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Chapter 2

The Physical Layer

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Transmission of light through fiber

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Attenuation depends on wavelength

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Transmission of light through fiber

• Attenuation in dB= 10log (transmitted power/recd. Power)

• 3 bands for comm. 0.85, 1.3, 1.55 micron

• dispersion: Light pulses sent down a fiber spread out in a manner that is wavelength dependent. Solution: 1) separate pulses 2) or raised hyperbolic cosines - solitons can propagate 1000 or more km

Optic fibers

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Fibre Cables

Optic fibers

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Humor

• Terrestrial fiber sheaths are normally laid in the ground within a meter of the surface, where they are occasionally subject to attacks by backhoes or gophers. Near the shore, transoceanic fiber sheaths are buried in trenches by a kind of seaplow. In deep water, they just lie on the bottom, where they can be snagged by fishing trawlers or eaten by sharks.

Optic fibers

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Fiber connections

• Method 1: Terminate in connectors and end up in fiber sockets. Loss = upto 20% of light

• Method 2: Spliced mechanically. Use a sleeve and align and calibrate.

• Method 3: Fuse 2 ends of fibre into solid connection.

Optic fibers

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Fibre Optic networks (ring)

Optic fiber networks

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Fibre Optic networks (star - broadcast)

Optic fiber networks

Silica cylinder

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Fibre optic vs Copper wire

Repeaters

External events (power surges)

Size (weight)

Security

Expense

Fiber Copper

30km 5km

Unaffected Affected

Light Heavy

Difficult to tap Easier

$$$ $

Optic fiber

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Wireless/Electromagnetic spectrum

Electromagnetic spectrum

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Wireless Transmission/Radio

• Advantage: Radio waves are easy to generate, can travel long distances, penetrate material objects easily and are omnidirectional.

• Disadvantage: 1/r^3 behavior. Absorbed by rain. Interference.

Wireless/Radio

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Wireless Transmission/Radio

Wireless/Radio

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Wireless/Microwave trans.• > 100MHz waves travel in straight lines. • Repeaters are needed. High towers are

constructed. • Do not pass through buildings• Multipath fading (late arriving waves are out

of phase with original wave, therefore fading).• Above 8GHz absorption by H2O occurs.• Some bands are free (e.g., 2.4-2.484 GHz)

Wireless/Micro

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Wireless/IR and mm waves

• Used in VCR’s stereos• Candidates for indoor wireless LAN’s e.g.,

portable computers with IR capability can be on the local LAN without a physical connection.

• Don’t pass through walls therefore security is good.

• Can’t use outdoors - sunlight washes it out.

Wireless/IR + mm

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Lightwave transmission

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Signal Transmission

Signal transmission

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Signal Transmission

• Square waves (digital signaling) have a wide spectrum.– Attenuation is frequency dependent therefore a large

range of frequencies is undesirable for long distance– Baseband (DC) is not suitable for long distance

transmission– Discovery - a continuous oscillating signal propagates

further.

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Modems

• Input serial bit stream is converted to a modulated carrier (and vice-versa) by MODEM (modulator-demodulator)

• Need to increase the # of bits / sample (per baud)

4 bits/baudon 2400 baudline

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Modems

• Patterns (phase and amplitude diagrams) like shown in previous slide are called constellation patterns.

• 6 bits/baud on a 2400 baud line (14,400 bps) is called V.32 bis. V.34 runs at 28,800 bps.

• Constellation pattern is complicated and small noise leads to large error.

Modems

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Modems

• Some modems have compression built in - thereby increasing the effective data rate.

• Popular compression scheme is MNP5 which uses run length encoding.

• Another coding scheme is V.42 bis which uses a Ziv-Lempel algorithm.

Modems

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Multiplexing

• Economies of scale.Building a trunk line is expensive, the cost of optic fibre is not (relatively speaking).

• Therefore there are many schemes for multiplexing many conversations over a single physical trunk.

• Two categories: Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM).

Multiplexing

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Multiplexing• Radio: Each station is given a frequency

and it broadcasts only on that frequency. Frequencies of radio stations are widely separated to minimize interference (FDM).

• Radio II: Radio sends ads followed by music (TDM).

• Two categories: Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM).

Multiplexing

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FDM

Multiplexing

4000Hz

Overlap

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FDM• A standard is 12, 4000Hz voice channels

(3000Hz + 2, 500Hz guard bands) multiplexed into the 60-108kHz band. This is called a group.

• 5 groups (60 voice channels) are multiplexed to form a super-group.

• 5-10 super-groups form a master-group.

Multiplexing

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FDM (WDM)• A variation on FDM for fibre-optic

channels is wavelength division multiplexing.

