected, noisy, dispersed, and collided frames and packets. Whenever a message has to be sent over a long distance, there are two problems to solve. The first problem is how to express the message by encoding or modulation. The second problem is which method to use to transport the message carrier. Encoding means converting binary data into a form that can travel on a physical communications link. Modulation means using the binary data to manipulate a wave. Encoding means converting 1s and 0s (zeros) into something real and physical, such as: an electrical pulse on a wire
a light pulse on an optical fiber a pulse of electromagnetic waves into space Two methods of accomplishing this are TTL encoding and Manchester encoding. TTL (transistor-transistor logic) encoding is the simplest. It is characterized by a high signal and a low signal (often +5 or +3.3 V for binary 1 and 0 [zero] V for binary 0 [zero]). In optical fibers, binary 1 might be a bright LED or laser light, and binary 0 (zero), dark or no light. In wireless networks, binary 1 might mean a carrier wave is present, and binary 0 (zero), no carrier at all. Closely related to encoding is modulation, which specifically means taking a wave and changing, or modulating it so that it carries information. To give you an idea of what modulation is, examine three forms of modifying, or modulating, a carrier wave to encode bits: AM (amplitude modulation) - the amplitude, or height, of a carrier sine wave is varied to carry the message FM (frequency modulation) - the frequency of the carrier wave is varied to carry the message PM (phase modulation) - the phase, or beginning and ending points of a given cycle, of the wave is varied to carry the message MEDIA ACCESS METHODS A network’s access method is a method of controlling how devices on the network access the communications medium. There are three (3) main access methods common in LANs: Contention based media access describes a way of getting data on to the network whereby systems ‘contend for’ or share the media. On a contention based network, systems can only transmit when the media is free and clear of signals. This way, devices listen to the media, and if no other system is transmitting, they can go ahead and send data. In cases where more than one system finds the network free and attempts to transmit, a data collision will occur, and systems will need to retransmit. The best example of a contention based access method is Ethernet, which uses a scheme called Carrier Sense Multiple Access with Collision Detection (CSMA/CD). Token Passing A more orderly scheme for moving data between network systems is token passing. In token passing media access environments, a special frame referred to as a token repeatedly circles the network, passed from system to system. If a system has control of the token, it can transmit data. If it doesn’t, it must wait for the token to become available again. The token moves around the network at incredibly high speeds. This type of access method virtually eliminates collisions. Examples of technologies that use token passing media access include Token Ring and Fiber Distributed Data Interface (FDDI). Polling While contention and token passing methods are by far the most popular ways in which PCs access LAN media, some technologies rely on a technique called polling. Polling based media access is a deterministic way of allowing systems access to the network while also avoiding collisions. When used, a central device referred to as the master polls systems to see if they have data to transmit. Some WLANs use polling based media access which uses a scheme called Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). NETWORK ARCHITECTURES Network architecture describes topologies, types of cables, protocols, access methods and other variables consisting a network. In other words I is the overall structure and all components that makes a network functional including hard ware and system software. The most common architectures are: Ethernet Apple networks (localtalk and appletalk) Token Ring ARCnet d ETHERNET The Ethernet is the most popular physical network architecture in use today. Ethernet uses an access method called CSMA/CD (Carrier Sense Multiple Access/Collision Detection). This is a system where each computer listens to the cable before sending anything through the network. If the network is clear, the computer will transmit. If some other node is already transmitting on the cable, the computer will wait and try again when the
Gigabit Ethernet The most recent development in the Ethernet standard is a protocol that has a transmission speed of 1 Gbps. Gigabit Ethernet is primarily used for backbones on a network at this time. In the future, it will probably be used for workstation and server connections also. It can be used with both fiber optic cabling and copper. The 1000BaseTX, the copper cable used for Gigabit Ethernet, is expected to become the formal standard in 1999. LOCALTALK AND APPLETALK LocalTalk is a network physical network architecture that was developed by Apple Computer, Inc. for Macintosh computers. The method used by LocalTalk is called CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). It is similar to CSMA/CD except that a computer signals its intent to transmit before it actually does so. LocalTalk adapters and special twisted pair cable can be used to connect a series of computers through the serial port. The Macintosh operating system allows the establishment of a peer-to-peer network without the need for additional software. With the addition of the server version of AppleShare software, a client/server network can be established. The LocalTalk protocol allows for linear bus, star, or tree topologies using twisted pair cable. A primary disadvantage of LocalTalk is speed. Its speed of transmission is only 230 Kbps. TOKEN RING IBM developed the Token Ring physical network architecture in the mid-1980s. The access method used involves token passing. In Token Ring, the computers are connected so that the signal travels around the network from one computer to another in a logical ring. A single electronic token moves around the ring from one computer to the next. If a computer does not have information to transmit, it simply passes the token on to the next workstation. If a computer wishes to transmit and receives an empty token, it attaches data to the token. The token then proceeds around the ring until it comes to the computer for which the data is