Physical Layer Issues Transmission Media and Network Cabling

What is Cable? Transmission Media Transmission medium is the physical path between the transmitter and receiver. n It is the Transmission medium through which information usually moves from one network device to another. n In some cases, a network will utilize only one type of cable, other networks will use a variety of cable types. n Understanding the characteristics of different types of transmission media and how they relate to other aspects of a network is necessary for the development of a successful network. n

Factors to Select Transmission Media n Data Rate and Bandwidth (BPS and Hz) n Distance and Attenuation (meters, d. B/km) n Interference Characteristics n Number of receivers (broadcast vs. point to point) n Cost - Remember cabling is a long term investment!

Transmission Impairments exist in all forms of data transmission media Analog signal impairments result in random modifications that impair signal quality l Digital signal impairments result in bit errors (1 s and 0 s transposed) l

Types of Media Two major classes n Conducted or guided media n l l n use a conductor such as a wire or a fiber optic cable to move the signal from sender to receiver. Energy is confined to the medium and guided by it Wireless or unguided media l l use radio waves of different frequencies and do not need a wire or cable conductor to transmit signals Energy spreads out and is not confined

Media Sub-types n Guided Media Unshielded Twisted Pair (UTP) Cable l Shielded Twisted Pair (STP) Cable l Coaxial Cable l Fiber Optic Cable l n Unguided Media Terrestrial microwave transmission l Satellite transmission l Broadcast radio l Infrared l

Twisted Pair Wires Consists of two insulated copper wires arranged in a regular spiral pattern to minimize the electromagnetic interference between adjacent pairs n Often used at customer facilities and also over distances to carry voice as well as data communications n Low frequency transmission medium n Two varieties n l STP (shielded twisted pair) ¬ the pair is wrapped with metallic foil or braid to insulate the pair from electromagnetic interference l UTP (unshielded twisted pair) ¬ each wire is insulated with plastic wrap, but the pair is encased in an outer covering

Twisted Pair One difference between the different categories of UTP is the tightness of the twisting of the copper pairs. The tighter the twisting, the higher the supported transmission rate and the greater the cost per foot. Each pair is twisted with a different number of twists per inch to help eliminate interference from adjacent pairs and other electrical devices.

Categories of Unshielded Twisted Pair The EIA/TIA (Electronic Industry Association/Telecommunication Industry Association) has established standards of UTP and rated five categories of wire. n n n n n Type Category 1 Category 2 Category 3 Category 4 Ring) Category 5 e Category 6 Category 7 Use Voice Only (Telephone Wire) Data to 4 Mbps (Local. Talk) Data to 10 Mbps (Ethernet) Data to 20 Mbps (16 Mbps Token Data to 100 Mbps (Fast Ethernet) Data to 1000 Mbps (Gigabit Ethernet) ?

Pros and Cons n Benefits of UTP Inexpensive and readily available l Flexible and light weight l Easy to work with and install l n Disadvantages of UTP Susceptibility to interference and noise l Attenuation problem l ¬ For analog, repeaters needed every 5 -6 km ¬ For digital, repeaters needed every 2 -3 km l Relatively low bandwidth (3000 Hz)

Twisted Pair - Applications n Telephone l Between house and local exchange (subscriber loop/local loop) n Within l buildings To private branch exchange (PBX) n For l network local area networks (LAN) 10 Mbps or 1000 Mbps

Unshielded Twisted Pair Connector n The standard connector for unshielded twisted pair cabling is an RJ-45 connector. This is a plastic connector that looks like a large telephone-style connector (See figure). n A slot allows the RJ-45 to be inserted only one way. RJ stands for Registered Jack, implying that the connector follows a standard borrowed from the telephone industry. This standard designates which wire goes with each pin inside the connector.

The RJ-45 Connector

Shielded Twisted Pair (STP) Cable n A disadvantage of UTP is that it may be susceptible to radio and electrical frequency interference (RFI, EFI). n Shielded twisted pair (STP) is suitable for environments with electrical interference; however, the extra shielding can make the cables quite bulky. n Shielded twisted pair is often used on networks using Token Ring topology. n More expensive, harder to work with.