Multiplexing

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TDM

Multiplexing

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TDM• Codec: Samples at 8000 samples/second

(125 microsec/sample) (Nyquist thm. States that we need sampling rate of 2 x max frequency).

• The above technique is called Pulse Code Modulation.

• Consider the T1 carrier (next slide).

Multiplexing

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TDM

Multiplexing

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TDM• Analog signals from modems etc. are

sampled in round robin fashion and then fed to the codec (rather than having 24 different codecs)

• Each of the 24 channels puts 8 bits into the output stream. 7 are data bits, 1 is a control bit. (7x8000 + 1x8000) = 64Kbps per channel.

• One frame = 24x8 + 1framing bit = 193bits

Multiplexing

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TDM• There are 193 bits / 125microsec. =

1.544Mbps

• 193rd bit has alternating zeros and ones 01010101010101010….. and is used for frame synchronization.

• Receiver checks 193rd bit to see if it is in sync

• 24th channel also has a special sync pattern

Multiplexing

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TDM• Variations: E1 - 32 channels (8 bit data

samples) in a 125 frame.

• 30 channels are used for info. Two for signaling.

• 4 frames together provide 2x8x4 signaling bits

Multiplexing

sec

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Multiplexing T1 streams (TDM)

Multiplexing

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TDM (statistical techniques)• Signals have characteristics that make them

amenable to compression through statistical techniques.

• DPCM (Differential Pulse Code Modulation) - the amplitude is not output but the difference between current value & prev. one. Large jumps are not likely so perhaps 5 bits should work. Wild jumps lead to error.

• Var: Modulation, predictive encoding

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TDM (statistical techniques)

Multiplexing

Variation on Differential Pulse Code Modulationis called DELTA MODULATION

Delta modulation

One bit is transmitted telling whether the new sample isbelow or above the previous one.

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Telephone networks• LAN’s are ok for computers in close

proximity. For longer distances companies prefer to use existing telecommunication facilities.

• Public Switched Telephone Network (PSTN) is therefore worth studying especially since they are going towards digital.

• PSTN was designed for voice communic.

Telephone networks

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Telephone networks• Voice communication is quite tolerant of

transmission errors but computer-computer communic. needs much less error. (Read example on page 102 of text)

• STRUCTURE (next slide)

Telephone networks

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Structure of the tel. network

Telephone networks

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Struct. of telephone network

• Calls from caller in end office 1 to caller in end office 1 go through end office 1

• Calls from end office 1 to end office 2 go through toll offices. In a tree there is only one minimal route.

• Some routes are busier than others (e.g., DC to NY) so direct trunks are created. Therefore many paths exist.

Telephone networks

CS481 Samir Chettri Telephone networks

Fiber to the curb (FTTC)

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Politics of telephone system

• Please read this section on your own section 2.4.2

Telephone networks

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SONET/SDH

• Synchronous Optical NETwork and Synchronous Digital Hierarchy.

• Goals of SONET– Different carriers (companies) need to work

together– Unify US, Japanese and European models– Multiplex several digital channels together (i.e.,

the T hierarchy - T1, T2 etc to gigabit/s lines)– Provide support for operations, admin. & maint.

SONET/SDH

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SONET/SDH• SONET is a traditional TDM system.

• Everything is controlled by a highly accurate clock (1x10E-9 accuracy) bits are sent out under clock control.

• SONET consists of switches, multiplexers and repeaters.– Fibre from one device to another is a section. – Between two multiplexers is a line.– Between source and destination is a path.

SONET/SDH

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SONET/SDH

SONET/SDH

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• SONET puts out a frame of 810 bytes every 125microseconds. The 8 frames/sec matches PCM channels used in all digital telephony systems.

• Each frame is described as a rectangle of bytes 90 columns wide and 9 rows high.

• SONET is synchronous since frames are emitted whether or not there is any data to send.

SONET/SDH

SONET/SDH

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SONET/SDH

SONET/SDH

System management info

Between two devices

Between muxes

Synchronous payloadenvelope starts any-where in frame

Synchronous PayloadEnvelope

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• First 3 columns are for system mgt. info.– First 3 rows (of first 3 cols) contain section

overhead– Next 6 contain line overhead

• Synchronous Payload Envelope (SPE) - the user data can start anywhere in the SONET frame.Useful when 1) a dummy frame is being constructed. 2) payload does not fit into a frame (we will study this in ATM)

SONET/SDH

SONET/SDH

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• Section, Line and path overheads contain bytes for operations, admin. and maint.The fields are described in – Bellamy, J. Digital Telephony, NY, JohnWiley

• Multiplexing of SONET streams are called tributaries. – Low speed input streams are converted to the

basic STS-1 (Synchronous Transport Signal-1) SONET rate much like muxing T1 lines.

