Physical Layer Issues - Transmission Media and

Network Cabling

Hitesh lad

What is Cable? Transmission Media

Transmission medium is the physical path between the transmitter and receiver.

It is the Transmission medium through which information usually moves from one network device to another.

In some cases, a network will utilize only one type of cable, other networks will use a variety of cable types.

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.

Factors to Select Transmission Media

Data Rate and Bandwidth (BPS and Hz) Distance and Attenuation (meters, dB/km) Interference Characteristics Number of receivers (broadcast vs. point to

point) Cost - Remember cabling is a long term

investment!

Transmission Impairments Impairments exist in all forms of data

transmission mediaAnalog signal impairments result in random

modifications that impair signal qualityDigital signal impairments result in bit errors (1s

and 0s transposed)

Types of Media Two major classes Conducted or guided media

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

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 Guided Media

Unshielded Twisted Pair (UTP) Cable Shielded Twisted Pair (STP) Cable Coaxial Cable Fiber Optic Cable

Unguided MediaTerrestrial microwave transmissionSatellite transmissionBroadcast radioInfrared

Twisted Pair Wires Consists of two insulated copper wires arranged in a

regular spiral pattern to minimize the electromagnetic interference between adjacent pairs

Often used at customer facilities and also over distances to carry voice as well as data communications

Low frequency transmission medium Two varieties

STP (shielded twisted pair) the pair is wrapped with metallic foil or braid to insulate the pair

from electromagnetic interference 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

Type Use Category 1 Voice Only (Telephone Wire) Category 2 Data to 4 Mbps (LocalTalk) 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) Category 5e Data to 1000 Mbps (Gigabit Ethernet) Category 6 Data to 1000 Mbps (Gigabit Ethernet) Category 7 ?

The EIA/TIA (Electronic Industry Association/Telecommunication Industry Association) has established standards of UTP and rated five categories of wire.

Benefits of UTPInexpensive and readily availableFlexible and light weight Easy to work with and install

Disadvantages of UTPSusceptibility to interference and noiseAttenuation problem

For analog, repeaters needed every 5-6km For digital, repeaters needed every 2-3km

Relatively low bandwidth (3000Hz)

Twisted Pair - Applications Telephone network

Between house and local exchange (subscriber loop/local loop)

Within buildingsTo private branch exchange (PBX)

For local area networks (LAN)10Mbps or 100Mbps or 1000 Mbps

Unshielded Twisted Pair Connector

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).

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

A disadvantage of UTP is that it may be susceptible to radio and electrical frequency interference (RFI, EFI).

Shielded twisted pair (STP) is suitable for environments with electrical interference; however, the extra shielding can make the cables quite bulky.

Shielded twisted pair is often used on networks using Token Ring topology.

More expensive, harder to work with.

Shielded Twisted Pair (STP) Cable

Coaxial Cable 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 “co-axial”

Coax Layers

copper or aluminum conductor

insulating material

shield(braided wire)

outer jacket(polyethylene)

Pros and Cons Coax Advantages

Higher bandwidth 400 to 600MHz up to 10,800 voice conversations

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 High attenuation rate makes it expensive over long distance Bulky - coaxial cabling is difficult to install

Coaxial Cable Applications Most versatile medium Television distribution

Ariel to TVCable TV

Long distance telephone transmissionCan carry 10,000 voice calls simultaneouslyBeing replaced by fiber optic

Short distance computer systems links Local area networks

Types of Coaxial Cable Thin Coax

Thin coaxial cable is also referred to as thinnet. Thin coaxial cable is popular in linear bus networks.

Thick Coax Thick coaxial cable is also referred to as thicknet.

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.

Coaxial Cable Connectors 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 Relatively new transmission medium used by

telephone companies in place of long-distance trunk lines

Also used by private companies in implementing local data communications networks

Require a light source with injection laser diode (ILD) or light-emitting diodes (LED)

Fiber Optic Cable Fiber optic cabling consists of a center glass core

surrounded by several layers of protective materials. 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.

It has also made it the standard for connecting networks between buildings, due to its immunity to the effects of moisture and lighting.

plastic jacket glass or plasticcladding

fiber core

Fiber Optic Layers consists of three concentric sections

Optical Fiber

Armored Cable

Facts About Fiber Optic Cables

Facts about fiber optic cables: Outer insulating jacket is made of Teflon or PVC. Kevlar fiber helps to strengthen the cable and

prevent breakage. A plastic coating is used to cushion the fiber center. Center (core) is made of glass or plastic fibers.

Fiber Optic Cable Fiber optic cable has the ability to transmit signals

over much longer distances than coaxial and twisted pair.

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.

The cost of fiber optic cabling is comparable to copper cabling; however, it is more difficult to install and modify.

