Lecture Note on Transmission Basic

Direct Connections in Communication Networks

• Building Blocks– links: coax cable, optical fiber...

– nodes: general-purpose workstations...

• Connectivity Types– point-to-point

– multiple access

• Connectivity Types– switched networks

=> switches

– inter-networks

=> routers

Indirect Connections in Communication Networks

Connectivity Examples

• Internet: – Best-effort (no performance guarantees)– Packet-by-packet

• Point-to-point link: – Always-connected– Fixed bandwidth– Fixed delay – Zero-jitter

Delays in Networks• Propagation time or propagation delay, tprop

– Time required for a signal or waveform to propagate (or move) from one point to another point. It is defined as d/v where d is the distance between the two points and v is the velocity of the signal.

– Electromagnetic signal sent through space from the Sun to the Earth.• Velocity is the speed of light, 3 × 108 meters per second.• Say distance is 148.8 million kilometers.• Propagation time is (148.8 /3) × 109 / 108 = 496 seconds.

• Transmission time, ttrans

– Links or channels for digital transmission are defined in bits/second or bps. This is called the Data Rate, R, and is the rate at which the transmitter can push bits onto the transmission line. Transmission time is the time it takes to send some number, say B bits, or B/R.

– What is the transmission time for a packet of 1500 bytes on a 10 Megabit link.

• 1500 × 8 / 107 = 1.2 milliseconds.

• Queuing time or queuing delay– Any time related to processing or waiting of packets in buffers.

Representation of Information• Block Oriented Information: specific length blocks

– Text files– Fax documents

• Example: A4 Page• 200 pixels per inch (horizontal) by 100 pixels per inch (vertical) requires 256

kilobytes prior to compression (a pixel is black or white). Compression reduces by a factor of 8 to 16.

– JPEG color images• 8 x 10 inch photo uncompressed is about 38.4 Mbytes before compression

and about 1.2 – 8 Mbytes after compression.– Note that each of 3 colors (RGB) requires one byte to represent the value

• Stream Oriented Information: continuous stream of data– Voice PCM: 4 kilohertz voice is 64 kbps, standard voice channel– Voice ADPCM, compression technique reduces rate to 16-32 kbps– Audio MPEG audio MP3 compression– Video MPEG 2

• 1920 x 1080 pixel frames at 30 frames /second requires about 1.5 Gbps (includes 20bits/pixel and vertical/horizontal blanking and embedded audio) uncompressed is reduced to 19-38 Mbps compressed

A4 Fax Document Standard

1/16 m2, 5 grams / page typically

210 millimeters x 297 millimeters (slightly bigger than 8 1/2 by 11)

Data Compression Techniques

• Why data compression: reduce amount of storage space required to represent the document– Compression ratio: size of uncompressed / size of compressed

– Lossless data compression• Example: run length encodings

– Sequences of likely symbols encoded into runs

– 25 0’s follows by a 1 is encoded as a fixed m-bit code word

– Example: encode 000000000000100001001 using a 3 bit code

– Lossy data compression• Compressed file is an approximation of the original data

• Better compression ratio is achieved.

• For example, MP3 can achieve a factor of 14 reduction.

• Example: unencoded MP3 requires about 1.4 Mbits/sec (2 channels, 16 bits per sample, at a rate of 44100 samples per second). Thus, 1 hour is about 317 MB (about ½ of an audio CD and 1/14 of common DVDs) Through MP3 compression, we can compress to say 128 kbits /second.

Analog Information

• Based on a wave that varies continuously with time

• Example– Sound

– Electromagnetic radiation

• Periodic signal repeats over time

• Representation of a analog signals – Amplitude versus time

– Frequency representation

– Representation as sums of sines and cosines

Frequency Spectrum• Frequency: cycles per second (Hertz) of a sinusoidal signal• Bandwidth: usable range of frequencies high – low: f2 – f1

• Typical telephone voice channel is 4k HzSome approximate frequencies300 Hz – 20,000 Hz human voice / sound50 kHz navigation (ships, submarines, etc)1 MHz AM radio (20 k Hz channels)10 MHz CB, short wave100 MHz FM radio, TV1 GHz UHF TV, mobile telephony10 GHz amateur satellite100 GHz upper microwave – many uses (up to about 300 GHz)10 T Hz = 1013 Hz Infrared, 1015 Hz Visible light, 1018 X-rays, . . . .