SONET/SDH

SONET/SDH

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SONET/SDH

SONET/SDH

Multiplexing is done byte by byte, i.e., a byte from first tributary is used then a byte from second and a byte from third in round-robinfashion. This is true for all levels of the hierarchy.

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Switching• The act of choosing a “physical copper

path” connection from transmitter to receiver is called circuit switching.

• In modern times the “physical copper” paths may well be microwave links.

• An end to end path needs to be set up before any data can be sent. For long distance communication long setup times (10-20sec) occur. Computer apps. don’t like this.

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Circuit Switching

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SONET/SDH

SONET/SDH

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• No physical copper path is established in advance between sender and receiver.

• Data is stored in a switching office (router) and forwarded one jump at a time. E.g. the old torn tape offices.

• There was no limit on block size which means routers need disks to buffer blocks.

• This limitation lead to packet switching networks

Message Switching

Message switching

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• Packet switching networks place a tight upper limit on block sizes.

• The transmission lines can only be obtained for millisecond intervals.

• Therefore no one person can dominate the transmission lines.

• Pkt switching is good for interactive traffic.• First pkt of multi-pkt message can be fwded

before second one has fully arrived (see fig)• With packet switching packets are delivered

in wrong order (sometimes) never happens with circuit switching.

Packet Switching

Packet switching

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Packet/Circuit Switching

Pkt/Circuit switching

Item Ckt-Switched Pkt-Switched

Dedicated “copper” path

Bandwidth available

Wasted bandwidth

Store-&-Forward Trans

Route each pkt takes

Call setup

When congestion occurs

Charging

Yes

Fixed

Potentially

No

Same

Required

At setup time

Per minute

No

Dynamic

No

Yes

Potentially diff

Not needed

Every packet

Per packet

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Crossbar switch

Q: For full duplex line and no self connections how many crosspoints are needed? (scaling problem)

Crosspoint

Switching

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Space division switch

Switching

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• Example: Build an NxN crossbar by staging.

• Use three stages. – First stage has (N/n) crossbars. Each crossbar

has n x k crosspoints, n input and k output.– Second stage has k crossbars with (N/n)x(N/n)

crosspoints.– Third stage is like first but with input and

output reversed.

• Number of crosspoints = 2kN + k(N/n)^2

Space division switch

Switching

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• TANSTAAFL:What happens in stage 2 when more than 8 calls come in? You get blocking. Figure b) is better but requires more crosspoints. Therefore we have to come to some kind of compromise.

• Having large k (in second stage) reduces the blocking probability but increases cost.

Space division switch

Switching

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Time division switch

Switching

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• n input lines are scanned in round-robin order. Each line contributes to an input frame of n slots each of k bits. (T1 lines have 8 bits, 125microsec/frame).

• Time slot interchanger takes input frames and outputs new frames where reordering of time slots occurs (using a mapping table). This goes to n output lines.

• The crux of the matter is the interchanger.

Time division switch

Switching

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• Table search is linear (in number of input lines). This is good.

• Need to access RAM - first store n slots, then read them out after accessing mapping table. This needs to take place in 125microsec.

• Time to process a frame is 2nTmicrosec or 2nT = 125 or n = 125/2T. This determines the number of lines given memory speed.

Time division switch

Switching

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Integrated Services Digital Network (ISDN)

• ISDN is a fully digital, circuit switched telephone system.

• Narrowband ISDN.– Attempted to replace the Plain Old Telephone

Service (POTS) with a digital one suitable for voice and non-voice traffic.

– Lacks bandwidth by 2 orders of magnitude for video (i.e., non voice traffic)

ISDN

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ISDN Arch. (home, small bus.)

ISDN

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ISDN Arch. (home, small bus.)• Digital bit pipe: ISDN is a conceptual full-

duplex pipe through which the bits flow between customer and carrier. Origin (tel., video) is irrelevant.

• Digital bit pipe does TDM.

• Network Terminating Device (NT1) placed at customers site. Connects customers equipment to ISDN exchange using twisted pair.

ISDN

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ISDN Arch. (home, small bus.)

• NT1 has a connector to which a bus connection can be put. Phones, terminals etc. can be put on the bus.

• Digital bit pipe does TDM.

• Network Terminating Device (NT1) placed at customers site. Connects customers equipment (up to 8 devices) to ISDN exchange using twisted pair.

ISDN

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ISDN Architecture (big business)

ISDN

R conn. betn non ISDN terminal and term-inal adapter.