Optical Fiber - Transmission Characteristics

Act as wave guide for 1014 to 1015 Hz Portions of infrared and visible spectrum

Light Emitting Diode (LED) Cheaper Wider operating temp range Last longer

Injection Laser Diode (ILD) More efficient Greater data rate

Wavelength Division Multiplexing

Fiber Optic Types Multimode step-index fiber

the reflective walls of the fiber move the light pulses to the receiver

Multimode graded-index fiberacts to refract the light toward the center of the fiber

by variations in the density

Single mode fiberthe light is guided down the center of an extremely

narrow core

Optical Fiber Transmission Modes

fiber optic multimodestep-index

fiber optic multimodegraded-index

fiber optic single mode

Fiber Optic Signals

Pros and Cons Fiber Optic Advantages

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 - 10s of km at least

Fiber Optic Disadvantages Expensive over short distance Requires highly skilled installers Adding additional nodes is difficult

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

Fiber Optic Connector 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

1000 THz1 THz1 GHz1 MHz

Optical Fiber

1 kHz0.1

3/8” Coaxial Cable

22-gauge Twisted Pair

Frequency

dB

/km

1

10

100

Wireless (Unguided Media) Transmission

Transmission and reception are achieved by means of an antenna

Directionaltransmitting antenna puts out focused beamtransmitter and receiver must be aligned

Omnidirectionalsignal spreads out in all directionscan be received by many antennas

Wireless (Unguided Media) Frequencies

Three general ranges of frequencies 30MHz to 1GHz

Broadcast radio Omnidirectional

2GHz to 40GHz microwave frequencies Microwave Highly directional Point to point Satellite

3 x 1011 to 2 x 1014

Infrared

Propagation of Radio Frequencies

Propagation of Radio Frequencies (continued)

Terrestrial Microwave Transmission

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

Distance between antennas: d = 7.14 (Kh)1/2 , d = distance in km, h is

antenna height in meters, K = constant = 4/3

Terrestrial Microwave Applications

Long-haul telecommunications service for both voice and television transmission

Short point-to-point links between buildings for closed-circuit TV or a data link between LANs

Terrestrial Microwave Communications

Pros and Cons Microwave Transmission Advantages

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 Line of sight requirement Expensive towers and repeaters Subject to interference such as passing airplanes and rain Frequency bands are regulated

Satellite Microwave Transmission

Satellite is a microwave relay station in space Can relay signals over long distances Geostationary satellites

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

The satellite then repeats those signals on a downlink The broadcast nature of the downlink makes it

attractive for services such as the distribution of television programming

dish dish

uplink station downlink station

satellitetransponder

22,300 miles

Satellite Transmission Process

Satellite Transmission Applications

Television distributiona network provides programming from a central

locationdirect broadcast satellite (DBS)

Long-distance telephone transmissionhigh-usage international trunks

Private business networks

Principal Satellite Transmission Bands

C band: 4(downlink) - 6(uplink) GHzthe first to be designated

Ku band: 12(downlink) -14(uplink) GHzrain interference is the major problem

Ka band: 19(downlink) - 29(uplink) GHzequipment needed to use the band is still very

expensive

Pros and Cons Satellite Advantages

Can reach a large geographical areaHigh bandwidthCheaper over long distances

Satellite DisadvantagesHigh initial costSusceptible to noise and interferencePropagation delay (0.25 sec) - requires

sophisticated flow control

Broadcast Radio Broadcast radio is omnidirectional Covers 30MHz to 1 GHz (FM, UHF, VHF) Need line of sight. Ionosphere is transparent

above 30MHz, hence no atmospheric reflection Advantages

Less sensitive to attenuation from rainfall

DisadvantagesMultipath interference is significant

Infrared Transceivers operate with line of sight or

reflection from light-colored surface Modulate noncoherent infrared light e.g. TV remote control, IRD port Advantages

Does not penetrate walls - enhanced securityNo licensing of frequencies

DisadvantagesOperate on limited distances

Summary

The RS-232 Serial Interface and Ethernet

10/100BaseT

DB-25 ConnectorsDB-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.

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?

A specially designed cable that allows you to connect two computers directly to each other via their communications ports (RS-232 ports).

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.

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).

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.

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

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

DB-25 DCE DB-91 AA x Protective Ground2 TXD 3 BA I Transmitted Data3 RXD 2 BB O Received Data4 RTS 7 CA I Request To Send5 CTS 8 CB O Clear To Send6 DSR 6 CC O Data Set Ready7 GND 5 AB x Signal Ground8 CD 1 CF O Received Line Signal Detector9 -- x Reserved for data set testing

10 -- x Reserved for data set testing11 x Unassigned12 SCF O Secndry Rcvd Line Signl Detctr13 SCB O Secondary Clear to Send14 SBA I Secondary Transmitted Data15 DB O Transmisn Signl Elemnt Timng16 SBB O Secondary Received Data17 DD O Receiver Signal Element Timing18 x Unassigned19 SCA I Secondary Request to Send 20 DTR 4 CD I Data Terminal Ready21 CG O Signal Quality Detector22 9 CE O Ring Indicator23 CH/CI I/O Data Signal Rate Selector24 DA I Transmit Signal Element Timing25 x Unassigned

RS-232 Signals

25 Pin 9 Pin 9 Pin 25 Pin

FG (Frame Ground) 1 - X - 1 FG

TD (Transmit Data) 2 3 - 2 3 RD

RD (Receive Data) 3 2 - 3 2 TD

RTS (Request To Send) 4 7 - 8 5 CTS

CTS (Clear To Send) 5 8 - 7 4 RTS

SG (Signal Ground) 7 5 - 5 7 SG

DSR (Data Set Ready) 6 6 - 4 20 DTRDTR (Data Terminal

Ready)20 4 - 6 6 DSR

Common Null-Modem Connection

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 LANAlso, 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 T1.605.

Pinouts for ISDNHere's an ISDN BRI U port pinout for a Cisco 750 series

router:

RJ-45 Pinout for RocketPort

The following chart shows the pinout for RJ-45 connectors used on certain RocketPort serial interface cards (manufactured by Comtrol).

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 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

Straight Through Cable vs. Crossover Cable

The Uses of RJ-45 Connector and Cat5 UTP Cable