data communication basics habib youssef, ph.d youssef@ccse.kfupm.edu.sa department of computer...

Data Communication Basics

Habib Youssef, Ph.D

youssef@ccse.kfupm.edu.sa

Department of Computer Engineering

King Fahd University of Petroleum and Minerals

Dhahran, Saudi Arabia

COMPUTER NETWORKS

Transmission

Electromagnetic spectrum for telecommunications

ELF VF VLF LF MF HF VHF UHF SHF EHF

102 103 104 105 106 107 108 109 1010 1011 1012 1013 1014 1015Frequency (hertz)

Power and telephone Rotating generators Musical instruments Voice microphone

Radio Radios and televisions Electronic tubes Integrated circuits

Microwave Radar Microwave antennas Magnetrons

Infrared Lasers guided missiles Rangefinders

visible light

106 105 104 103 102 101 100 10-1 10-2 10-3 10-4 10-5 10-6 Wavelength in space (meters)

Twisted Pair

Coaxial Cable

AM Radio FM Radio and TV

Terrestrial and Satellite Transmission

optical fiber

Transmission Medium Guided (P-T-P, Multipoint)

» Twisted Pair» Coaxial Cable» Optical Fiber

Unguided» Air» Vacuum» Seawater

Simplex (Signal One direction) Half Duplex (1 Station at a time) Full-Duplex (2 Stations TX & RX) ** CCITT Simplex = ANSI HD

Duplex = ANSI HD

Guided Transmission Configurations

Transmitter/ Receiver

MediumAmplifier

or repeaterMedium Transmitter/

Receiver

Point-to-Point

0 or more

Guided Transmission Configurations

MediumAmplifier

or repeaterMedium

Multipoint

0 or more

Point-to-point transmission characteristics of guided

The medium itself is more important than other factors in determining transmission limitations

For unguided media, range of frequencies is of more importance.

Transmission medium Total data rate Bandwidth Repeater spacing

Twisted pair 4 Mbps 3 MHz 2 to 10 km

Coaxial Cable 500 Mbps 350 MHz 1 to 10 km

Optical fiber 2 Gbps 2 GHz 10 to 100 km

Twisted-Pair Cables The least expensive media (unshielded) Capable of handling up to 100 Mbps May be used with voice and data

» Private Automatic Branch eXchange (PABX)

Unshielded Twisted Pair (UTP)» Data capacity grades defined by EIA/TIA 568» Categories that can be used for data

– Category 3 to 10 Mbps– Category 4 to 20 Mbps– Category 5 to 100 Mbps

» Characteristic impedance of 100 to 120 ohms

Twisted-Pair Cables (cont.) Shielded Twisted Pair (STP)

» Primarily used by IBM» Should be better than UTP

– Shields prevent interference from outside signals– Also prevent interference to outside signals

Token Ring environments may include a mix of UTP and STP cabling

Coaxial Cables Very high cable bandwidth

» Up to 400 MHz

Low noise (low bit error rate) Used in a variety of networking applications

» In IBM networks (e.g., cluster controllers)» In Ethernets (10Base2 and 10 Base5)» In cable television (used in broadband LANs)

Termination resistance (impedance)» 50 ohms for Ethernet cables» 75 ohms for broadband LANs» 93 ohms in some other cables

Baluns Baluns provide a BALanced-to-Unbalanced interconnect Balanced cables typically are twisted pairs Unbalanced cables typically are coaxial cables Baluns are often used to allow twisted pairs to replace more

expensive coaxial cables Impedance match Connector match

Fiber-Optic Cables Extremely high data rates

» More than 100 Mbps for LAN uses» More than 10 times that for telephone company links

Usage is typically in unidirectional links, with one fiber in each direction

Convert electrical to light and back to electrical

//Electrical ElectricalLight

Electrical Electrical//

Fiber-Optic Cables Very small size

» Hair-like fiber-optic strand (125-micron outer diameter)» Light-conducting core size of typically 62.5 micron» Called “62.5/125-micron” fiber» Other sizes are also used

