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Data Communication Basics
Sadiq M. Sait, Ph.D
Department of Computer Engineering
King Fahd University of Petroleum and Minerals
Dhahran, Saudi Arabia
Transmission
Media
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
Transmitter/ Receiver
MediumAmplifier
or repeaterMedium
Transmitter/ Receiver
Multipoint
0 or more
Transmitter/ Receiver
Transmitter/ Receiver
Point-to-point transmission characteristics of guided
media
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 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.
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 detection 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 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)
Switched Voice-Grade Telephone Channels
(Cont.)
Switched telephone networkModem Modem
Analog AnalogDigital Digital
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
Digitized Voice Channels
Digitized voice channels can also be used for digital data
Analog voice signals are digitized
Time
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
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
T
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
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 (Contd.)
Each character is framed with these control bits
Memory
Serial
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
10
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
(DTE)
Data circuit-terminating equipment
(DCE)
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
PSTN
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
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 3rd [-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
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 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)
a
b
c
d
e
f
g
h
a
b
c
d
e
f
g
h
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)
Cable
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
1
2
3
4
5
6
7
8
Pin 1
Pin 8
T2 White/Orange
R2 Orange/White
T3 White/Green
R1
T1
R3 Green/White
T4
R4
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)
1 TX+
2 TX-
3 RX+
4
5
6 RX-
7
8
MDI
Computers, Routers
1 RX+
2 RX-
3 TX+
4
5
6 TX-
7
8
MDI-X
Hubs, Switches
LAN Interfaces and Cables Cable Types (10/100BASE-
T)
Straight Through» MDI<-> MDI-X or otherwise
1 TX+
2 TX-
3 RX+
4
5
6 RX-
7
8
MDI
Computers, Routers
1 RX+
2 RX-
3 TX+
4
5
6 TX-
7
8
MDI-X
Hubs, Switches
LAN Interfaces and Cables Cable Types (10/100BASE-T)
(contd.)
Cross Over» MDI<->MDI, MDI-X<->MDI-X
1 TX+
2 TX-
3 RX+
4
5
6 RX-
7
8
MDI
1 TX+
2 TX-
3 RX+
4
5
6 RX-
7
8
MDI
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.
Multiplexing of Communications Links
MUX
Modem
Modem
MUX
CPU
Remote terminal
s
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.
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
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
(5)
(8)
Frame of tagged
characters
» Example
Character
Statistical TDM (Cont.)
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.
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
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 Ack 2
send 2 receive 2 Ack 3
send 1 window size =3 receive 1 send 2 receive 2 send 3 receive 3 Ack 4 send 4
sender receiver
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
X
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
