Chapter 2 – Topics in Data Communications

Data Communications ConceptsIntroduction

Essential definitions for Data Communications– Data, Signaling, & Transmission Systems– Analog & Digital

Data are entities that convey meaning, while signaling is the transfer of encoded data thru a transmission system

Analog versus digital signaling– Digital signaling usually less expensive than analog but care

must be taken to properly engineer system (e.g. - attenuation)– Combinations of analog & digital data and signals

Analog data -> Analog signals Digital data -> Analog signals (Key equipment is a modem) Analog data -> Digital signals (Key equipment is a codec) Digital data -> Digital signals

Data Communications ConceptsAnalog versus Digital Transmission Systems

Analog systems transmit analog signals without regard for the content of the signal

– Amplifiers are used to boost the energy of the signal – Amplifiers also boost the strength of any noise on the line,

introducing the possibility that the signal could be lost

Digital Transmission Systems are concerned with the content of the signal

– Repeaters used to regenerate the signal, overcoming attenuation

– Repeaters output a new copy cleansed of any noise, so noise is not cumulative (however, bit errors can still occur if the signal is not regenerated before it degrades too much)

Data Encoding TechniquesIntroduction

Encoding is the process of mapping digital data into the appropriate signal elements for transmission

– Encoding may be very complex or as simple as using binary signal elements (0s and 1s)

Encoding schemes are chosen to assist the receiver in its two key tasks:

– Determining when the signal element begins and ends (so sampling is done at the proper time)

– Determining the value of the signal element (Is it a one? A zero?)

Attenuation, data rate, & noise all play a role at receiver With analog data the encoding scheme also plays a key

role in system performance but the details are a little different

Data Encoding TechniquesAnalog encoding of digital data

The basis for analog encoding is a base signal called the carrier signal

Digital data is encoded (and decoded at the other end) by a device called a modem

Three basic schemes for analog encoding of digital data:– Amplitude Shift-keying (ASK)– Frequency Shift-keying (FSK)– Phase Shift-keying (PSK)

These schemes can be combined for more sophisticated digital transmission systems that carry more data per signal element

Data Encoding TechniquesAnalog encoding of digital data

Amplitude-shift Keying– Data represented by different amplitude levels of the carrier

signal– Simplest scheme, but inefficient and prone to noise– Most valuable use is in optical systems

Frequency-shift Keying– Data represented by different frequency values near the

carrier signal frequency– Less prone to errors but requires more complex circuitry

Phase-shift Keying – Data represented by different phase shifts to the carrier

frequency– More efficient and noise resistant than ASK or FSK but requires

more complex circuitry

Data Encoding TechniquesDigital Encoding of Digital Data

The most common way to encode digital data is to use a binary signaling scheme consisting of two voltage levels– NRZ-L (Non-Return to Zero Level)

Each voltage level defines the value of the digital data Used only in very short connections

– NRZ-I (Non-Return to Zero Inverted) A transition at the beginning of a signal unit denotes a

binary one This type of signaling is known as differential signaling;

it is usually easier to detect a transition out of the background noise and the signals are polarity insensitive

Clocking and DC current are usually problems

Data Encoding TechniquesDigital Encoding of Digital Data

Manchester Encoding– Example of a bi-phase coding; up to two signaling transitions

per signal element (needs more bandwidth to transmit a given data rate)

– The mid-signal transition provides clocking as well as the data value (a zero data element is a high-to-low transition and a one is a low-to-high transition)

– Used in Ethernet LANs (IEEE 802.3) Differential Manchester Encoding

– Another bi-phase code– The mid-signal transition provides clocking; the transition at

the beginning of the signal element represents data (a zero data element has no transition at the beginning of a bit time while a one does)

– Used in Token Ring LANs (IEEE 802.5)

Data Encoding TechniquesDigital Encoding of Analog Data

Pulse Code Modulation (PCM) is an example – used in the phone system to transmit analog data across digital networks

Sampling rate based on the Nyquist theorem Digitized into 8 bit samples based on a nonlinear scale

that provides good reproduction of the human voice Other digital-to-analog encoding schemes:

