Local Area Networks , 3rd Edition David A. Stamper

1. Local Area Networks, 3rd EditionDavid A. Stamper Part 2: Hardware Chapter 3 Hardware Introduction and LAN Media

2. Chapter Preview In this chapter you will study: • What makes up a LAN system • Several of the Leading LAN media • Characteristics of LAN media • Error sources, detection, and correction

3. Three Major LAN Components • LAN software • Topology • The media access control (MAC) protocol

4. Things to Consider When Building a LAN • A variety of media—twisted-pair wires, coaxial cable, fiber optic cable, and several lesser used wireless media • Three basic topologies—ring, bus, and star • Two basic media access control protocols—contention and token passing • Hardware from many vendors • Network operating systems from several vendors • Network utilities • Application software

5. Two Major Classes of LAN Media • Conducted Media • uses a conductor like a wire or a fiber optic cable to move the signal from sender to receiver • includes twisted-pair wires, coaxial cables, and fiber optic cables • Wireless Media • uses radio waves of different frequencies or infrared light broadcast through space • does not need a wire or cable conductor to transmit signals

6. Conducted Media • Twisted-Pair Wires • Twisted-pair wires are classified in several ways • by American wire gauge (AWG) rating • by shielding, either unshielded twisted-pair (UTP) or shielded twisted-pair (STP) • by categories that define the wire’s rated acceptable speed and error characteristics • AWG Rating • The AWG rating is a measure of the thickness of the copper conductor in the cable. The higher the AWG rating, the smaller the diameter of the wire. • Twisted-pair wiring for LANs have an AWG rating of 22-26.

7. Conducted Media (cont.) • UTP and STP • Shielded twisted-pair (STP) • These wires have a metal foil or wire mesh wrapped around individual wire pairs with a metal braided shield around the twisted-pair wire bundle itself. • Twisting pairs of wires helps eliminate interference from neighboring wires; the metal shielding helps prevent ambient distortion like heavy-duty motors, electrical or magnetic fields, and fluorescent lights. • Unshielded twisted-pair (UTP) • These wires have no protective metal covering. UTP wires are more susceptible to environmental noise that can disrupt the signal. • UTP is used because it is cheaper than STP, and it may safely be used in environments where external disruptions are rare.

8. Twisted-Pair Wire Category Summary Cost (Relative to Category 1) Category Typical Use Maximum Data Rate 1 2 3 4 5 1 Mbps 4 Mbps 10 Mbps 16 Mbps 100, 155, and 1,000 Mbps Telephones Token Ring LANs Ethernet LANs Token ring LANs Ethernet, fast ethernet, and gigabit ethernet LANs, CDDI LANs and asynchronous transfer mode (ATM) 1 1.5 2 3 4

9. Coaxial Cable • Most early microcomputer-based LAN implementations used coaxial cable as the medium. • Coaxial cable comes packaged in a variety of ways, but essentially it consists of one or two central data transmission wires surrounded by an insulating layer, a shielding layer, and an outer jacket. • Coaxial cable is most commonly used in two types of LANs, ethernet and ARCNET.

10. A Single Conductor Coaxial Cable Outer Insulation Insulation Conductor Mesh Shielding

11. Fiber Optic Cable • Fiber optic cables come in two varieties, multimode and singlemode, each with a different way of guiding the light pulses from source to destination. • Fiber optic links for very short distances cost more than wires, but as distance or the required transmission rate increases, fiber optic cables become cost effective. • Fiber optic cables will not corrode, so they can be used in environments unsuited for copper media.

12. Views of a Fiber Optic Cable Glass Cladding Glass Conductor Plastic Covering

13. Wireless Media • Broadcast Radio • When broadcast radio is used with local area networks, cables connecting each microcomputer are eliminated. • Microwave Radio • For networks where installation of conducted media is difficult or too expensive, microwaves provide a high-speed medium alternative. • Spread Spectrum Radio • Its reliability in environments where signal interference is likely makes SSR well suited for LAN transmissions.

14. Wireless Media (cont.) • Infrared Transmission • Infrared transmission is a line-of-sight technology. It can be used to provide LAN connections between buildings and also is the medium used in some wireless local area networks.