– May use 50/125 (especially in Europe)

Many different types of connectors are available LAN usage is usually “multimode”, “graded index”

» Multimode supports different light modes, which may travel at different speeds

» Graded index resists pulse spreading due to different transmission speeds

Fiber-Optic Cables Approximately the same cost as good-quality coaxial cable

» Optical interfaces are the most expensive component» Transmission by Light Emitting Diodes (LEDs) or laser diodes» Reception by Positive Intrinsic Negative (PIN) diodes or avalanche

diodes

Best available communications media» Excellent electrical noise immunity» Difficult to tap (security)» Lightweight» Small size (frequency fits in existing cable trays)

Wireless Communications There are several different forms of wireless

communications Point-to-point microwave

» Requires “line of sight” between antennas» Antennas are often mounted on towers» Requires a license

Cellular» Uses the frequency range assigned to the cellular telephone» Shares the frequency range with other transmissions

Wireless Communications Wireless LANs

» Have been used for some time (e.g., in grocery store inventory scanners)

» Spread spectrum technology– Standards are being developed (IEEE 802.11)

Satellite Links

Satellite dish

Satellite

Satellite Links Potential of

» Multiples of 56-to-64 Kbps data rates» Low cost» Large area of reception (broadcast)» Distance-independent charging

Large propagation delay» 1-nsec/foot (3-nsec/meter) delay (speed of light)» 250-msec one-way delay for geosynchronous orbit

Moderate-cost earth stations are possible

Physical Interconnection Requirements

Communication Requirements

Essential issues in a data communication system:Physical Interface Connectors :

Shape, size, no. of pins, serial/parallel.Protocols

Rules of communication at various layers.

Codes/formats.

Communication Requirements (Cont.)

Basic concept behind a protocol is Handshaking (hardware) Syntax, semantics, and procedure rules

(software)

Communication Requirements (Cont.)

The protocol allows each party to show the other end that it has something to send, it is ready to accept messages, a message has been received, and the reception has been successful.

If any of the communication steps fails, the protocol should indicate this, and each party follows a predefined set of rules to handle the exception.

Purpose of Physical Layer Connections

The basic purpose of the OSI Physical Layer is»To adapt the digital signals to allow them to be communicated across the physical medium.

Examples include»Convert digital signals to tones for communications across a voice grade telephone circuit.

»Convert digital signals to light (on/off) for communications across a fiber-optic circuit.

Purpose of Physical Layer Connections (Contd.)

The communications circuit may need to be» Established (initially)» Controlled or maintained» Released when no longer needed

The Physical Layer may also be responsible for sharing (multiplexing) the communications circuit.

Inter- vs. Intracomputer Communications

Data communications characteristics differ from those within a computer system.» Bit serial transmission» Handling control information (inband control)» Higher error rate (need error detectioon and

correction) These issues are discussed on the

following slides

Inter- vs. Intracomputer Communications(Cont.)

Internal Bus

External Communications Line

Serial vs. Parallel Transmission

Internal computer buses transfer many bits in parallel.

Address

Timing & Control

Inband Control

Bit Serial transmission line

Which bits are data, which are address, and which are control ?

How is timing (clocking) determined at the receiver?

Framing Control

A sequence of bits on the line is called frame

There is a known format of the serial data frame

Control Information Data

Framing Control (Cont.)

Need to determine the beginning of the frame

Start Frame

Known format then provides separation of control and data

Start Control Information Data

Some Examples of Framing Control

Using the “flag” pattern of the data link protocols

Flag FrameFlag Flag

Using the Ethernet preamble/start pattern

Frame101010…1011

(Null)

The Token Ring start and stop indicators

Start EndFrame

Error Rates

The physical lines have inherently different error properties.

The average error rate: the fraction of bits delivered with errors; e.g.,one in 105 for telephone channels» For lengthy transmissions, this error rate is

often unsatisfactory» It must be improved by higher level protocol

mechanisms

Error Rates (Cont.)