meant. At this point, the data is captured by the receiving computer. The Token Ring protocol requires a star-wired ring using twisted pair or fiber optic cable. It can operate at transmission speeds of 4 Mbps or 16 Mbps. Due to the increasing popularity of Ethernet, the use of Token Ring in school environments has decreased. DATA TRANSMISSION Encoding and Modulation Techniques In communications and information processing, encoding is the process by which information from a source is converted into symbols to be communicated. Decoding is the reverse process, converting these code symbols back into information understandable by a receiver. In telecommunications, modulation is the process of conveying a message signal, for example a digital bit stream or an analog audio signal, inside another signal that can be physically transmitted. Encoding and Modulation illustrated Digital Signaling Versus Analog Signaling Digital signaling Digital or analog data is encoded into a digital signal Encoding may be chosen to conserve bandwidth or to minimize error
Analog Signaling Digital or analog data modulates analog carrier signal The frequency of the carrier fc is chosen to be compatible with the transmission medium used Modulation: the amplitude, frequency or phase of the carrier signal is varied in accordance with the modulating data signal by using different carrier frequencies, multiple data signals (users) can share the same transmission medium Digital Signaling Digital data, digital signal Simplest encoding scheme: assign one voltage level to binary one and another voltage level to binary zero More complex encoding schemes: are used to improve performance (reduce transmission bandwidth and minimize errors). Examples are NRZ-L, NRZI, Manchester, etc. Analog data, Digital signal Analog data, such as voice and video Often digitized to be able to use digital transmission facility Example: Pulse Code Modulation (PCM), which involves sampling the analog data periodically and quantizing the samples Analog Signaling Digital data, Analog Signal A modem converts digital data to an analog signal so that it can be transmitted over an analog line The digital data modulates the amplitude, frequency, or phase of a carrier analog signal Examples: Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK), Phase Shift Keying (PSK) Analog data, Analog Signal Analog data, such as voice and video modulate the amplitude, frequency, or phase of a carrier signal to produce an analog signal in a different frequency band Examples: Amplitude Modulation (AM), Frequency Modulation (FM), Phase Modulation (PM) Digital Data, Digital Signal Digital signal discrete, discontinuous voltage pulses each pulse is a signal element binary data encoded into signal elements Interpreting Signals Need to know timing of bits: when they start and end signal levels: high or low factors affecting signal interpretation Data rate: increase data rate increases Bit Error Rate (BER) Signal to Noise Ratio (SNR): increase SNR decrease BER Bandwidth: increase bandwidth increase data rate encoding scheme: mapping from data bits to signal elements Comparison of Encoding Schemes Signal spectrum Lack of high frequencies reduces required bandwidth, lack of dc component allows ac coupling via transformer, providing isolation, should concentrate power in the middle of the bandwidth
(ASK-Amplitude Shift Keying , FSK-Frequency, PSK-Phase) AMPLITUDE SHIFT KEYING In ASK, the two binary values are represented by to different amplitudes of the carrier frequency The resulting modulated signal for one bit time is Susceptible to noise Inefficient modulation technique used for up to 1200bps on voice grade lines very high speeds over optical fiber Example BINARY FREQUENCY SHIFT KEYING The most common form of FSK is Binary FSK (BFSK) Two binary values represented by two different frequencies ( f 1 and f 2 ) less susceptible to noise than ASK used for up to 1200bps on voice grade lines high frequency radio (3 to 30MHz) even higher frequency on LANs using coaxial cable Example 00110100010 PHASE SHIFT KEYING (PSK) Phase of carrier signal is shifted to represent data Binary PSK (BPSK): two phases represent two binary digits Example Reading Assignment - Quadrature Amplitude Modulation (QAM)
Transmission modes A given transmission on a communications channel between two machines can occur in several different ways. The transmission is characterised by: the direction of the exchanges the transmission mode: the number of bits sent simultaneously synchronisation between the transmitter and receiver Simplex, half-duplex and full-duplex connections There are 3 different transmission modes characterised according to the direction of the exchanges: A simplex connection is a connection in which the data flows in only one direction, from the transmitter to the receiver. This type of connection is useful if the data do not need to flow in both directions (for example, from your computer to the printer or from the mouse to your computer...). A half-duplex connection (sometimes called an alternating connection or semi-duplex ) is a connection in which the data flows in one direction or the other, but not both at the same time. With this type of connection, each end of the connection transmits in turn. This type of connection makes it possible to have bidirectional communications using the full capacity of the line. A full-duplex connection is a connection in which the data flow in both directions simultaneously. Each end of the line can thus transmit and receive at the same time, which means that the bandwidth is divided in two for each direction of data transmission if the same transmission medium is used for both directions of transmission. Serial and parallel transmission The transmission mode refers to the number of elementary units of information (bits) that can be simultaneously translated by the communications channel. In fact, processors (and therefore computers in general) never process (in the case of recent processors) a single bit at a time; generally they are able to process several (most of the time it is 8: one byte), and for this reason the basic connections on a computer are parallel connections. Parallel connection Parallel connection means simultaneous transmission of N bits. These bits are sent simultaneously over N different channels (a channel being, for example, a wire , a cable or any other physical medium). The parallel connection on PC-type computers generally requires 10 wires.