Shielded Twisted Pair (STP) Cable

Coaxial Cable n Coaxial cabling has a single copper conductor at its center. A plastic layer provides insulation between the center conductor and a braided metal shield (See figure). The metal shield helps to block any outside interference from fluorescent lights, motors, and other computers. Both conductors share a common center axis, hence the term “coaxial”

Coax Layers outer jacket (polyethylene) shield (braided wire) insulating material copper or aluminum conductor

Pros and Cons n Coax Advantages l Higher bandwidth ¬ 400 to 600 MHz ¬ up to 10, 800 voice conversations l l l n Can be tapped easily (pros and cons) Much less susceptible to interference than twisted pair Greater cable lengths between network devices than twisted pair cable. Coax Disadvantages l l High attenuation rate makes it expensive over long distance Bulky - coaxial cabling is difficult to install

Coaxial Cable Applications n Most versatile medium n Television distribution Ariel to TV l Cable TV l n Long distance telephone transmission Can carry 10, 000 voice calls simultaneously l Being replaced by fiber optic l n Short distance computer systems links n Local area networks

Types of Coaxial Cable n Thin Coax l n Thin coaxial cable is also referred to as thinnet. Thin coaxial cable is popular in linear bus networks. Thick Coax l Thick coaxial cable is also referred to as thicknet. l 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. l One disadvantage of thick coaxial is that it does not bend easily and is difficult to install.

Coaxial Cable Connectors n The most common type of connector used with coaxial cables is the Bayone-Neill-Concelman (BNC) connector (See figure). 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.

Fiber Optic Cable n Relatively new transmission medium used by telephone companies in place of long-distance trunk lines n Also used by private companies in implementing local data communications networks n Require a light source with injection laser diode (ILD) or light-emitting diodes (LED)

Fiber Optic Cable n Fiber optic cabling consists of a center glass core surrounded by several layers of protective materials. n It transmits light rather than electronic signals, eliminating the problem of electrical interference. This makes it ideal for certain environments that contain a large amount of electrical interference. n It has also made it the standard for connecting networks between buildings, due to its immunity to the effects of moisture and lighting.

Fiber Optic Layers n consists of three concentric sections plastic jacket glass or plastic fiber core cladding

Optical Fiber

Armored Cable

Facts About Fiber Optic Cables n Facts about fiber optic cables: Outer insulating jacket is made of Teflon or PVC. l Kevlar fiber helps to strengthen the cable and prevent breakage. l A plastic coating is used to cushion the fiber center. l Center (core) is made of glass or plastic fibers. l

Fiber Optic Cable n Fiber optic cable has the ability to transmit signals over much longer distances than coaxial and twisted pair. n It also has the capability to carry information at vastly greater speeds. This capacity broadens communication possibilities to include services such as video conferencing and interactive services. n The cost of fiber optic cabling is comparable to copper cabling; however, it is more difficult to install and modify.

Optical Fiber - Transmission Characteristics n Act as wave guide for 1014 to 1015 Hz l n Light Emitting Diode (LED) l l l n Cheaper Wider operating temp range Last longer Injection Laser Diode (ILD) l l n Portions of infrared and visible spectrum More efficient Greater data rate Wavelength Division Multiplexing

Fiber Optic Types n Multimode l the reflective walls of the fiber move the light pulses to the receiver n Multimode l graded-index fiber acts to refract the light toward the center of the fiber by variations in the density n Single l step-index fiber mode fiber the light is guided down the center of an extremely narrow core

Optical Fiber Transmission Modes

Fiber Optic Signals fiber optic multimode step-index fiber optic multimode graded-index fiber optic single mode

Pros and Cons n Fiber Optic Advantages l l l n Greater capacity - data rates of hundreds of Gbps Smaller size and lighter weight Lower attenuation Electromagnetic isolation - immunity to environmental interference and highly secure due to tap difficulty and lack of signal radiation Greater repeater spacing - 10 s of km at least Fiber Optic Disadvantages l l l Expensive over short distance Requires highly skilled installers Adding additional nodes is difficult

Optical Fiber - Applications n Long-haul trunks n Metropolitan trunks n Rural exchange trunks n Subscriber loops (FTTH, FTTC) n LANs (generally backbone connections)

Fiber Optic Connector n The most common connector used with fiber optic cable is an ST connector. It is barrel shaped, similar to a BNC connector. A newer connector, the SC, is becoming more popular. It has a squared face and is easier to connect in a confined space.

