Chapter 4: Physical Layer—Transmission media

Physical medium carries data in either electrical form or optical form. We can classify the media into two categories:

a) Guided media—Electrical signals are guided along the solid wires made up of twisted cables, coaxial cables and optical signals are guided through optical fibers.

b) Unguided media (=wireless)—Space (air) carries the signals, either electrical or optical.

4.1 Guided Media

There are three common media used for communications: Twisted pairs, coaxial cables, and fiber optic cables.

Twisted Pair Cables

There are two major types of twisted pair cables; Unshielded Twisted Pair (UTP) and Shielded Twisted Pair (STP). In STP, the inner wires (twisted pairs) are encased in a sheath of foil or braided wire mesh.

UTP Cables

· UTP commonly has RJ-45, RJ-11, RS-232, and RS-449 connectors.

Patch cable with RJ45 connectors (used in Ethernet cabling)

· Maximum length is 100 meters for all the cables listed below.

· Maximum speed is up to 100Mps

· Cheap, easy to install, length can become a limiting factor.

· There are several different qualities of cables. According to EIA/TIA 568(Commercial Building Telecommunications Cabling Standard), UTP cables are classified into Category 1 to Category 7

· CAT 1 & CAT 2 cables are called “sub-voice grade” which were used for telephone wiring prior to 1980s

· CAT 3 cables are made up of 4 twisted pairs, each twisted 3 times per foot. They are called “voice grade” and used in telephone wiring. CAT 3 is certified to transmit data up to 10Mbps. Used in Ethernet cabling (10BaseT cabling).

· CAT 4 cables are made up of 4 twisted pairs, certified to transmit data up to 16 Mbps. Used in Token Ring cabling.

· CAT 5 cables are made up of 4 twisted pairs, certified to transmit data up to 100 Mbps. Used in 100Mbps Ethernets. CAT 5 cables have much more twists (about 30 twists per foot) than CAT 3 cables(3 twists per foot).

· CAT 5e (“e” stands for enhanced) This is the newest version of CAT5 cables. CAT 5e is completely backward compatible with current Category 5 equipment. CAT 5e has a higher bandwidth and it can be used for 10/100Base-TX and 1000Base-TX (Gigabit Ethernet). For applications such as Gigabit Ethernet or analog video, CAT 5e should be chosen over CAT 5 cable. All aspects of performance are enhanced: capacitance, frequency, resistance, attenuation, impedance, and NEXT (Near End Cross Talk).

· CAT 6 has the bandwidth of 250 MHz and can run up to Gigabit Ethernets.

· CAT 7 has the bandwidth of 600MHz.

ANSI / TIA / EIA 568 Cabling Standards and LAN Applications
	CAT 3	CAT 5	CAT 5e	CAT 6	CAT 7
Bandwidth	16MHz	100MHz	100MHz	250MHz	600MHz
Type	UTP	UTP/FTP	UTP/FTP	UTP/FTP	SSTP
Data Rate & LAN Applications	16Mbps 10BaseT	100Mbps 100BaseT, ATM, CDDI	1000Mbps 1000BaseT (Gigabit Ethernet)	Not specified	Not specified
Length	100 meters	100 meters	100 meters	100 meters	100 meters
Link Cost (Cat 5 =1)	0.7	1	1.2	1.5	2.2

FTP: Foil Twisted Pair

SSTP: Shielded Screen Twisted Pair

STP Cables

STP cables are more expensive than UTP cables but they have better noise characteristics and higher data rate. They are used in special applications such as noisy environment and longer distance.

· Uses RJ-45, RJ-11, RS-232, and RS-449 connectors

· Max length is 100 meters

· Speed is up to 500Mps.

· Not as cheap as UTP, easy to install, length can become a limiting factor.

· Can be CAT 2,3,4 or 5 quality grades with shields

Coaxial Cables

Coaxial cables are made up of a center wire surrounded by insulation material and then covered by a grounded shield of braided wire. The shield minimizes electrical and radio frequency interference. Coaxial cables are commonly used for thick Ethernet, thin Ethernet, cable TV. Coaxial cables use BNC connectors. Heavy shielding protects data, but expensive to manufacture and the BNC connectors are costlier too.

Coaxial cable with shield exposed and BNC connectors

· Coaxial cables have a higher bandwidth than twisted pair cables. Two physical aspects provide the higher bandwidth.