Adaptive Differential Pulse Code Modulation (ADPCM) - used with voice transmission

Delta Modulation - used rarely but also for voice transmission systems

Code Excited Linear Prediction (CELP) - used in very low-bandwidth voice and multimedia communication systems

Multiplexing Introduction

Allows a transmission system to carry multiple independent signals simultaneously for higher efficiency

Two general schemes are in use: FDM and TDM– Frequency Division Multiplexing (FDM)

Takes advantage of the fact that the useful bandwith of the transmission system exceeds the required bandwidth of a given signal

Allows frequency spectrum to be divided & allocated to different signal sources

Most commonly used with analog signaling and transmission

– Time Division Multiplexing (TDM)

MultiplexingTechniques

Allows a transmission system to carry multiple independent signals simultaneously for higher efficiency

Two general schemes are in use: FDM and TDM– Time Division Multiplexing (TDM)

Takes advantage of the fact that the maximum bit rate of the system exceeds the required bit rate of the digital signal

Each source is allocated a ‘time slot’ in the multiplexer Analog signals can be time division multiplexed, but it is

very uncommon Two varieties of TDM: statistical and fixed time-slot

– Both FDM and TDM can be used in a synchronous or asynchronous manner

Transmission Media Introduction

The transmission media is the physical signal path between the transmitter and the receiver

Can be guided (cables, waveguides, etc.) or unguided (open air)

Our key concerns for transmission systems are data rate and distance

Influencing factors: Bandwidth of the media Transmission impairments Interference Number of Receivers

Transmission Media (2)Twisted Pair Cable

Consists of a minimum of two copper wires twisted together and enclosed within a protective sheath

Advantages: inexpensive, easy to work with, may already be installed where needed

Disadvantages: limited in distance, data rate, and bandwidth; susceptible to interference

Comes in two general varieties: shielded twisted pair (STP) and unshielded twisted pair (UTP)

– Shielding provides more noise immunity, especially at lower data rates

– STP costs more and is more difficult to work with

Transmission Media (3)Twisted Pair Cable

Category 3 and Category 5 UTP– Rating standards devised by the Electronic Industries

Association (EIA)– The higher the category the better the cable; Cat 3 designed

to support 10Mbps Ethernet while Cat 5 will support 100Mbps Ethernet

– The key difference between the two categories is the number of twists per unit length of cable

– Near-end Crosstalk (NEXT) is a key transmission impairment to minimize in any twisted pair cabling system

– While these are regarded as the most commonly found UTP installations, there are higher performance UTP choices

Transmission Media Twisted Pair Cable

High-performance Twisted Pair– Category 5e (or enhanced Category 5): supports 125-MHz

bandwidth on all four pairs, allowing Gigabit Ethernet to run over UTP up to 100 meters

o Attenuation (db/100m)=12.3 for 100 MHz, 12.3=-10Log(Vin/Vout) gives Log(Vin/Vout)=-1.23 or Vin/Vout is about 1/10 (tenth of signal magnitude exits from the UTP.

o Crosstalk=32db=-10Log(Vneighbor/Vsignal), about 1/1000 crosstalk.

o Cat 5 UTP provides 100 Mbps over 100m.– Category 6: supports over 200-MHz bandwidth on all four

pairs; could potentially run high data rate ATM connections– Category 7: will require special shielding and will likely

support up to 700-MHz bandwidth on each pair

Transmission Media Coaxial Cable

Provides a two conductor transmission system where one conductor is situated inside the outer hollow conductor with an insulating dielectric in between

Because of its structural characteristics coaxial cable is more resistant to noise than twisted pair

Harder to work with and more expensive than twisted pair Coax systems can be grouped in three categories based

on the type of signaling used:– Baseband: digital signaling occupies the entire spectrum of

the cable– Broadband: carrier-band analog signaling is used, allowing

multiple channels on the cable– Carrierband: carrier-band analog signaling with low-end

components; signal occupies entire spectrum of cable– Coaxial cable provides 100 Mbps over 1Km.