Comparison of the Three Guided Media Types d. B/km 100 22 -gauge Twisted Pair 3/8” Coaxial Cable 10 1 Optical Fiber 0. 1 1 k. Hz 1 MHz 1 GHz 1 THz Frequency 1000 THz

Wireless (Unguided Media) n Transmission l Transmission and reception are achieved by means of an antenna n Directional transmitting antenna puts out focused beam l transmitter and receiver must be aligned l n Omnidirectional signal spreads out in all directions l can be received by many antennas l

Wireless (Unguided Media) Frequencies n n Three general ranges of frequencies 30 MHz to 1 GHz l l n 2 GHz to 40 GHz microwave frequencies l l n Broadcast radio Omnidirectional Microwave Highly directional Point to point Satellite 3 x 1011 to 2 x 1014 l Infrared

Propagation of Radio Frequencies

Propagation of Radio Frequencies (continued)

Terrestrial Microwave Transmission n n Uses the radio frequency spectrum, commonly from 2 to 40 GHz Transmitter is a parabolic dish, mounted as high as possible Used by common carriers as well as by private networks Requires unobstructed line of sight between source and receiver Curvature of the earth requires stations (called repeaters) to be ~30 miles apart

Terrestrial Microwave Transmission n Distance between antennas: n d = 7. 14 (Kh)1/2 , d = distance in km, h is antenna height in meters, K = constant = 4/3

Terrestrial Microwave Applications n Long-haul telecommunications service for both voice and television transmission n Short point-to-point links between buildings for closed-circuit TV or a data link between LANs

Terrestrial Microwave Communications

Pros and Cons n Microwave Transmission Advantages l l l n No cabling needed between sites Wide bandwidth Multi-channel transmissions Used for long haul or high capacity short haul Requires fewer amplifiers and repeaters Microwave Transmission Disadvantages l l Line of sight requirement Expensive towers and repeaters Subject to interference such as passing airplanes and rain Frequency bands are regulated

Satellite Microwave Transmission n Satellite is a microwave relay station in space Can relay signals over long distances Geostationary satellites l l remain above the equator at a height of 22, 300 miles (geosynchronous orbit) travel around the earth in exactly the time the earth takes to rotate Earth stations communicate by sending signals to the satellite on an uplink n The satellite then repeats those signals on a downlink n The broadcast nature of the downlink makes it attractive for services such as the distribution of television programming n

Satellite Transmission Process satellite transponder dish 22, 300 miles uplink station downlink station

Satellite Transmission Applications n Television distribution a network provides programming from a central location l direct broadcast satellite (DBS) l n Long-distance l telephone transmission high-usage international trunks n Private business networks

Principal Satellite Transmission Bands n. C l band: 4(downlink) - 6(uplink) GHz the first to be designated n Ku l rain interference is the major problem n Ka l band: 12(downlink) -14(uplink) GHz band: 19(downlink) - 29(uplink) GHz equipment needed to use the band is still very expensive

Pros and Cons n Satellite Advantages Can reach a large geographical area l High bandwidth l Cheaper over long distances l n Satellite Disadvantages High initial cost l Susceptible to noise and interference l Propagation delay (0. 25 sec) - requires sophisticated flow control l

Broadcast Radio n Broadcast radio is omnidirectional n Covers 30 MHz to 1 GHz (FM, UHF, VHF) n Need line of sight. Ionosphere is transparent above 30 MHz, hence no atmospheric reflection n Advantages l Less sensitive to attenuation from rainfall n Disadvantages l Multipath interference is significant

Infrared n Transceivers operate with line of sight or reflection from light-colored surface n Modulate noncoherent infrared light n e. g. TV remote control, IRD port n Advantages Does not penetrate walls - enhanced security l No licensing of frequencies l n Disadvantages l Operate on limited distances

Summary

The RS-232 Serial Interface and Ethernet 10/100 Base. T

DB-25 Connectors v. DB-25/DB-9 is an old standard family of connectors which are used for various interface standards such as RS-232 (Serial), Parallel and SCSI interfaces. v. The DB-25 male and female connectors are shown in the figure. Please note the way the pins are numbered.

What is a Null Modem Cable? n. A specially designed cable that allows you to connect two computers directly to each other via their communications ports (RS-232 ports). n Null modems are particularly useful with portable computers because they enable the portable computer to exchange data with a larger system.