Some media may have error rates as low as one in 1014 » May be adequate for many purposes; e.g.,

digitized images» Still typically have higher level protocol

recovery mechanisms

Switched Voice-Grade Telephone Channels

Direct-dial analog telephone channels» Dial-up modem use

Normal voice line» Limited to about 3000 Hz bandwidth

The local loop is a two-wire circuit» To the central office(exchange)

(Cont.)

Switched telephone networkModem Modem

Analog AnalogDigital Digital

(Cont.)

Home PC

Leased Voice-Grade Telephone Channels

Leased (dedicated) analog telephone channels» Sometimes called “conditioned” lines

Often used for 19.2-kbit/s transmission Fixed monthly cost, independent of

Leased Voice-Grade Telephone Channels

(Cont.)

4-wire modem4-wire modem

Two one-way analog circuits

Router

ModemModem

Analog Communications Channels

Voice-grade telephone channels have a 3kHz bandwidth» 300 to 3300 Hz

Data rate depends on BandWidth (BW)» The bit/s data rate is usually two to three time the

» For example, 9600 bit/s over 3000 Hz (3 kHz) Data rate also depends on the signal-to-

noise ratio

Digitized Voice Channels

Digitized voice channels can also be used for digital data

Analog voice signals are digitized

Samples 8000 samples per second

56 kbit/s or 64 kbit/sSend digitized value of each sample

7 or 8 bits per sample

Digitized Voice Channels (Cont.)

Digitized samples are placed in a slot in each frame

001..0

Frame N Frame N+1

001..0

Slot no. 2

Digitized Voice Channels (Cont.)

The frames for digitized voice have two different forms :T1 has 24 slots per frame

» 24 slots at 56 kbit/s (or 64 kbit/s)» A total of 1.544 Mbit/s

“E1”» 32 slots at 64 kbit/s» 2.048 Mbit/s

Digital Telephone Channels

Digital (instead of analog) telephone communications channels are also available

» 56 or 64 kbit/s channels (or a multiple)» 1.544 Mbit/s (US, Canada, and Japan) or

2.048Mbit/s (Europe) channels

Digital Telephone Channels (Cont.)

Instead of modem, Data Service Unit / Channel Service Unit (DSU/CSU) adapter devices are needed.» The DSU adapts the digital signal (transmit

and receive voltages and timing)» The CSU normalizes voltage levels,

provides maintenance capabilities, and protects the public network.

Digital Telephone Channels (Cont.)

Computer ComputerDSU/CSU DSU/CSU

DSU/CSU DSU/CSU

Inter-central office/exchange links

(high data rates)

Central office or exchange

Reason for Going Digital Computer data are inherently digital

» Adapt more easily to digital transmission

Easier to multiplex» Time Division Multiplexing (TDM)

Easier to switch Better error rate

» Noise is not cumulative, since repeaters can reject most induced noise

Repeater

Direction of Data Flow

Simplex

Half Duplex

Duplex (or Full duplex)

Synchronous /Asynchronous Transmission

Asynchronous Timing

Asynchronous means no predefined timing between characters

The sending and receiving ends provide their own clocking

The timing of asynchronous characters is

Character

Start bit

Next Character

Start bit

Asynchronous Timing (Contd.)

The receiver does not know when the next unit of data is coming » The term async frequently is used this way

Clocking at the Sending End

The sending device determines when to transmit the “start bit”» The start bit indicates the beginning of a character» The bits of the character follow with a well-

defined timing (LSB first)» A party (error-check) bit is generated and sent» There is at least one stop bit» There is an arbitrary time before the next

character is sent

Clocking at the Sending End (Contd.)

Each character is framed with these control bits

Memory

Serial

I/O hardware

Character

Start bit

Stop bit

Hardware generated

I/O = input/output

Synchronous Transmission

Has a known timing relationship between bits and characters

Characters are sent one after the other The receiver recovers this timing from

transitions in the arriving data

Start End

Characters

Modulation

Method used to transmit digital data across analog channels.