Transmission MediaFiber Optic Cabling

A transmission system composed of a guided medium that allows the propagation of optical rays

A range of fiber optic cabling exists for various needs, from ultra-pure fused silica (expensive but high data rate) to plastic (cheap with lower data rate for short runs)

Advantages– Huge bandwidth capacity– Smaller size and lightweight– Lower attenuation– Electromagnetic isolation (high security & minimal

interference)– 100 Gbps over 10 Km (multimode fiber)

Common transmitters used are LEDs for (low-cost & low-speed systems) or Injection Laser Diodes (long-haul high-speed systems)

Transmission MediaFiber Optic Cabling

Basic fiber types– Step-index multimode: cheapest to manufacture but allows

light to travel different paths down the fiber, causing signal distortion & lowering the maximum data rate.

– Graded-index multimode: Higher grade of fiber with a varying refractive index that limits distortion of the signal.

– Singlemode: contains a core with a diameter close to the wavelength to be transmitted; allows only a single transmission path down the fiber which practically eliminates distortion

Three wavelength ‘windows’ provide the best light propagation: 850, 1300, & 1550 nm

– Most multimode systems use the 850 nm window– Long-haul transmission systems use the 1550 nm window

because loss is lower at higher wavelengths

Transmission Media Unguided Media

Microwave– Occupies the frequency spectrum from 1GHz to 30GHz; can

provide either a highly directional or omni-directional system– There are 3 main challenges to using microwave for data

transmission: Frequency Allocation and licensing Interference Security

Infrared– Uses light in the infrared spectrum for data transmission– Must be used line-of-sight or in an environment that allows

infrared waves to be reflected– Less issues associated with microwave but only for

specialized uses

Data Communication NetworksIntroduction

For most WAN and MANs, transmission of data usually involves a number of intermediate switching nodes that move the data between source and destination

The complete set of end nodes, data links, & intermediate switches is known as a communications network

There is a spectrum of communication switching techniques; the two main variations are circuit and packet switching

Data Communication NetworksCircuit Switching

Communication between end nodes is via a dedicated communications channel

Communications via circuit switching involves three phases:

– Circuit establishment: the path is established before any data is transferred. The path is digital or analog and may include internal links operated using TDM or FDM.

– Data transfer– Circuit disconnect: release of resource dedicated to the

connection The fixed capacity of the channel is allocated for the

duration of the connection; can be very inefficient with bursty traffic (repeatedly ON during T and OFF later)

Circuit switching is best suited for synchronous data such as voice or real-time video

Data Communication NetworksPacket Switching

Packet switching breaks data up into a series of packets, each appended with enough control information to ensure the packet transits the network successfully from source to destination

Developed to address problems certain data sources have with circuit switching:

– Bursty data transmission– Source and destination must operate at the same data rate– Inefficient resource allocation– Connection setup can be too slow for certain applications(set up Virtual Circuit along logical connection path links, send packets

without routing decision over VC) In addition to addressing the above problems, packet switching

also has other benefits for data transmission:– Under heavy load the network will accept packets but delay increases– Priorities for transmission of the packets can be set– Data rate conversion along links with short store and forward

Data Communication NetworksPacket Switching

Two main varieties: datagrams or virtual circuits Datagram Approach: Each packet is routed independently of all others,

leading to the following consequences:– Packets don’t take the same routes, may arrive out of sequence– Routing based on neighboring info on traffic and failure– Possible packet discard (overflow in queues) and no control-flow– No circuit setup time, so data flow begins without delay– Data can easily flow around problems in the network

Virtual Circuit Approach: a preplanned route through the network is established before any data is sent

– Requires logical circuit setup and teardown but routes along the connection are shared (identical) with other packets

– A routing decision does not have to be made for every packet– May provide enhanced services such as error & flow control, and packet

sequencing not available in a datagram environment

Problem:M bit to be transmitted using UDP and Virtual Circuit (VC).

t(UDP) and t(VC) is routing overhead for UDP and VC for each link. There are K links on the path. The largest packet has only N bits and data rate is R.

Under what condition UDP and VC have the same transmission time.

Using UDP: T(UDP)= kM/N(1/R+t(UDP))Using VC: T(UDP)= k(t(VC) + M/NR)

The same transmission time when kMt(UDP)/N =kt(VC).However, UDP produces shorter time when M/N is small.