Bandwidth Versus Channel Rate

• Shannon's theorem: maximum bit rate for noisy channel C = Bandwidth * log2 (1 + S/N)

• 4 kHz voice channel equates to 56 or 64 k bits / second.

– This is achievable by modern modems

• Some sample bit rates

Radio LAN in the 2.4GHz band: 2 Mbps

Fast Ethernet: 100 Mbps, Gigabit Ethernet,

Optical fiber transmission: 2.4 – 9.6 Gbps over one wavelength

Multiplexing

• Multiplexing is fundamentally sharing some amount of bandwidth by many different users, channels, etc.– FDM: Frequency Division Multiplexing: dividing up the frequency

spectrum into multiple channels, each channel dedicated to a particular use

– TDM: Time Division Multiplexing: using the entire frequency bandwidth allotted but uses “logical slots” to transmit different channels at specific points of relative time within a logical frame that is transmitted.

• A digital telephone speech signal (corresponding to a 4 kHz channel) is obtained as follows– Sample the signal at twice the maximum frequency or 8000 times a

second

– Quantize the sampled value ( the amplitude of the signal) to 7 bits or 128 different value.

– This gives a bit rate of 56 kbits/second

T-1 Carrier System

• T-1 was developed to carry digitally multiplexed channels between central offices using TDM.

• T-1 multiplexed 24 voice channels in a T-1 frame as follows

Channel 1 Channel 2 . . . . . . Channel 24

7 bit data,

1 bit control

1 bit framing

Sampling rate: 1 frame is sent every 125 µsec (8000 frames /sec)

8 bits / channel x 24 channels + 1 bit = 193 bits per frame.

Therefore, 193 bits / 125 µsec = 1.544 Mbps

This standard signaling rate became known as DS-1 (digital signal 1)

DS Hierarchy

• DS-1: basic block, 1.544 Mbits/sec

• DS-2: 6.312 Mbits/sec (4 DS-1 or 96 voice, plus some control)

• DS-3: 44.736 Mbits/sec

• International standards (based on the ITU hierarchy developed in Europe) is somewhat different– E1: 2.048 Mbits/sec supporting 30 voice channels at 64 kbits/sec each

with two channels for control.

SONET Digital Hierarchy

• OC-1 51.84 Mbps• OC-3 155.52 Mbps• OC-12 622.08 Mbps• OC-48 2.48832 Gbps• OC-192 9.95328 Gbps• OC-768 40 Gpbs

• Compare to the evolution of standard rates for Ethernet– 10 Mbps– 100 Mbps– 1 Gbps– 10 Gbps– 40 Gbps

Wavelength Division Multiplexing (WDM)

• Combining multiple wavelengths over the same optical fiber

• For example 16 wavelengths, each at 2.5 Gbps can provide a rate of about 40 Gbps. (OC-48 x 16)

• Using optical add drop multiplexers

• Optical switches

• A lightpath is a logical connection from a start node to an end node going through intermediate nodes.– Same wavelength throughout

– Reallocating of a wavelength at intermediate nodes (switching)

– Protection paths

– Wavelength switching capabilities

Parallel transfer of information (bits)

0

1

2

3

4

5

6

7

0

1

2

3

4

5

6

7

Parallel Wires

8 bits, 16 bits, etc.

Serial Transfer (Bit Serial)

SOURCE DEST

Transfer, for example, an octet a single bit at a time using a fixed time interval for each bit. A clock signal helps determine when to sample the bit.

Why use a serial line? As distances increase, multiple wire costs increase, complexity of line drivers and receivers increase, etc.

Thus, to send a character, first serialize the bits, send over the line, and then receive the bits, and convert back to character.

Control line / info

Data line

0 0 0 0 1 1 0 0 1 0 0 0 0

Communication Modes

• Simplex: Data is transmitted in one direction only

• Half duplex: Alternating exchange of data between two devices. Need to switch between sender and receiver mode

• Full duplex: Data can be exchanged in both directions simultaneously

Determining Received Bit Pattern

Determine• Start of each bit (or center of bit). Bit or clock synchronization

• Start and end of the unit, a character, a byte, etc. Byte synchronization

• Start and end of message unit or frame. Frame synchronization

Asynchronous Transmission• Receiver and transmitter clocks are not synchronized. They are

independent. Receiver resynchronizes at the start of each unit, say a character.

• To determine beginning of character, have a “start” bit

• To be sure about end of character, have a stop bit.

• Know how many bits are in a character, say 8.

• Clock frequency for sampling is usually 16 times bit rate. Receiver attempts to sample in the center of the bit.