S interface between PBX & ISDN equip T connector between customer, NT1

U connector between NT1, exchange

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ISDN Architecture (big business)

• For larger concerns, NT1 is inadequate.

• Therefore we have NT2 aka PBX (Private Branch Exchange)

ISDN

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ISDN Interface

• Bit pipe supports multiple channels interleaved by TDM. Standard channels are– A 4kHz analog telephone channel– B 64kbps digital PCM channel or voice data– C 8 or 16 kbps dig. channel for out of band sig.– D 16 kbps dig. chan for out of band signaling– E 64 16 kbps dig. channel for ISDN signaling– H 384, 1536, 1920 kbps digital channel

ISDN

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ISDN Interface• Digital bit pipe consists of standard

combinations of A through H channels.– Basic rate: 2B + 1D– Primary Rate 23B + 1D (US, Japan) 30B + 1D

(Europe)– Hybrid 1A + 1C

ISDN

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• Telephone companies want to invent a single new network for the future that will replace the entire telephone system with a single integrated network.

• The new network is called B-ISDN (Broadband Integrated Services Digital Network) and will have a huge data rate.

• Underlying B-ISDN is ATM (Asynchronous Transfer Mode)

Broadband ISDN & ATM

ISDN

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• In ATM all transmission occurs in small 53 byte packets called cells. 5 bytes are header and 48 bytes are payload.

• ATM networks are connection oriented but is implemented internally with packet switching.

• SPEED: 155.52Mbps and 622Mbps (4 155Mbps channels). Gigabit speeds are to follow.

B-ISDN & ATM

B-ISDN + ATM

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B-ISDN & ATM

B-ISDN + ATM

REFERENCE MODEL

Read on own (p64)

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• Standard twisted pair cannot be used (Category 5 twisted pair can be). Therefore we need re-wiring or fibre.

• Space and time-division switches cannot be used for ATM packet switching. Therefore need new switches. (more on that later).

• Wide area fibre trunks can be used.

B-ISDN & ATM

B-ISDN + ATM

CS481 Samir Chettri B-ISDN + ATM

CS: Convergence SublayerSAR: Segmentation and Reassembly

PMD: Physical Medium DependentTC:Transmission Convergence

Generate packetslarger than a cell

Interfaces to cable. Differenthardware is required for diff.cables and fibres.

Transmitting end - streams of bits to PMD. Receivingend - streams of bits from PMD to cell stream.

to applications

Segments packets, transmitscells and reassembles them atother end

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Virtual circuits vs. circuit switching

B-ISDN + ATM

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• Permanent Virtual Circuits - are requested by the customer and remain in place as long as customer pays the rent.

• Switched Virtual Circuits - setup like telephone calls, i.e., allocated dynamically and then torn down.

• ATM: a route is chosen from source to dest. & switches make table entries to route pkts on virtual ckt. (Fig shows H1 to H5)

Virtual circuits vs. circuit switching

Virtual circuits

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• When pkt arrives it switch it examines pkts header to determine what virtual circuit it belongs to.

• Virtual ckt between H1, H5 means that switches (routers) will hold table entries for a particular destination - regardless of the last time traffic occurred (costly, but no setup time). App - credit card verification.

Virtual circuits vs. circuit switching

Virtual circuits

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Transmission in ATM networksThe rate is governed by a master clock. T1 is synchronous

Strict alternation between different sources not adhered to. Cells arrive randomly from different sources.

Transmission in ATM

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• ATM permits cells to be enclosed in a carrier such as T1, T3, SONET etc. In each case a published standard is available

Transmission in ATM networks

Transmission in ATM

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ATM switches

• We mentioned earlier that time division and space division switches do not work with ATM. Generic switch shown below

ATM switches

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• Switches may be pipelined, i.e., several cells from one input line may be collected before being sent to its output line.

• Cells arrive on input line asynchronously so there is a master clock that marks the beginning of the cycle.

• Any cell (53 bytes) that arrives before the clock ticks is eligible for switching. If not the cell is made to wait for the next cycle.

ATM switches

ATM switches

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• Cells come in at approx. 150Mbps = 36E04 cells/sec - therefore cycle time of switch is 1/36E04 = 2.7777microsec.

• Switch may have from 16 to 1024 input lines. Thus anywhere between 16 to 1024 cells are to be switched every 2.7microsec.

• At 622Mbps the time is measured in nanoseconds.

ATM switches

ATM switches

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• All ATM switches have 2 common goals– GOAL 1: Drop cells but only in emergencies -

1E-12 cell loss is permissible. This translates to 1 or 2 cells per hour.