A primary example of analog channels is the telephone company’s voice-grade circuit.

There is one primary reason to use modems» To be compatible with the voice-grade channel

Modulation (Cont.)

The process of converting digital data into analog form is called modulation.

AnalogDigital

Generally, we get about 2 to3 bit/s per Hz of bandwidth of the analog channel (more or less based on complexity)

Data Communications Interfacing

Transmission line

interface device

Digital data

transmitter/ receiver

Transmission line

interface device

Digital data transmitter/ receiver

Bit-serial transmission line

(or bit-serial interface to

network

Data terminal equipment

Data circuit-terminating equipment

Generic interface to transmission medium

Data Communications Interfacing (Contd.)

Network

EIA 232/ V.24

interface

Modem Modem

External Modem Connections

Typical Modem Capabilities

Many modern modems can operate in a number of modes, which are negotiated when the connection is established.» V.32 operation at 9600 bit/s» Or V.32 bis at 14400 bit/s» Or V.42 bis at 2400 bit/s

Typical Modem Capabilities (Contd.)

Modems can automatically dial the telephone number» V.25 bis sync/async autodial» Or the non-CCITT Hayes AT command set

(discussed later) Modems can perform operations previously

done by software» V.42 error correction» V.42 bis error compression

Typical Modem Capabilities (Cont.)

Modems can “fall back” to a lesser data rate if needed for communications, and some can later “fall forward” when possible

Leased-line modems can automatically dial a backup line as needed.

The Hayes AT Command Set

The Hayes AT command set is an industry standard» Controls modem operation» Initiates dial sequence» Hangs up» Runs diagnostics» Selects data compression feature» Etc.

For more than 50 such modem commands

The Hayes AT Command Set (Contd.)

The AT commands start with an escape sequence and AT(tention)

An example AT command is to dial a number

+++ATDT18007654321 <cr>

When “D” is for “dial”, “T” is for “tone”, and “18007654321” is the telephone number

CCITT V.42 and V.42 bis Capabilities

The CCITT V.42 recommendation provides a reliable data transfer capability (error correction)» There are actually two forms (CCITT

couldn’t agree on only one)» The preferred approach s Link-Access

Procedure for Modems (LAPM)» MNP 4 is also included (see next slide)

CCITT V.42 and V.42 bis Capabilities (Contd.)

The CCITT recommendation V.42 bis builds on V.42» V.42 bis is a data compression standard» Uses an automatic adaptation algorithm

that handles different degrees of randomness in the data

» V.42 bis achieves a data compression factor of up to 4X

Microcom Network Protocol (MNP)

The Microcom Network Protocol (MNP) is a set of communications protocols for enhancing modem communications» Some are industry standards» Others are proprietary to Microcom

Three protocols are identified by terms such as » MNP 4, MNP class 4, or MNP level 4

Microcom Network Protocol (MNP) (Contd.)

MNP 4 is a reliable public-domain delivery protocol» MNP 4 is built into hundreds of thousands

of modems» MNP 4 is part of the CCITT V.42

recommendation

XMODEM File Transfer Protocol (1978)

XMODEM was the first file transfer protocol for use with PCs (XMODEM actually predates PCs and DOS)

XMODEM is available from many bulletin boards Transfers are limited in many ways

» Transfers data in small (128-byte) blocks (8-bit code)» Operates as a simple “stop and wait” ACK/NAK

protocol» Inefficient use of links in excess of 1200 bit/s

XMODEM File Transfer Protocol (Contd.)

There are many variations : YMODEM, ZMODEM, etc.» Larger block sizes» Better error detection

XMODEM File Transfer Protocol (Contd.)

The operating mode is negotiated at connection establishment

Kermit (1981)

Kermit is available on many bulletin boards

Kermit was developed at Columbia University» Well documented» Intended for use between different

computers– Mainframes, minis, PCs

Kermit (Contd.)

All transmitted bytes are printable ASCII (except ASCII “SOH” start) 7-bit code» Avoids problems with control characters, for

example, which might affect PAD operation.