In a synchronous connection , the transmitter and receiver are paced by the same clock. The receiver continuously receives (even when no bits are transmitted) the information at the same rate the transmitter send it. This is why the transmitter and receiver are paced at the same speed. In addition, supplementary information is inserted to guarantee that there are no errors during transmission. During synchronous transmission, the bits are sent successively with no separation between each character, so it is necessary to insert synchronisation elements; this is called character-level synchronisation. The main disadvantage of synchronous transmission is recognising the data at the receiver, as there may be differences between the transmitter and receiver clocks. That is why each data transmission must be sustained long enough for the receiver to distinguish it. As a result, the transmission speed can not be very high in a synchronous link.
Synchronous Vs Asynchronous Communication
Asynchronous Synchronous Data is transferred one char at a time (at random times). Each char is surrounded by a start and stop bit. Frequently used for terminal communications. Hardware logic is simple. Problem : If start bit is missed then the whole char is lost. Rarely used in high speed communications. Data is transmitted in blocks of characters. Header and trailer mark the beginning and the end of the block. E.g. File transfers from disk. More efficient for high speed communication. More difficult logic to implement. ERROR DETECTION AND CORRECTION Integrity of information is ensured in two steps:
- Detecting errors when they occur at the receiver during transmission
- Triggering retransmission or performing an error correction in the event that an error is detected Parity Checking
- One of the simplest error-detection schemes
- It refers to the use of parity bits to check that data has been transmitted accurately
- There two types of parity: odd and even
- Parity checking has limitations
- It cannot detect an error when an even number of bits change in the same data unit Parity Checking (with parity bits shown in boxes) Longitudinal Redundancy Check (LRC)
- Operates on a group of bytes
- Create a cross-grid matrix pattern to pinpoint a bad bit
- Produces a Block Check Character (BCC) to provide extra error-detection capabilities for a block of data
- Advantage
- It is simple and improves the odds of detecting errors
- Provides error correction at the receiver, which simply inverts the bad bit
- Disadvantage
- It has significant overhead Example of LRC Implementation
Hamming Code
- Hamming Code is a error-detection-and-correction scheme for single-bit errors
- Generates several parity bits that are interspersed with data in a specific pattern
- One data bit affects more than one parity bit, so the bad bit can be detected
- Its error correction capability eliminates the need for retransmission
- It uses forward-error-correction
- Error corrected by the receiving device Cyclic Redundancy Check (CRC)
- One of the most widely used, reliable, and efficient error-detection schemes
- Used in synchronous transmission where blocks of data can be several thousand bytes and where single bit errors occur less frequently as compared with multiple or burst errors
- CRC uses a unique mathematical algorithm, which is known to both the transmitter and the receiver
- The bit pattern (16 or 32 bits) that is used to verify the data is called a Frame Check Sequence (FCS) CRC Process Flowchart TRANSMISSION M EDIA Data transmission is the process of conveying data between two points by way of a communication medium. A wide variety of media are available, but they fall into two classes: bounded and unbounded. Bounded media confine the data to specific physical pathways. Common examples of bounded media are wire and optical fiber cables. Cable TV uses bounded media. Unbounded media transmit the data-carrying signal through space, independent of a cable. Broadcast radio and television are examples of unbounded media.