• Start bit is usually 1 bit long and stop bits are often 1, 1.5, or 2 bits

(idle) start 1 0 0 1 1 1 1 0 stop (idle)

Synchronous Transmission

• Sender and receiver clocks are synchronized (often through the sending of a clock signal on a control line)

• Start and stop bits are not used. Characters can be sent one after the other without the overhead of start and stop bits.

• Framing is still important.

1850 – Invention of telegraph by Morse

1874 - Invention of time-division multiplexing by Baudot

1876 - Invention of telephone by Bell

1899 - Invention of radio by Marconi

1928 – Development of sampling theory by Nyquist

1936 - Invention of pulse code modulation by Reeves

1948 – Development of channel capacity theory by Shannon

Invention of transistor by Bell Laboratories

1962 – First 1.544 Mb/s T1 cable carrier system by Bell System

Telestar, first communications satellite be Bell System

1965 – Early Bird, first geostationary communications satellite by INTELSAT

1966 – Low-loss optical fiber proposed by Kao

1980 – AT&T introduces fiber optic transmission at T3 data rate

1984 – Divestiture of the Bell System increases competition and accelerates introduction of digital

transmission in the United States

1988 – First transatlantic fiber optic cable (TAT-8) installed

1989 – First transpacific fiber optic cable (HAW-4/TPC-3) installed

1992 – Conversion of major U.S. networks to digital transmission completed

1990s- Deployment of digital transmission worldwide in support of the Synchronous Digital Hierarchy

and Integrated Services Digital Network.

Performance of Transmission System

Circuit length Type of service

Multiplex hierarchy

Analog vs.digital

Performance objectives

Reference circuit

Bit error rate

AvailabilityQuality

Performance objectives

Bit count integrity

Jitter

Delay Propagation reliability

Equipment reliability

Voice Transmission

For digital transmission applications, voice signal must first be converted to a digitalrepresentation by the A/D process

- PCM (Pulse Coded Modulation)- Different PCM- Delta modulation

Block Diagram of Analog-to-Digital Converter

Low-pass filter

Quantizer Coder

Predictor

\ ´

Digital output

Analog input

Standard for voice digitization

• For single channel: PCM at a transmission rate of 64 kb/s (8 kHz sampling with 8 bit quantization)

• For multi-channel:

North American standard: 24 voice channels at a total transmission rate of 1.544 Mb/s

European CEPT standard: 30 voice channels at a total transmission rate of 2.048 Mb/s

Transmission quality of digitized voice channels

• A/D technique

• Number of tandem A/D and D/A conversions (accumulation of quantization noise)

• Characteristics of transmission impairment

Data Transmission

Data Transmission Network Interfaces

Data terminal equipment

Data circuit terminating equipment

Communications Network

DTE DCE DCE DTE

Data circuit terminating equipment

Data terminal equipment

data

Interface timing

control

data

Interface timing

control

Timing Signals

Asynchronous transmission

Synchronous transmission

(a) Codirectional timing

(a) Contradirectional timing

DTE DCECommunication Network DCE DTE

Timing Timing

DataData

DTE DCECommunication Network DCE DTE

Timing Timing

DataData

(c) Buffered timing

Synchronous Data Transmission Configuration

DTE Buffer

DCE

Timing

Data

Timing

Data

Communication Network

DTE Buffer

Timing

Data

Timing

Data

•••

•••

Interface standards

End-to-end______________ Layer7_______________

End-to-end______________ Layer6_______________

End-to-end______________ Layer5_______________

End-to-end______________ Layer4_______________

________________________ Layer3_______________

________________________ Layer2_______________

________________________ Layer1_______________

DTE DCE

Architecture of Open Systems Interconnection Model

PhysicalCommunications Equipment

NetworkData Link

Network

Transport

Session

Presentation

Application

Video Transmission

• Compression schemes are essential because of very large amounts of information to be transmitted. (e.g.) 8-bit PCM applied to 6-MHz TV signal results in 96 Mb/s

• Compression schemes - Compression in space: transform coding, sub-band coding - Compression in time: motion estimation/compensation

Failure Model

Failure : departure from specified performance

• Failure rate ( ): average rate at which failures can be expected to occur throughout useful life of equipment

• Reliability (R(t)): probability that equipment will perform without failure for a period of time t given by

• Note that the above equation is based on the Poisson assumption on the failure process.

tetR )(

Performance Objectives• Availability is defined as the probability or fraction of time that circuit continuity is

maintained and usable service is provided.