· First is the “skin effect”. When electrical signals flow through a solid copper, the signals tend to concentrate on the outer surface of the wire. This phenomenon is more and more prominent as the frequency increases (higher data rate). Since only the outermost areas of the copper cable carry the signal, we do not need the inner part. Therefore, we can cut out the middle part. This wire with the middle part cut out makes the outer shield of a coaxial cable. The skin effect is utilized in coaxial cables to achieve higher “effective bandwidth” without having a thick cable.

· Second is “shielding effect”. We need another wire(other than the shield described above) to carry electrical signal. So, a little wire is put inside the shield. When we do this, the electromagnetic interference is greatly reduced by the shield.

· Coaxial cables have the bandwidth in the range from few hundred to 1000MHz.

· Coaxial cables are used in Thick or Thin Ethernets and Cable TV(a portion of the bandwidth is used for cable modem).

· Has better noise immunity than twisted pairs.

· More expensive than twisted pairs

Optical Fiber

Optical fiber is a cable made by a thin(2 to 125 micrometer) glass fiber and is capable of data in the form of light.

Light Emitting Diode

* How optical fibers work?

An optical fiber is made up of several layers of different materials. Two most important layers are the core and the cladding. The core is the carrier of light. The cladding is a layer which completely surrounds the core. The core and the cladding have different refractive index. The refractive index of the core is slightly higher (about 0.5% higher) than the cladding. The sole purpose of the cladding is to reflect the light back into the core so that once the light enters the core it stays inside the core. Fiber cables are identified by the dimensions of the core over cladding. For example, a cable rated as 62.5/125 microns has the core diameter of 62.5 microns and the cladding diameter of 125 microns.

Fiber optic transmission medium works as follows:

1. An electrical signal (usually digital signal of 0’s and 1’s) is fed into a device that changes the electrical signal to light. There are two kinds of devices that can perform the conversion. One is LED(Light Emitting Diode) made up of Silicon chips. The other is Laser device.

2. The light signal, essentially On (for binary 1) or Off (for binary 0), will travel through the fiber.

3. At the receiving end, a Photo Detector often called as Photodiode detects the existence or non-existence of light and converts the light into electrical pulse (0 or 1).

Light Emitting Diode

Photo Diode Detector

* Light sources

· LED(Light Emitting Diode)

§ Lower cost than Laser

§ Lower data rate than Laser

§ Longer life than Laser driven fibers

§ Less sensitive to temperature changes

§ Lower light power than Laser

· Laser

§ Higher data rate than LED based systems

§ Higher light power output with Laser

§ Higher cost for Laser system

§ Shorter life for cables than LED

          A Laser Diode

The data rate of an optical fiber system is determined by how fast we can detect the On/Off switching of the incoming light signal, in other words, how fast a Photodiode can detect the On/Off pulses of light. This speed obviously depends upon the quality of the signal (clarity of the light) which in turn depends upon the capability/quality of the fiber. A fiber is not a perfect medium. It has its own limitations. The main factors for the limitations are the “attenuation” and “dispersion”.

· Attenuation—Light energy gets reduced as it travels through a fiber since some energy is absorbed by the fiber (absorption) and some light escapes the fiber. So, the light attenuates. Attenuations are measured by decibels (dB). The decibel is a logarithmic unit that indicates the ratio of output power to input power. Each optical fiber has a characteristic attenuation that is normally measured in decibels per kilometer (dB/km). For example, a 10-Gigabit Multimode, 50-Micron Fiber Optic Bulk Cable is listed to have the attenuation of 3.0 dB/km. Fiber optic cables are known to have much lower attenuation compared to other media such as twisted pairs and coaxial cables.

          Absorption

· Dispersion—When light is sent along the fiber in a form of a pulse, the pulse spread out. This is known as “dispersion” and it occurs because the pulse travels in the form of numerous light beams and each light beam travels in a different path than others. Depending upon the length of each path, each beam travels at different speed. A beam which happens to travel on a shorter path arrives faster than the ones with a longer path.

In the figure, Light Signal 1 takes different path from Light Signal 2. The result is that pulses begin to spread into one another and the data bits become indistinguishable.

Figure 1a—LP01 Mode Distribution

Figure 1b—LP11 Mode Distribution Figure 1—Dispersion

Effect of Dispersion

* Bandwidth Limitation—Bandwidth of an optical fiber determines the data rate. Attenuation and dispersion together limit the data rate.