Data Communication NetworksHybrids

Multi-rate Circuit Switching (to improve circuit switching)– Extends circuit switching to allow one or more fundamental

channels to be bundled together to provide a range of data connection rates

– Examples of multi-rate switching are ISDN (2x64 Kbps and 1x16Kbps channels) and inverse multiplexing

Frame Relay (FR)– Packet switching was operating under high error rate and

overhead added to enhance redundancy and reduce errors which produced the FR scheme (from 64 Kbps to 2 Mbps by removing overhead).

– WAN service based on a connection-oriented packet data protocol– Frame Relay evolved from X.25; the new protocol was

streamlined by eliminating features necessary on earlier, less reliable X.25 data communications networks

Cell Relay (ATM)– A further evolution of connection-oriented packet data services– Unlike frame relay fixed length data units (cells) are used which

allow high-speed hardware based switching– Connection oriented, fixed cell size, fast switching devices.

Comparison

Packet Switching vs. Circuit SwitchingIs packet switching a “clear winner?” Great for bursty data

– Resource sharing– No call setup

Excessive congestion: packet delay and loss– Protocols needed for reliable data transfer, congestion control

Q: How to provide circuit-like behavior?– Bandwidth guarantees needed for audio/video apps– Still an unsolved problem

Routing

Routing in Packet-Switched Networks Goal: move packets among routers from source to

destination– We’ll study several path selection algorithms (chapter 5)

Datagram network: – Destination address determines next hop– Routes may change during session– Analogy: postal service

Virtual circuit network: – Each packet carries tag (virtual circuit ID), tag determines

next hop– Fixed path determined at call setup time, remains fixed

through call– Routers maintain per-call state

Taxonomy

Telecommunication networks

Circuit-switched networks

Packet-switched networks

Datagram networks

VC Based networks

TDMFDM

End hosts are connected to edge routers through access networks

Types of access networks:

– Residential access– Company access– Mobile access

Types of physical media technologies for access networks:

– Fiber– Coaxial cable– Twisted-pair telephone

wire– Radio spectrum

Network Access

Access Network

Access Network: Residential Access Connects home end systems to the network edge

– Typically, through an ISP– End hosts are PCs– AKA last mile

Means of residential access: dialup, DSL, Cable, etc. Dial-up modem

– Uses POTS line twisted pair copper wire– Calls ISP’s number– Max. data rate: 56 Kbps– Phone line is tied up when connected to ISP

Digital Subscriber Line (DSL)– Does not tie up the phone line– Uses existing twisted-pair line– Asymmetric upstream and downstream data rates

Downstream: 384 Kb/s—1.5 Mb/s Upstream: 128—256 Kb/s

Hybrid Fiber Coaxial (HFC) Cable – Utilizes distribution network of video broadcast cable– Cable modem uses two channels for data transmission

Shared among subscribers 10 Base-T Ethernet port

LAN access

LAN access Company/university local area network (LAN) connects end system to

edge router Ethernet:

– Shared or dedicated cable connects end system and router– 10 Mbs, 100Mbps, Gigabit Ethernet

Deployment: institutions, home LANs soon LANs: Link layer (chapter 5)

Wireless Access Networks Shared wireless access network connects mobile end system to router at

a base station– Laptops, PDAs, etc.

Wireless LANs:– Radio spectrum replaces wire– Wireless LANs are based on IEEE 802.11 b standard (11 Mbps)

Wider-area wireless access– CDPD (Cellular Digital Packet Data): wireless access to ISP router via cellular

network– Third Generation (3G) wireless: packet-switched wide-area Internet access at

384 Kbps

How long will it take to send a file of 640,000 bits from host A to host B over a circuit-switched network.

Suppose all links in the network are TDM with:– 24 slots and – have a bit rate of 1.536Mbps

It takes 500 msec to establish an end-to-end circuit before host A begins transmitting to B

How long will it take to send file?