A wide variety of coax cable is available. You must use cable that exactly matches the requirements of a particular type of network. Coax cables vary in a measurement known as the impedance (measured in a unit called the ohm), which is an indication of the cable's resistance to current flow. The specifications of a given cabling standard indicate the required impedance of the cable. The two types of coaxial cabling are thick coaxial and thin coaxial. Thin coaxial cable is also referred to as thinnet. 10Base2 refers to the specifications for thin coaxial cable carrying Ethernet signals. The 2 refer to the approximate maximum segment length being 200 meters. In actual fact the maximum segment length is 185 meters. Thin coaxial cable is popular in school networks, especially linear bus networks. Thick coaxial cable is also referred to as thicknet. 10Base5 refers to the specifications for thick coaxial cable carrying Ethernet signals. The 5 refer to the maximum segment length being 500 meters. Thick coaxial cable has an extra protective plastic cover that helps keep moisture away from the center conductor. This makes thick coaxial a great choice when running longer lengths in a linear bus network. One disadvantage of thick coaxial is that it does not bend easily and is difficult to install. Here are some common examples of coaxial cables used in LANs, along with their impedances, and the LAN standards with which they are associated: RG-8 and RG-11 are 50 ohm cables required for thickwire Ethernet. (10Base5 - ThinkNet) RG-58 is a smaller 50 ohm cable required for use with thinwire Ethernet. (10Base2 - ThinNet) RG-59 is a 75 ohm cable most familiar when used to wire cable TV. RG-59 is also used to cable broadband 802.3 Ethernet. RG-62 is a 93 ohm cable used for ARCnet. It is also commonly employed to wire terminals in an IBM SNA network. Some advantages of coaxial cable are as follows: Highly insensitive to EMI Supports high bandwidths Heavier types of coax are sturdy and can withstand harsh environments Represents a mature technology that is well understood and consistently applied among vendors Coax also has some disadvantages including the following: Although fairly insensitive to EMI, coax remains vulnerable to EMI in harsh conditions such as factories. Coax can be bulky. Coax is among the most expensive types of wire cables. Coaxial Cable Connectors The most common type of connector used with coaxial cables is the Bayone-Neill-Concelman (BNC) connector (See fig below). Different types of adapters are available for BNC connectors, including a T-connector, barrel connector, and terminator. Connectors on the cable are the weakest points in any network. To help avoid problems with your network, always use the BNC connectors that crimp, rather than screw, onto the cable. BNC Connector TWISTED PAIR Two wires are twisted together to form the wire type known as twisted pair (TP). Cables can be constructed of multiple pairs of cables contained by a common jacket. The twists in the wire pairs are an important part of the electrical characteristics of TP cable. Twists reduce the cable's sensitivity to outside EMI and the degree to which the cables radiate radio frequency signals. Remember that the frequencies at which LANs operate fall into the range of radio signals. If TP cable is insufficiently
twisted, it can function as an antenna and radiate significant amounts of radio signals that can interfere with local broadcast reception equipment. Until recently, twisted pair cable used in networks was most frequently surrounded by a braided shield that served to reduce both EMI sensitivity and radio emissions. STP cable, however, is expensive and bulky, and manufacturers of network equipment have devoted extensive research to enabling high-speed networks to work with unshielded twisted pair (UTP). UTP is the cost leader among network cables. The 10Base-T & 100Base-TX standards defines an Ethernet configuration that utilizes UTP. Recent work by IBM and other vendors also has developed network equipment that can use UTP even for high speed 16 megabit per second Token Ring. In most cases, UTP cable is implemented using modular telephone-type connectors such as the RJ-11 (2 pair) and RJ- (4 pair) connectors. Telephone modular connectors are inexpensive and easy to install, serving to further reduce the cost of UTP cabling systems. NOTE: UTP looks much like the wire used to wire voice telephones. In newer telephone installations, it may indeed be possible to use wiring installed for the voice telephone system as cable in a network. UTP cable comes in a variety of grades, ranging from level 1 (lowest quality) to level 5 (highest quality). When investigating the use of UTP cabling, be sure to determine the wire quality required for your network. When utilizing UTP cable, it is necessary to ensure that all components in the data network are data grade. Voice grade components used in voice telephone systems are not of sufficiently high quality. Shielded twisted pair cable (STP) is the standard cable specified for IBM Token Ring networks and for Apple's LocalTalk. Unshielded twisted pair cables (UTP) can be utilized for some configurations of Token Ring, Ethernet, and ARCnet networks. Here are some advantages of twisted pair wiring: Telephone cable standards are mature and well established. Materials are plentiful, and a wide variety of cable installers are familiar with the installation requirements. It may be possible to use in-place telephone wiring if it is of sufficiently high quality. UTP represents the lowest cost cabling. The cost for STP is higher and is comparable to the cost of coaxial cable. Some disadvantages of twisted pair are as follows: STP can be expensive and difficult to work with. Compared to fiber optic cable, all TP cable is more sensitive to EMI. UTP especially may be unsuitable for use in high-EMI environments. TP cables are regraded as being less suitable for high-speed transmissions than coax or fiber optic. Technology advances, however, are pushing upward the data rates possible with TP. Cable segment lengths are also more limited with TP. UTP figure below
Categories of Unshielded Twisted Pair
Type Use Category 1 Voice Only (Telephone Wire) Category 2 Data to 4 Mbps (Local Talk) 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)