Remote-Control Software

The idea is that the remote PC takes over control of the office PC» Remote keyboard and screen “mirrors” the

other PC operations» For access to your office PC from a remote

PC; e.g. a laptop» Or, to assist a remote user without having

to go to that location

Remote-Control Software (Contd.)

Remote-control software is required in both PCs» A typical configuration is shown in our

example internetwork

Remotely controlled

Roving laptop

Terminal Emulation

A terminal-emulation program allows your PC to appear to be a terminal that a remote host knows how to talk to » It may appear to be a scroll-mode terminal

(e.g., VT100)» It may appear to be a page-mode terminal

(e.g., an IBM 3270)

Terminal Emulation (contd.)

Terminal emulation is a common approach» To log in at a host or server» To log in at any other device to access

services» For network management

–To read and write network management objects (variables)

Fax Modem Facts

Some modems provide facsimile (fax) as well as data capabilities

Two commonly used recommendations for fax transmission» V.29at 9600bit/s» V.17 at 14400 bit/s

Fax Modem Facts (contd.)

Flow is unidirectional Support software is required

» Class 1: Minimal processing on the fax board

» Class 2: More on-board processing, less required by the PC

ANALOG AND DIGITAL PHYSICAL INTERFACES

The RS-232/CCITT V.24 and V.28 Interface

Out of Band

Control

Computer

DTEDCE

Out of Band

Control

DCEDTE

Computer

RS-232/CCITT V.24

DTE : Data Terminal Equipment

DCE : Data Circuit Termination Equipment

Data processing (DTE) to modem (DCE) interface

The CCITT V.24 Recommendation defines the interchange circuits» V.28 defines the electrical characteristics

The RS-232/CCITT V.24 and V.28 Interface (Cont.)

In EIA, known as RS-232-C (the 3rd [-C] version of RS-232)» More recent version of RS-232-D (now EIA-

232-D)» Sometimes TIA-232-D

(Telecommunications Industry Association)

A 25-pin connector/interface» ISO 2110 is used» Is not part of the RS-232-C standard

Bit serial data (full duplex) Out of band control lines

The RS-232/CCITT V.24 and V.28 With Null

Modems

Req to Send

Clear to Send

Data Set Ready

Signal Detect

Data Terminal Ready

Req to SendClear to Send

Data Set Ready

Signal Detect

Data Terminal Ready

Signal Ground

Note : There are many variations to Null Modem Cross Connection

DCEDTE Null Modem

Pin Assignments for V.24/EIA-232

14 15 2116 2017 1918 22 23 24 25

1 2 3 4 5 6 7 8 9 10 11 12 13

Shield

Tx Data

Rx Data

Reg to Send

Clear to Send

DCE Ready

Carrier Detect

Reserved for testing

Unassigned

Secn. Recv. Line Signal

Detector

Secn. CTS

Secondary Tx Data

Transmitter signal element

timing

Secondary received data

Transmitter signal element

timing

Local Loopback

Secondary RTS

DTE Ready

Remote Loopback

Ring Indicator

Data Signal Rate Select

Transmit signal element

timing

Test Mode

RS-232/CCITT V.24 & V.28 Related Products

It is often convenient to switch RS-232/V.24 signals from a computer to one of several devices» For example, to different types of printers

Simple “multiple” switches are available for this purpose

Specialized companies have been developed to handle the interface market with products such as» Multiple switches» RS-232/V.24 cables» Null modems» RS-232/V.24 “gender changers”

Breakout boxes to monitor control signals

RS-232/CCITT V.24 & V.28 Related Products (Cont.)