* Three modes of fiber

* Multimode fiber vs Single mode fiber

There are two types of fiber in use. Both have 125 microns (one micron is one-millionth of a meter) in outside diameter(core plus cladding). The 125 micron (0.005 inches) is a bit larger than a typical human hair.

· Multimode fiber has two modes; Step-index and Graded-index. Light travels in numerous rays in multimode fiber, called modes. The size of the core is almost always 62.5 microns (in some cases 50 microns). Multimode fibers have greater dispersion than the single mode fibers. Therefore, they have lower data rate. Multimode fibers are mainly used with LED sources for slower LANs. They are sometimes used with laser for few gigabits speed. Step-index multimode fibers were the first generation fiber optic cables and they too slow for most applications and rarely used nowadays. Graded-index multimode fibers are constructed with multilayered fiber material with varying refraction indexes so that many rays entering the fiber will bend at different angles depending upon their angles of entries. Therefore, after certain distance, the rays gather together as illustrated in the figure as “eye like patterns”. The end result is that the light pulse (composed of numerous rays) behaves as one coherent ray. This behavior improves the quality of light pulse, thus increases data rate.

· Single mode has the core size of about 9 microns. Since the core is so small, in a single mode fiber, light travels in only one ray, thus the name ‘single mode”. Single mode fibers are used for telephony and Cable TV. They are driven by laser sources. The current practical limit for their bandwidth is about 100,000 Giga Hz. Single mode fibers are more expensive due to its delicate size, higher cost for connections, and the higher cost for laser drivers.

For more information, visit http://www.corning.com/opticalfiber/discovery_center/tutorials/fiber_101/multi_vs_single.asp

* Fiber connectors

Optical fibers are flimsy and are easily broken. Therefore, fibers need to be terminated (encased in a strong housing) with connectors. There are several connectors. They are:

The ST connector is the most common connector. A bayonet locking system is used.

The SC connector uses a molded body and has a push- pull locking system.

The FDDI connector two fiber mold.

The MT-RJ connector, a small-form RJ-style connector, features a molded body and uses cleave-and-leave splicing.

The LC connector, a small-form factor connector, features a ceramic ferrule and looks like a mini SC connector.

The VF-45 connector is another small-form factor connector. It uses a unique "V-groove" design.

* Connector and Splicing Loss

When a fiber is terminated to a connector and it is spliced (connected to another fiber), it loses some of the light. Therefore, to minimize the loss, fibers should be carefully, precisely terminated or spliced. Some of the common bad splicing are shown in the following figure.

From http://www.lanshack.com/fiber-optic-tutorial-termination.asp

Examples of bad splicing

* Advantages of Fiber Optics

Fiber optic systems are emerging as the dominating medium of the future. The advantages over copper wires are:

· More cost effective—Data rate/cost ratio is higher for fiber

· Higher bandwidth—Copper cables reach their maximum at about one gigabits/sec speed, fiber can be used up to few hundred gigabits/sec.

Data rate increase of Fiber optic cables

· No electromagnetic interference—Light signal is not affected by electromagnetic energy.

· Less attenuation, therefore greater repeater spacing—Light in fiber travels much longer distance before it needs to be regenerated. Typical copper cable needs a repeater every 3 miles and for a fiber the spacing is 30 miles or more.

· Thinner—Fibers are about one-tenth in dimension

· Lightweight—Lighter than copper wires and takes less space in the ground

· No danger of electrical fire

· Harder to tap, therefore more secure—Fibers are much harder to tap and eavesdrop since the fiber when spliced loses its power dramatically and can be detected easily.

* WDM (Wave Division Multiplexing) & DWDM (Dense WDM)

Demand for more bandwidth increases continuously even for optical fiber networks. There are three options for a solution: (a) Install more fiber (b) Increase the data rate as physics allow (c) Increase the number of lights beams on a fiber.

WDM is a technology of multiplexing many light beams of different wavelengths on to one fiber, thus increases the aggregate data rate. Let’s first look at a conceptual diagram.

WDM system

Each transmitter generates a narrow wavelength light signal. Each has different colors. The outputs from many transmitters(many light beams) are fed into a multiplexer which combines the light beams into a mixed light beam. This light beam (beams mixed) is sent on a fiber and amplified as needed. At the receiving end, a demultiplexer separates the beam into many beams. Another conceptual diagram looks like:

WDM with 4 beams; note that there is only one fiber running

The following figure shows the prism which splits (demultiplexes) a light into many beams at the receiving end of a fiber.