Example 1

Example 1

Transmission rate for each circuit = 1.536 Mbps / 24 = 64 Kbps Time to send 640 Kbits file = (640000 bits)/(64 Kbits/sec) = 10

seconds Including circuit setup overhead, time to send file is 10.5 seconds

This calculation is independent of the # of end-to-end links and does not include propagation delays

How long will it take to send a file of 640,000 bits from host A to host B over a circuit-switched network.

Suppose all links in the network are TDM with:– 24 slots and – have a bit rate of 1.536Mbps

It takes 500 msec to establish an end-to-end circuit before host A begins transmitting to B

How long will it take to send file?

Packet Switching: Two forwarding mechanisms:

– No segmentation message switching– With segmentationpipelining

Example: 7.5 million bits message sent over 3 links, each of 1.5 Mbps

– Time required without segmentation = (7.5/1.5)x3=15 sec– Now segment packet into 5000 chunks each of 1500 bits

Time for whole packet = 5.002 sec

Pipelining results in reduction of delays as all links are being utilized simultaneously

Example 2

Delay in Packet-Switched networks

Transmission delay: R=link bandwidth (bps) L=packet length (bits) Time to send bits into link = L/R

Propagation delay: d = length of physical link s = propagation speed in medium (~2x108 m/sec) Propagation delay = d/s

Example 3

Packet Switching Calculation of delay: A packet of L bits Q links between source and destination hosts Each link has a data rate of R bits/sec Assume:

– No queuing delays– No end-to-end propagation delays– No connection establishment is required

How long it takes to send this L bit packet from source to destination?

– Time to traverse the first link from source host: L/R seconds Q-1 more such links are traversed before reaching destination

– Thus, total delay: QL/R seconds more delay for larger packets

Example: LAN Hierarchy

N stations to be arranged in a LAN hierarchy Each ST transfers on the average M bits per second, of

which qM are to STs that are local to its segment and (1-q)M to STs in other segments

How to arrange the LAN Hierarchy so that traffic in one LAN segment does not exceed the segment capacity B, where B is 0.8xData.Rate of a LAN segment, i.e. a reference to a congestion point.

Solution

Suppose there are K segments each consists of N/K stations, where k rages from 1 (1 seg) to N (N segs).

In a segment, the locally destined traffic by is qMN/K In a segment, the exported traffic by is (1-q)MN/K In a segment, the imported traffic by is (1-q)MN/K, because:

– there are (K-1) segments each generates (1-q)MN/K external traffic, – Total external traffic for K-1 segments is (K-1)(1-q)MN/K destined to

(k-1) segments.– Thus, a segment imports (1-q)MN/K data.

Total traffic for a segment is Q= qMN/K (local) + (1-q)MN/K (exported) + (1-q)MN/K (imported)=MN/K + (1-q)MN/K= MN(2-q)/K

Let’s derivate Q w.r.t. K to find its minimum value: dQ/dK=-MN(2-q)/K^2 which is minimum for K maximum or K=N.

We may incease the number of STs in a segment until Q(K)=0.8xData.Rate or MN(2-q)/K <= 0.8xData.Rate or

MN(2-q)/(0.8xData.Rate) <= K.

wireless transmission • Wireless local area networks (e.g IEEE 802.11) – 2.4 GHz (microwave band) 1-2 Mbps, 150 m – Infrared (IR) link 1-10 Mbps, 10 m • Earth based cellular data – Basic individual connection, 13 Kbps, 3 km – Higher rate PCS (e.g. EDGE), 384 Kbps, 3 km • Satellite links (GHz frequencies) – geosynchronous, 600-1000 Mbps, continent – Low earth orbit (LEO) 13 kbps - 400 Mbps, 800 km (e.g., Motorola Iridium, 66 satellite constellation)

services available from carriers Integrated Services Digital Network (ISDN) 144 Kbps Asymmetric Digital Subscriber Lines 1.5-8 Mbps/16-640 Kbps Cable modems 0.5-2 Mbps T1 (old electronic telephony standard) 1.544 Mbps T2 6.312 Mbps T3 44.736 Mbps Synchronous Transport Signal-1 (STS-1) 51.840 Mbps STS-3 (a common ATM rate) 155.250 Mbps STS-12 (another common ATM rate) 622.080 Mbps STS-24 1.244160 Gbps etc.