Limitations of RS-232/V.28

An upper data rate of about 20 kbit/s An upper cable length of about 50 to 100

feet (about 20 to 40 m) Some products are available to extend

these, but a new approach is needed

The Evolution of RS-232-C

RS-232-C

EIA-232-D (1987)

• Unbalanced circuits

RS-530 (1987)

•Balanced Circuits

RS-449 signals

RS-422/423 electrical (1977)

RS-442 balanced circuits

RS-443 unbalanced circuits

Synchronous Transmission

Has a known timing relationship between bits and characters

Characters are sent one after the other The receiver recovers this timing from

transitions in the arriving data

Start End

Characters

RS-423/CCITT V.10Single Ended Interchange Circuit

Signal return

Trans Recvr

ErrorNoise

Note: V.10 is the same as X.26 or RS-423-A (unbalanced)

RS-422/CCITT V.11 Differential Interchange Circuit

Trans Recvr

NoiseSensitive to

differential signal

Termination resistor

Noise was rejected

Note : V.11 is the same as X.27 or RS-422-A (balanced)

CCITT X.21 Interface

Physical-level interface between DTE and DCE

For synchronous operations on public data networks

X.21 uses control transitions and ASCII characters rather than using separate signal lines

CCITT X.21 Interface (Cont.)

The X.21 electrical characteristics are» CCITT X.27 (balanced; same as V.11 and

RS-422)» CCITT X.26 (unbalanced; V.10 and RS-

423)(Note: For operation above 9600 bit/s, X.27 is required)

X.21 mechanical characteristics are» 15-pin connector per ISO Standard 4903

Switched 64 kbit/s

DSU Bridge

Circuit Name Direction

To DCE / To DTEG Ground,Common Return

Ga DTE Common Return X

Gb DCE Common Return X

T Transmit X

R Receive X

C Control X

I Indication X

S Signal Timing X

B Byte Timing (Optional) X

CCITT X.21 bis

As an interim (perhaps longer term) provision, we have X.21 bis

X.21 bis utilizes RS-232 for use with X.25

Particularly used in countries where X.21 has not yet become available

CCITT X.21 bis (Cont.)

RS-232 signals are used to represent X.21 events» To initiate the call

Some X.21 features are not supported» Call progress signals

ISDN Interface

Power Source 3

Transmit

Receive

Power Source 2

Terminal Equipment (TE)

Transmit

Receive

Power Sink2

Network Equipment (NE)

LAN Cables and Interfaces

Primary Cable Types» Coaxial» Twisted Pair

– Unshielded Twisted Pair– Shielded Twisted Pair

» Fiber Optic

LAN Interfaces and Cables

Coaxial Cable

Thick Net (0.5 inch in diameter) Thin Net (0.25 inch in diameter) Connection Hardware

» BNC (British Naval Connector)» BNC T» Terminator

LAN Interfaces and Cables Unshielded Twisted Pair (UTP)

Cat 1 and Cat 2: Suitable only for voice and low data rates (less than 4 Mbps).

Cat 3: Suited for data rates up to 10 Mbps. Uses 4 twisted-pairs. Some schemes may support data rates up to 100 Mbps. Standard for most telephone installations.

Cat 4: Consists of 4 twisted-pairs. Suitable for data rates up to 16 Mbps.

Cat 5: Consists of 4 twisted-pairs. Suitable for data rates up to 100 Mbps.

Cable connector RJ-45

LAN Interfaces and Cables Physical Ethernet

Standards

10 BASE 5 » thick net, 10 Mbps, 500 m, bus

10 BASE 2 » thin net, 10 Mbps, 185 m, bus

10 BASE T» 2-twisted pair,10 Mbps, 100m, star

10 BASE F » fiber-optics,10 Mbps,500-2000m, star

LAN Interfaces and Cables Physical Ethernet Standards

(contd.)

100BASE TX» 2-twisted pairs (cat 5), 100 Mbps, 100m, star

100 BASE T4» 4-twisted pairs (cat 3,4,5), 100 Mbps, 100m, star

100 BASE-FX» fiber-optic, 100 Mbps, 2000m, star

LAN Interfaces and Cables Types of Ethernet

connectors

British Naval Connector (BNC) (used with coax cables)

Attachment Unit Interface (AUI)» DIX : It is a 15-pin connector (AUI) used to

interface Ethernet components. RJ-45 connector (used with twisted-pair

cables)» RJ-11 are used in telephone installations

LAN Interfaces and Cables 10BASE2 (Thinnet)

It uses the onboard transceivers of the NIC to translate the signals to and from the rest of the network.