* Speed of WDM

The following is from

http://www.bell-labs.com/news/1999/november/10/2.html

Bell Labs Uses Ultra-Dense WDM to Transmit 1,022 Channels over Fiber

---

MURRAY HILL, N. J. (Nov. 10, 1999) -- Pushing the frontiers of optical-networking technology, scientists from Bell Labs have set a new world's record -- transmitting data over 1,022 wavelengths, or colors, of light, through a single optical fiber -- with each wavelength carrying a distinct stream of information.

The experimental transmitter uses a single ultra-high-speed laser to generate signals over all 1,022 wavelengths, instead of using a separate laser for each, as is done in conventional multi-wavelength systems.

Today's commercial optical systems combine up to 100 wavelengths on an optical fiber, using a technique called dense wavelength division multiplexing (DWDM), or transmission over many wavelengths of light. Conventionally, DWDM signals are separated by 50 gigahertz (GHz) frequency spacing, but the 1,022-channel system operates at a record high density of 10 GHz channel spacing. Each channel occupies only 10 GHz of frequency bandwidth, making this an ultra-dense WDM (UDWDM) transmitter.

The 1,022-channel transmitter carries information at the rate of 37 megabits (million bits) of information per second, for a total system capacity of more than 37 gigabits (billion bits) per second. The researchers believe the system can be scaled up to OC-48 data rates, for a capacity of several terabits (trillion bits) per second.

Gilder’s Law

Computing power, often calculated by the number of transistors on a chip, follows Moore’s law: Doubles every 18 months. There is Gilder’s law for fiber optics. Gilder’s law states that the Fiber bandwidth triples every year. The difference between the two laws is that Gilder’s law only applies to the base technology advances not to the installed or used capacity. Most installed fibers are thousands of times slower than the base technology.

The cost for fiber is not dropping as rapidly as the advance of the base technology because only 5% of capacity is sold and the carriers are losing money. Cost will drop only when you can sell it all.

Gilder’s law: Capacity triples every year

Summary of Cable Characteristics
Cable Type	Cable Cost	Installation Cost	EMI Sensitivity	Data Bandwidth
UTP STP Coax Fiber Optic	Lowest Medium Medium Highest	Lowest Moderate Moderate Highest	Highest Low Low None	Lowest Moderate High Almost infinite

Lifecycle of Cables:

First figure: Problems with networks

Second figure: Life cycle of cables compared to others

Cabling is very important in networking as the above figure illustrates: First figure shows that about 70% of all the network problems are cable related. Second figure shows the longevity of cables compared to other networking or computing equipments.

Let’s look at an wiring example for Ethernet LANs.

From http://ppewww.ph.gla.ac.uk/~flavell/struwire/whatisit.html

4.2  Unguided Media (Wireless)

There has been increasing needs for mobile users to connect to a network. The answer for their needs is wireless. In wireless communications, space (air) is the medium for the signals.

Wireless networking has some advantages over wired networking:

· No wires needed. Running wires can be difficult in some cases; such as wiring an existing building, wiring between buildings, wiring across mountains, etc.

· Staying connected is important for mobile users. Wireless networks allow users stay connected more hours each day. Users with laptops may roam their work space without losing network connection and without logging into another machine. This increases the productivity of workers.

· Wireless networks can grow without much difficulty compared with wired networks. Making a wired network larger often involves wiring and usually costly.

· Wireless networks are not confined to an area. There is no long term commitment as in the wired networks.

4.2.1 Bandwidth for wireless transmission

The principle of wireless communication is to send and receive electromagnetic wave using antenna. Several frequency bands are used for wireless communications.

· Radio—Frequencies between 30 MHz to 1 GHz

· Microwave—Frequencies between 1 GHz to 40 GHz

· Infrared—Frequencies between 3 x 10¹¹ to 2 x 10¹⁴ Hz

The Electromagnetic spectrum used in communications

(From Tanenbaum Figure 2.11)

As you noticed from the above figure, there are some overlap between the bandwidths for wired media and wireless. The only difference is whether they have solid wires carrying signals or not.

· Radio transmission: These are systems for AM or FM radio. They are one form of communications and not used for computer networks.

