Physical Interconnection Requirements

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

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 too 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 (Cont.)

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.)

Host

Internal Bus

External Communications Line

Serial vs. Parallel Transmission

Internal computer buses transfer many bits in parallel.

Data

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

Frame

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 physically 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)

Switched Voice-Grade Telephone Channels

(Cont.)

Modem Modem

Analog AnalogDigital Digital

Switched telephone network

Switched Voice-Grade Telephone Channels

(Cont.)

PSTN

PAD

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

usage

Leased Voice-Grade Telephone Channels

(Cont.)

4-wire modem4-wire modem

Two one-way analog circuits

Router

Router

PSTN

Modem

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

BW

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

noise ratio

Analog Communications Channels (Cont.)

300 3300

3 kH bandwidth

Pass

100%

Frequency, Hz

kHz=1000 hertz

Digitized Voice Channels

Digitized voice channels can also be used for digital data

Analog voice signals are digitized

Time

Samples 8000 samples per second56 kbit/s

or 64 kbit/s

Send 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” or CEPT» 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 protect the public network.

Digital Telephone Channels (Cont.)

DSU/CSU DSU/CSU

DSU/CSU DSU/CSU

Inter-central office/exchange links

(high data rates)

Central office or exchange

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)

ANALOG AND DIGITAL PHYSICAL INTERFACES

COMPUTER NETWORK

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

Data

Out of Band

Control

Computer

DTEDCE

Modem

Data

Out of Band

Control

DCEDTE

Computer

Modem

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 third [-C] version of RS-232)» More recent version of RS-232-D (now EIA-

232-D)» Sometimes TIA-232-D

(Telecommunications Industry Association)

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

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 Interface (Cont.)

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

Modems

2

3

4

5

6

8

20

7

2

3

4

5

6

8

20

7

Data

Data

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

Data

Data

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

GND

Carrier Detect

Reserved for testing

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

V.35

•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

1

0

Characters

V.24/EIA-232 dial-up operation

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

CCITT X.21 Interface (Cont.)

X.21

Switched 64 kbit/s

DSU Bridge

4

CCITT X.21 Interface (Cont.)

Circuit Name DirectionTo DCE/To DTE

G Ground, Common ReturnGaGb

DTE Common ReturnDCE Common Return

X X

TR

TransmitReceive

X X

CI

ControlIndication

X X

SB

Signal TimingByte Timing (Optional)

X 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)

a

b

c

d

e

f

g

h

a

b

c

d

e

f

g

h

Transmit

Receive

Power Sink2

Network Equipment (NE)

COMPUTER NETWORK

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

T

Character

Start bit

Next Character

Start bit

Asynchronous Timing (Cont.)

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

X.25

PAD

Async

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 (Cont.)

Each character is framed with these control bits

MemorySerial

I/O hardware

Character

Start bit

P

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

1

0

Characters

Modulation

We will explore methods 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

(DTE)

Data circuit-terminating equipment

(DCE)

Generic interface to transmission medium

Data Communications Interfacing (Cont.)

Network

EIA 232/ V.24 interface

Modem Modem

External Modem Connections

CCITT Modems

Typical Modern 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 Modern Modem Capabilities (Cont.)

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 (discussed later)» V.42 bis error compression (discussed later)

Typical Modern 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 (Cont.)

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 Modern 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 Modern Capabilities (Cont.)

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) (Cont.)

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» Operates as a simple “stop and wait” ACK/NAK

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

XMODEM File Transfer Protocol (Cont.)

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

DOS = disk operating system

ACK = acknowledgement

NAK = negative acknowledgement

XMODEM File Transfer Protocol (Cont.)

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 (Cont.)

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 (Cont.)

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

example internetwork

PSTN

Remotely controlled

Roving laptop

Terminal Emulation

A terminal-emulation program allows your PC to appear to be a terminal hat a remote host knows how 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 (Cont.)

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 (Cont.)

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

COMPUTER NETWORK

MULTIPLEXING

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.

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.)

Multiplexing (Cont.)

» Time Division Multiplexing

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

Multiplexing of Communications Links

MUX

Modem Modem

MUX

CPU

Remote terminals

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 NUser1 User2 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”

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 Time Division Multiplexing (Cont.)

Port no.

Character

Data fieldControl field

(5) (8) Frame of tagged

characters

» For example

Statistical Time Division Multiplexing (Cont.)

Statistical multiplexing can be generalized to produce packet switching

» More control information» Multiple characters of data

Typical Statistical Multiplexer (STAT MUX)

Example

CPU

MUX

Supervisory Terminal

Modem

Modem

MUX

Terminal

Printer