Thinnet uses BNC T-connectors that directly attach to the NIC.

Each end of the cable should have a terminator, and a terminator at one end should be grounded.

LAN Interfaces and Cables 10BASE5 (Thicknet)

It uses an external transceiver to attach to the NIC. The external transceiver clamps to the thicknet

cable.

An AUI cable must run from the transceiver to a DIX connector on the back of the NIC.

Each segment should be terminated at both ends, with one terminator grounded.

LAN Interfaces and Cables 10BASE-T

It is based on the IEEE 802.3 standard. 10Base-T supports a data rate of 10 Mbps

using baseband. 10Base-T cabling is wired in a star topology,

because nodes are wired to a central hub. A 10Base-T network functions logically as a

linear bus. The cable uses RJ-45 connections, and the

NIC can have RJ-45 jacks built into the back of the card.

LAN Interfaces and Cables 100BASE-X (Fast Ethernet)

It uses a star bus topology. It provides a data transmission speed of 100

Mbps using baseband. Other specifications:

» 100Base-TX: 2 TP of cat 5 UTP or STP» 100Base-T4:4 TP of cat3, 4, or 5 UTP

Compatible with 10Base-T systems.

LAN Interfaces and Cables RJ-45 Connector Specifications

EIA/TIA 568B or AT&T 258A RJ-45

T2 White/Orange

R2 Orange/White

T3 White/Green

R3 Green/White

LAN Interfaces and Cables Configuration mode of the

ports Normal (MDI-X). Uplink (MDI).

Types of cabling– Straight through cables.– Crossover cables.– Roll-over cables.

LAN Interfaces and Cables Configuration mode of the Ports

(contd.)

MDI (Medium Dependent Interface) MDI-X (Medium Dependent Interface-X)

Computers, Routers

Hubs, Switches

LAN Interfaces and Cables Cable Types (10/100BASE-

Straight Through» MDI<-> MDI-X or otherwise

Computers, Routers

Hubs, Switches

LAN Interfaces and Cables Cable Types (10/100BASE-T)

(contd.)

Cross Over» MDI<->MDI, MDI-X<->MDI-X

Multiplexing

It costs about the same amount of money to install and maintain a high bandwidth cable as a low bandwidth wire between two stations

Need for multiplexing techniques to share a single communication channel between multiple stations.

Multiplexing of Communications Links

Remote terminal

Two classes of multiplexing schemes :» Frequency Division Multiplexing (FDM)

The frequency spectrum is divided among the logical channel, with each station having exclusive possession of its frequency band. Filters limit the usable bandwidth per channel.

Multiplexing (Cont.)

» Time Division Multiplexing

The stations take turns, each one periodically getting the entire bandwidth for a short interval of time.

Time Division Muxes

A TDM combines signals onto a high speed link, and then sends those signals sequentially at fixed time intervals.

Each user interface is allocated a time slot within which its data is transmitted.

Data is usually sent one char at a time

Combined signal rates > 100 Mbps.

Time Division Muxes

Ethernet

Token Ring

MainframeEthernet

Token Ring

Mainframe

MTEMTE

Aggregate pathway

Muxing

De-Muxing

Time Division Multiplexing

Each user gets the channel’s full capacity for a period of time

Each user gets a time slot in each frame

Start User NUser1User2 User3 Start User1

One Frame One character of user data is sent in each slot If a user has nothing to send, the slot contains “null”

TDM Strengths

+ Dedicated bandwidth partitions

=> Guaranteed throughput & no loss.

+ Versatile & scaleable.

+ Low cost compared to Stat. TDM.

+ Proven Reliable data transport.

TDM Weaknesses

-- Bandwidth of idle sources is lost.

-- Minimal internetworking capability.