· Microwave transmission:  We can classify them into three categories; Terrestrial microwave, Satellite microwave and ISM(Industrial, Scientific, Medical)

§ Terrestrial microwave: Uses dish-type antennas to achieve point-to-point line-of-sight communication. Microwave antennas are typically installed at high level from ground such as on top of a building, on top of a hill or a mountain. Line-of-sight means that there should be no obstacles between antennas. Terrestrial microwave systems are primarily used for long distance telephone networks carrying voice and video (TV) signals. They also are used for connecting buildings in close ranges. Another important usage of them is for cellular telephone systems.

§ Satellite Microwave: A satellite is a microwave repeater. It receives a signal from ground antennas(earth stations) in a certain frequency band(uplink) and repeats(relays) the signal in another frequency(downlink). Usually, a satellite is equipped with several circuits and each circuit operates in difference frequency bands. These circuits are called as “transponders”. Depending upon the distances from the Earth, satellites can be classified into two categories; Geosynchronous satellites and Low Orbit satellites.

v Geosynchronous satellites: When a satellite is placed on a orbit 22,241 miles (35,786km) directly above equator, the satellite's orbital speed exactly matches the rate at which the earth rotates. Therefore, the satellite appears stationary to the earth stations. These satellites are called as “Geosynchronous satellites”. Typical usages are TV broadcasting, long distance telephones, and relaying network traffic. One disadvantage is the delay due to the distance. The 22,241 miles(35,786 Km) results in the delay of 270 milliseconds for one way(either downlink or uplink). This delay can create problems for voice communications and also networking.

Satellite frequency bands

DOWN                  UP-LINK     SHORT-HAND

----------------------------------------------------------

3.7-4.2                  5.925-6.425           4/6 (c-band)

11.7-12.2               14.0-14.5               12/14(ku)

17.7-21.2               27.5-31                  20/30(k)

All numbers are in GHz

From: http://liftoff.msfc.nasa.gov/academy/rocket_sci/satellites/geo-high.html

v Low Orbit satellites: Satellites orbiting at a distance between 800-1400Km. Since they are not synchronous with Earth’s rotation speed, typically many satellites works together as relaying signals. If a satellite disappears from an earth station’s sight, then another one should be insight and the handover is performed. Search the Internet with "Iridium” or “Teledesic”.

§ ISM(Industrial, Scientific, Medical): For all of the above microwave systems, their bandwidth assignments are regulated by national and international bodies and they are fixed assignments. A totally different way of assignment is to let everyone freely use the bandwidth but their power(signal strength) is regulated. These frequency bands for unlicensed usages are called as ISM bands. Each country regulates the ISM bands and they are different from country to country. In U.S., any device with its power under 1 watt can use the ISM bands but all devices must use “spread spectrum” technique. We will cover this technique in Local Area Networks(Wireless LAN).

From Tanenbaum

These bands  (900 MHz band and 2.45 GHz bands) has been used for license-free error-tolerant communications applications such as Wireless LAN(IEEE 802.11b) and Blutooth. The 5.7 GHZ band is used in IEEE 802.11a.

· Infrared: These are in the range of 10¹² to 10¹⁴ Hz. Typically used for the remote controls on TVs, VCRs, other home equipments.

· Laser: Uses same frequency bands as Infrared devices except the transmitted signal is generated by a laser device. Therefore, they can be used in a longer distance, typically up to few kilometers.

From http://www.infraredsystems.net/installs/install12.htm

Location: Washington DC
Customer Type: Power Utility
Application: Inter building network connection
System: 1000 Meter 155Mbps ATM

As a summary, let us compare the four main media.

Media	Advantages	Disadvantages	Bandwidth
Twisted Pair	Inexpensive Easy to install Experience	Sensitive to EMI Short distance(100M) Limited bandwidth Easily tapped	Up to 600 MHz Up to 1 Gbps
Coaxial cable	Higher bandwidth Better noise immunity then Twisted Pair Longer span than Twisted Pair	More expensive than Twisted Pair Bigger than Twisted Pair Easily tapped	Up to 1 GHz
Optical Fiber	Huge bandwidth Longer repeater        spacing No EMI High security Small size	Expensive Splicing/termination difficult	Up to few hundred GHz
Microwave	No wires	Cost high	1 M – 10 Gbps
Satellite	No limit for location	Cost high Long delay	1 M – 10 Gbps
Cellular	Small, convenient	Cost high	9.6 – 19.2 Kbps