– GOAL 2: Cells arriving at a switch in a particular order must leave in that order without exception.

• PROBLEM: What does the switch do when the cells arriving at 2 or more input lines want to go to the same output port? (PTO)

ATM switches

ATM switches

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ATM switches

ATM switches

Problem: Head-of-line blocking. When cell is held up all the cells behind it get held up. PTO for another solution.

Input queuing

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ATM switches

ATM switches

Output queuing

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• Read Knockout Switch on your own.

ATM switches (Batcher-Banyan)

ATM switches

Banyan Switch

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• Routing is done by looking up the output line for each cell (using routing tables)

• Each switching element has two inputs and two outputs (=0,1. Reasons given below).

• Example: 6 (110) arrives at input line 0. 3 stage banyan switch. Binary number is read from L-R. 1 means use lower port 0 means use upper port. Thus input cell 0 with 001 as output port ends up on output port 6.

ATM switches (Batcher-Banyan)

ATM switches

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Batcher-Banyan (Collisions)

ATM switches

Collisions: STAGE 1: (5,7), (0,3), (6,4), (2,1) (Resolve for 5,0,4,1) STAGE 2: (0,1), (5,4) STAGE 3: Only 1, 5 win.Depending on input we could have collisions and therefore good orbad routing.

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Batcher-Banyan (Cont’d)

ATM switches

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• Batcher switch placed before banyan switch. Element consists of 2x2 cells.– When an element receives 2 cells it does a

numerical comparison. – Higher output address goes in direction of

arrow. – Lower output address in opposite direction– If only one cell it goes to output port that is

opposite to the direction of the arrow.

Batcher-Banyan (Cont’d)

ATM switches

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Batcher-Banyan (Cont’d)

ATM switches

• After leaving Batcher switch the cells are shuffled and passed to a banyan switch.

• This is shown two slides previously.

• A more concrete example is given on the next slide.

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Batcher-Banyan (Cont’d)

ATM switches

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Cellular Radio

• Advanced Mobile Phone System. (AMPS)– In AMPS a geographical region is divided into

cells. – These cells are circular but are modeled as

hexagonal regions.– Frequencies are reused in cells that are not

adjacent.– Small cells lead to less power requirements for

devices.

Cellular radio

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Cellular Radio

Cellular radio

Base Station

Subdivision of basic cells

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Cellular Radio

• At any time a phone is in one cell and therefore working with the base station that sits in the center of that cell.

• When a phone moves to a new cell ownership of the phone is transferred to the new bas station.

• If a call is in progress, this transfer takes 300ms. This transfer is called handoff.

Cellular radio

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Cellular Radio

• There are 832 full duplex channels in the AMPS system.– 832 simplex channels going from 824-849MHz

for transmission– 832 simplex channels going from 869-894MHz

for reception.– Each simplex channel is 30KHz wide.

• Echo occurs when radio waves bounces off trees, bldgs. as well as travels in a st. line.

Cellular radio

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• In each city 416 channels given to B-side carrier (i.e., AT&T) and 416 channels are given to A-side carrier (entrant in cellular business). This is done to promote competition.

• There are 4 categories of channels– Control– Paging– Access (for call setup and channel assignment)– Data (for voice, fax or data)

Cellular Radio

Cellular radio

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• SECURITY– Anyone with an all band receiver (radio) can

tune into and hear everything that is going on in a call.

– With an all band receiver connected to a computer, the thief can record the 32 bit phone number and 34 bit SN. A DB can built up of these numbers and used.

– Solution - use encryption. But police don’t like this.

Cellular Radio

Cellular radio

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Communication Satellites

• Weather balloons (metallized on the outside) were used as radio reflectors. US Navy used the moon!

• Artificial satellites have been used since 1962.– Receives signals at one frequency – Rebroadcasts at another frequency (to avoid

interference with incoming frequency)

Comsats

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Communication Satellites

• Geosynchronous: – Beams are usually single spatial beam that

illuminated entire earth. Now spot beams are also available.

– VSAT (Very Small Aperture Terminals). These are low cost microstations with 1m antennas with 1watt power output.

– Communications take place as shown in next slide (due to the low power).

Comsats

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Communication Satellites

Comsats

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Communication Satellites

Comsats

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Communication Satellites• Low Orbit Satellites.

– Individual low orbit satellites are not useful for communication satellites.

– However groups of satellites in low earth orbit could be useful. One such project is Iridium.

– Here the cell phones would be mobile as would the cells (since the satellites are moving)

– Each satellite has 48 spot beams. 66 satellites. So 1628 cells. Uplink and downlink on L band (1.6GHz).

Comsats