Statistical Time Division Multiplexing (STDM)

Few users fill every slot assigned to them

This results in wasted slots A better approach is statistical TDM It operates as follows

» A user character is “tagged” with the port number

Statistical TDM Based on the premise that stations rarely

need to transmit data constantly at full available speed.

Attempts to move as much data as possible across the common channel.

Combined bandwidth of all sources exceeds the available bandwidth.

Allocates time slots on-demand, constantly evaluating traffic needing to be sent (based on priority).

Statistical TDM (Cont.)

Port no.

Data fieldControl field

Frame of tagged

characters

» Example

Character

In case demand exceeds capacity, lower-priority traffic is off-loaded into a buffer and delayed for retransmission during a non-peak period

=>More complex front-end management.

Greater degree of intelligence.

Greater computer power.

Statistical multiplexing can be generalized to produce packet switching

» More control information» Multiple characters of data

Statistical TDM strengths

+ Supports more data than available bandwidth

=> better bandwidth utilization.

+ Critical data can be given higher priority.

Statistical TDM Weaknesses

-- Requires more management and more expensive to operate.

-- Low priority data can suffer excessive delays.

-- Data may get lost.

(No guaranteed bandwidth)

Data Link Control

Role of Data Link Layer

Provide reliable communication between adjacent nodes

DLL Design Issues

Framing and frame synchronization Sequenced Delivery of Frames Error and Flow Control Addressing (multi-access link) Link Management

Link Management

sender receiver

synchronize

Negotiate connection

synchronize

Acknowledge

Connection established

Data transfer (send segments)

Link Management

sender receivertransmit

not ready

Resume Transmission

Buffer full process segments

Buffer OK

Flow Control with Windowing

In the most basic form of reliable connection-oriented transfer, data segments must be delivered to the recipient in the same sequence that they were transmitted.

Windowing is a method to control the amount of information transferred end-to-end. Some protocols measure information in terms of number of packets

Windowing (contd.)

send 1 window size =1 receive 1

send 2 receive 2

send 1 window size =3 receive 1 send 2 receive 2 send 3 receive 3 Ack 4 send 4

sender receiver

Error Control

Reliable delivery guarantees that a stream of data sent from one machine will be delivered through a functioning data link to another machine without duplication or data loss. Positive acknowledgement with retransmission (PAR) is one technique that guarantees reliable delivery of data streams.

The sender keeps the record of each segment it sends and waits for an acknowledgement.

The sender also starts a timer when it sends a segment, and it retransmits a segment it the timer expires before an acknowledgement arrives.

An Acknowledgement Technique

send 1 send 2 send 3 Ack 4 send 4 send 5

send 6Ack 5

send 5Ack 7

sender receiver

1 2 3 4 5 61 2 3 4 5 6

Error Control (contd.)

An error check is appended to each PDU.» Typically a Cyclic Redundancy Check (CRC)» CRC is 16 bits in length

Good PDUs are ACKed Bad PDUs are discarded (Rejec mode) or NAKed

(Selective Reject mode). If ACK (or NAK) is not received with a timeout interval,

the PDU is retransmitted.

Examples of DLL Protocols

Binary Synchronous Communication (BSC, also known as bisync.» Character-oriented link protocol from the 60’s» Utilizes special characters to delimit the frames» Frame length is an integral number of characters» Uses ASCII, EBCDIC, or transcode character sets» Half-duplex operation» For point-to-point or multipoint operation

SYN DLESYN SYN STXDLE Data ETX CRC

Examples of DLL Protocols (contd.)

High-Level Data Link Control

HDLC is an umbrella specification» There are many variations of HDLC» There are variations to support

– X.25 WANs (Link Access Procedure Balanced LAPB)

– ISDN (LAPD)

– LANs

– MODEM operations

» HDLC is a bit-oriented protocol and is independent of any code set.

Flag Flag Address Control Data CRC Flag

LAN Data-Link Sublayers

Network LLC

Data Link MAC

Physical

Logical Link Control

Media Access Control

MAC Frame 802.2 LLC Packet or datagram

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