15. The Frequencies of Various Wireless Media Frequency (Hz ) X rays, gamma rays Ultraviolet light Visible light Infrared light 1016 1015 1014 1013 1012 1011 1010 109 108 107 106 105 104 103 102 101 Millimeter waves Microwaves UHF television VHF television VHF TV (high band) FM radio VHF TV (low band) Short-wave radio AM radio Very low frequency

16. Cost Speed or Capacity Availability Expandability Error Rates LAN Media Selection Criteria • Security • Distance • Environment • Application • Maintenance

17. Media Selection Criteria • Cost • The costs associated with a given transmission medium include not only the costs of the medium but also ancillary fees, such as the costs for additional hardware like repeaters that might be required. • Speed • Response time • Aggregate data rate • Expandability • Some LAN media, for example, coaxial cable, are easier to expand than others, for example, fiber optic cables.

18. Media Selection Criteria (cont.) • Error Rates • The propensity for error influences not only the quality of the transmission but also its speed. • Security • Although most of the hacker incidents reported relate to wide area networks, similar concerns occur on LANs. • Distance • Before deploying a medium, LAN designers need to determine the distances that need to be covered and ensure that the wiring configuration or wireless configuration does not exceed the distance limitations of the technology being used.

19. Media Selection Criteria (cont.) • Environment • The constraints of environment can eliminate certain types of media. • Application • In some applications, the characteristics of the required equipment may dictate the type of medium and interfaces to be used.

20. Characteristics of Common LAN Media Medium Type Common Speeds (Mbps) Error Characteristics Unshielded twisted-pair Shielded twisted-pair Coaxial Cable Fiber optic cable Broadcast radio Spread spectrum radio Microwave radio Infrared light 1, 4, 10, 16, 100, 1000 1, 4, 10, 16, 100, 1000 10, 16, 50 10, 16, 50, 100, 1000, 2000 2 2, 10, 16 5.7 4, 10, 16 Less capable than other conducted media Better than unshielded; less capable than fiber optic or coaxial cables Good; less capable than fiber optic cable Excellent Subject to interference Good Subject to interference Objects can block transmission

21. Error Sources • White Noise • White noise, also referred to as thermal noise and Gaussian noise, result from the normal movements of electrons and is present in al transmission media at temperatures above absolute zero. • Impulse Noise • In LANs, it can be caused by lightning striking the medium, by jarring loose connections, or by transient electrical impulses such as those occurring on a shop floor. • Crosstalk • Crosstalk occurs when signals from one channel distort or interfere with the signals of a different channel.

22. Error Sources (cont.) • Echo • Echo is essentially the reflection or reversal of the signal being transmitted. • Attenuation • Attenuation is the weakening of a signal as a result of distance and characteristics of the medium.

23. Error Detection • Parity Check • A parity check (also known as vertical redundancy check [VRC]) involves adding a bit—known as the parity bit—to each character during transmission. • Longitudinal Redundancy Check (LRC) • With LRC, an additional, redundant character called the block check character (BCC) is appended to a block of transmitted characters, typically at the end of the block. • Cyclic Redundancy Check (CRC) • A CRC can detect bit errors better than either VRC or LRC or both. The transmitting station generates the CRC and transmits it with the data.

24. Error Detection (cont.) • Sequence Checks • Sequence check numbers can be assigned to each block of data so that the ultimate receiver can determine that all blocks have indeed arrived, and the blocks can be put back into proper sequence. • Error Correction Codes • Some error-detection schemes allow the receiving station not only to detect errors but also to correct some of them. Such codes are called forward error-correcting codes, the most common of which are called Hamming codes.

25. Error Correction • Message Acknowledgment • The mechanism used to effect retransmission is the positive or negative acknowledgment, often referred to as ACK and NAK, respectively. • Retry Limit • To cut down on continual retransmission of messages, a retry limit—typically between 3 and 100—can be set. A retry limit of five means that a message received in error will be retransmitted five times; if it is not successfully received by the fifth try, the receiving station either disables the link or disables the sending station itself.