• Quality is considered only when the circuit is available and the quality parameters commonly used include bit error rate, timing slips, jitters, and delay.

• Design objectives (DO) vs. Operation and Maintenance (O&M) standards of performance.

DO=O&M + Margin

• Mean Time Between Failure (MTBF): ratio of total operating time divided by the number of failures in the same period.

0

1)(

dtetMTBF t

• Mean Time To Repair (MTTR): average time to repair the failed equipment including fault isolation, equipment replacement or repair, and test time, but excluding administrative time for travel and locating spare modules of equipment.

• Mean Time to Service Restoration (MSTR): average time to restore service including repair and administrative time.

• Availability (A): probability that equipment will be operable at any given point in time or the fraction of time that the equipment performs the required function over a stated period of time, which can be defined as follows:

• Unavailability (U): complement of availability :

• For large MTBF and small MTSR (usually the case)

MTSRMTBF

MTSRAU

1

MTBF

MTSRU

MTSRMTBF

MTBFA

• Outage (O): condition whereby the user is deprived of service due to failure within the communication system.

• Mean Time Between Outages (MTBO): ratio of total operating time divided by the number of outages in the same period.

Redundant equipment

• MTBO1: If the equipment consists of n identical units, each with the same MTBF, for which at least r must be operable for the system to be operable, the first type of outage is given by

where the sensing of a failure and resulting switchover is itself to occur within failure.

MTBFMTTRfor

rnrnn

MTTRMTBFMTBFMTBO

rn

,)!()!1/()!1(

)/(1

System availability• Series combination:

• Parallel combination:

• Series parallel combination:

• Parallel-series combination:

.1

,

,

1

1

1

i

N

iN

N

iiN

i

N

iN

AU

A

AA

II

II

.11

i

M

iM UA II

.111

ij

M

i

N

iNM UA IIII

.1111

ij

N

j

M

iMN AA IIII

Series Combination

Parallel Combination

N21

1

2

N

Series-Parallel Combination

B1

B2

A1

A2

AM

N1

NMBM

N2

Parallel-Series Combination

B2

A1

B1

M1

AN

MNM2

BN

A2

Error Objectives Error parameters

• Bit Error Rate (BER): ratio of erred bits to the total transmitted bits in a measurement interval

• Error-Free Second (EFS) or Error Seconds (ES): percentage or probability of one-second measurement intervals that are error free (EFS) or in error (ES)

• Error-Free Blocks (EFB): percentage or probability of data blocks that are error free.

Relationships between error parameters• Basic assumption: statistically independent bit errors with an average probability

of error

p=BER, R = bit rate

• EFS and BER:

• P (block error)

R-p)(1100%EFS

n

nnknk

pEFB

npnppppk

n

)1(

1for )1(1)1( 1k

Block length

Error second probability

Pro

babi

lity

of b

lock

err

or1

Bit error probability

R(data rate)10

Bit Count Integrity

Definitions• Bit Count Integrity (BCI): preservation of precise number of bits (or

characters or frames) that are originated in a message or unit of time.

• Slip: loses of BCI caused by a shift of a digital signal.

Performance Impact of One SlipService Potential Impact

Encrypted Text Encrypted key must be resent

Video Freeze frame for several seconds - Loud pop on audio

Digital Data Deletion or repetition of data - Possible reframe

Facsimile Deletion of 4-8 scan lines – Drop call

Voice Band Data Transmission errors for 0.01 to 2 sec. – Drop call

Voice Possible click

CCTTT Performance Objectives for slip rate on 64-kb/s International Connection (Rec. G.822)(13)

Performance Mean Slip Rate Measurement Percentage of

Classification Thresholds Average Period Total Time

Unacceptable >30 slips in 1 hr 1yr <0.1%

Degraded >5 slips in 24 hrs and 1yr

<30 slips in 1 hr

Acceptable <5 slips in 24 hrs 1yr >98.9%

Jitter

Definitions• Jitter: short-term variation of sampling instant from its intended position in time or phase

• Wander (or drift): long-term variation of sampling instant fro its intended position in time or phrase

Transmission impairment• By jitter:

* Degradation in system error rate performance

* Slips

* Distortion in the recovered analog signal

• By wander:

* Buffer overflow (fill) or underflow (empty)

Sources of jitter

• Regenerative repeaters: imperfect clock recovery process• Digital multiplexers: insertion/deletion of overhead of

stuffing bits

Sources of wander

• Oscillator instability• Propagation delay variation