What is a Null Modem Cable? The purpose of a null-modem cable is to permit two RS-232 "DTE" devices to communicate with each other without modems or other communication devices (i. e. , "DCE"s) between them. n To achieve this, the most obvious connection is that the TD signal of one device must be connected to the RD input of the other device (and vice versa). n Also, however, many DTE devices use other RS-232 pins for out-of-band (i. e. , "hardware") flow control. One of the most common schemes is for the DTE (the PC) to assert the RTS signal if it is ready to receive data (yes, it DOES sound backwards, but that's how it works), and for the DCE (the modem) to assert CTS when it is able to accept data. By connecting the RTS pin of one DTE to the CTS pin of the other DTE, we can simulate this handshake. n Also, it is common convention for many DTE devices to assert the DTR signal when they are powered on, and for many DCE devices to assert the DSR signal when they are powered on, and to assert the CD signal when they are connected. By connecting the DTR signal of one DTE to both the CD and DSR inputs of the other DTE (and vice versa), we are able to trick each DTE into thinking that it is connected to a DCE that is powered up and online. As a general rule, the Ring Indicate (RI) signal is not passed through a null-modem connection n

DB-25 and DB-9 male and female connectors, as viewed from the pin side (not the solder side).

DB-9 connectors on both ends

DB-25 connectors on both ends

DB-9 connector on one end, DB-25 on the other end

RS-232 Signals DB-25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 DCE TXD RTS CTS DSR GND CD DB-9 3 2 7 8 6 5 1 AA BA BB CA CB CC AB CF --- SCF SCB SBA DB SBB DD SCA DTR CG CD 9 CH/CI DA 4 CE x I O O x x x O O I O O O x I I O O I/O I x Protective Ground Transmitted Data Received Data Request To Send Clear To Send Data Set Ready Signal Ground Received Line Signal Detector Reserved for data set testing Unassigned Secndry Rcvd Line Signl Detctr Secondary Clear to Send Secondary Transmitted Data Transmisn Signl Elemnt Timng Secondary Received Data Receiver Signal Element Timing Unassigned Secondary Request to Send Data Terminal Ready Signal Quality Detector Ring Indicator Data Signal Rate Selector Transmit Signal Element Timing Unassigned

Common Null-Modem Connection 25 Pin 9 Pin FG (Frame Ground) 1 TD (Transmit Data) 2 3 RD (Receive Data) 3 2 RTS (Request To Send) 4 7 CTS (Clear To Send) 5 8 SG (Signal Ground) 7 5 DSR (Data Set Ready) 6 6 DTR (Data Terminal Ready) 20 4 X - 9 Pin 25 Pin 1 2 3 3 2 8 5 7 4 20 6 6 FG RD TD CTS RTS SG DTR DSR

RJ-45 Connector • The RJ-45 connector is commonly used for network cabling and for telephony applications. • It's also used for serial connections in special cases.

RJ-45 Connector for LAN Also, please note that it is very important that a single pair be used for pins 3 and 6. If one conductor from one pair is used for pin 3 and a conductor from another pair is used for pin 6, performance will degrade. See the following figure.

RJ-45 Connector in Non LAN Applications The following chart shows the pinout for RJ 45 connectors used on certain ISDN S/T interfaces. For more info, see ANSI T 1. 605. RJ-45 Pinout for Rocket. Port Pinouts for ISDN The following chart show Here's an ISDN BRI the pinout for RJ-45 U port pinout for a connectors used on Cisco 750 series certain Rocket. Port seria interface cards router: (manufactured by Comtr

Straight Through Cable vs. Crossover Cable In general, the patch cords that you use with your Ethernet connections are "straight-through", which means that pin 1 of the plug on one end is connected to pin 1 of the plug on the other end. In this particular case it is not then important to wire them as above. Pin 1 is Pin 1 etc. However for the sake of uniformity it may be best to wire your cables with the same colour sequence. Cross-Over cables are "crossed" end to end data cables aren't. If you have a network hub that has an uplink port on it then you do not need to make (or purchase a cross-over cable). Just switch the port on the hub to the 'uplink' mode. If your hub does not have an 'uplink' port on it then the only way to cascade another hub or attach a cable modem is to use a cross-over cable. It helps for future reference to mark or attach a tag to the cross-over cable so that you do not attempt to use it as a 'normal' patch lead at some time in the future.

Straight Through Cable vs. Crossover Cable

The Uses of RJ-45 Connector and Cat 5 UTP Cable