Data Communication Basics

1
Data Communication Basics

• Habib Youssef, Ph.D
• youssef_at_ccse.kfupm.edu.sa
• Department of Computer Engineering
• King Fahd University of Petroleum and Minerals
• Dhahran, Saudi Arabia

COMPUTER NETWORKS
2

• Transmission
• Media

3
Electromagnetic spectrum for telecommunications
Frequency (hertz)
102 103 104 105 106 107
108 109 1010 1011
1012 1013 1014 1015
ELF VF VLF LF MF HF VHF
UHF SHF EHF
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
Coaxial Cable
optical fiber
Terrestrial and Satellite Transmission
106 105 104 103 102 101
100 10-1 10-2 10-3
10-4 10-5 10-6
Wavelength in space (meters)
4
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

5
Guided Transmission Configurations
6
Guided Transmission Configurations
Transmitter/ Receiver
Transmitter/ Receiver
Transmitter/ Receiver
Transmitter/ Receiver
Amplifier or repeater
Medium
Medium
0 or more
Multipoint
7
Point-to-point transmission characteristics of
guided media
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

• The medium itself is more important than other
  factors in determining transmission limitations
• For unguided media, range of frequencies is of
  more importance.

8
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

9
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

10
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

11
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

12
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

Light
//
//
Electrical
Electrical
13
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

14
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)

15
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

16
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)

17
Satellite Links
18
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

19

• Physical Interconnection Requirements

20
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.

21
Communication Requirements (Cont.)

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

22
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.

23
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.

24
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.

25
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

26
Inter- vs. Intracomputer Communications(Cont.)
Host Internal Bus
External Communications Line
27
Serial vs. Parallel Transmission

• Internal computer buses transfer many bits in
  parallel.

Data
Address
Timing Control
28
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?

29
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
30
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
31
Some Examples of Framing Control

• Using the flag pattern of the data link
  protocols

Flag
Frame
Flag
Flag

• Using the Ethernet preamble/start pattern

Frame
1010101011
(Null)

• The Token Ring start and stop indicators

Start
End
Frame
32
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

33
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

34
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)

35
Switched Voice-Grade Telephone Channels (Cont.)
Analog
Analog
Digital
Digital
Switched telephone network
Modem
Modem
36
Switched Voice-Grade Telephone Channels (Cont.)
Home PC
PSTN
PAD
37
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

38
Leased Voice-Grade Telephone Channels (Cont.)
Two one-way analog circuits
4-wire modem
4-wire modem
Router
Router
Modem
Modem
Modem
PSTN
39
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

40
Digitized Voice Channels

• Digitized voice channels can also be used for
  digital data
• Analog voice signals are digitized

Signal Level
Time
56 kbit/s or 64 kbit/s
8000 samples per second
Samples
Send digitized value of each sample 7 or 8 bits
per sample
41
Digitized Voice Channels (Cont.)

• Digitized samples are placed in a slot in each
  frame

Frame N
Frame N1
001..0
001..0
Slot no. 2
42
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

43
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

44
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.

45
Digital Telephone Channels (Cont.)
Computer
Computer
DSU/CSU
DSU/CSU
Inter-central office/exchange links (high data
rates)
DSU/CSU
DSU/CSU
Central office or exchange
Central office or exchange
46
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
47
Direction of Data Flow

• Simplex

• Half Duplex

• Duplex (or Full duplex)

48

• Synchronous /Asynchronous Transmission

49
Asynchronous Timing

• Asynchronous means no predefined timing between
  characters
• The sending and receiving ends provide their own
  clocking
• The timing of asynchronous characters is

Start bit
Start bit
Character
Next Character
T
50
Asynchronous Timing (Contd.)

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

Async
X.25
PAD
51
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

52
Clocking at the Sending End (Contd.)
Stop bit
Start bit
Serial I/O hardware
P
Character
Memory
Hardware generated

• Each character is framed with these control bits

I/O input/output
53
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

1
0
Start
End
Characters
54
Modulation

• Method used to transmit digital data across
  analog channels.
• A primary example of analog channels is the
  telephone companys voice-grade circuit.
• There is one primary reason to use modems
• To be compatible with the voice-grade channel

55
Modulation (Cont.)

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

Analog
Digital

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

56
Data Communications Interfacing
Bit-serial transmission line (or bit-serial
interface to network
Transmission line interface device
Digital data transmitter/ receiver
Transmission line interface device
Digital data transmitter/ receiver
Data terminal equipment (DTE)
Data circuit-terminating equipment (DCE)
Generic interface to transmission medium
57
Data Communications Interfacing (Contd.)
EIA 232/ V.24 interface
Network
Modem
Modem
58
External Modem Connections
59
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

60
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

61
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.

62
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

63
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 ltcrgt
• When D is for dial, T is for tone, and
  18007654321 is the telephone number

64
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 couldnt
  agree on only one)
• The preferred approach s Link-Access Procedure
  for Modems (LAPM)
• MNP 4 is also included (see next slide)

65
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

66
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

67
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

68
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

69
XMODEM File Transfer Protocol (Contd.)

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

70
XMODEM File Transfer Protocol (Contd.)

• The operating mode is negotiated at connection
  establishment

71
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

72
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.

73
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

74
Remote-Control Software (Contd.)

• Remote-control software is required in both PCs
• A typical configuration is shown in our example
  internetwork

Roving laptop
PSTN
Remotely controlled
75
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)

76
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)

77
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

78
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

79

• ANALOG AND DIGITAL PHYSICAL INTERFACES

80
The RS-232/CCITT V.24 and V.28 Interface
RS-232/CCITT V.24
Computer
Computer
Data
Data
Modem
Modem
DTE
DTE
DCE
DCE
Out of Band Control
Out of Band Control
DTE Data Terminal Equipment DCE Data Circuit
Termination Equipment
81
The RS-232/CCITT V.24 and V.28 Interface (Cont.)

• Data processing (DTE) to modem (DCE) interface
• The CCITT V.24 Recommendation defines the
  interchange circuits
• V.28 defines the electrical characteristics

82
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)

83
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

84
The RS-232/CCITT V.24 and V.28 With Null Modems
DCE
DTE
Null Modem
Data
Data
2
2
Data
Data
3
3
Req to Send
Req to Send
4
4
Clear to Send
Clear to Send
5
5
Data Set Ready
Data Set Ready
6
6
Signal Detect
Signal Detect
8
8
Data Terminal Ready
Data Terminal Ready
20
20
Signal Ground
7
7
Note There are many variations to Null Modem
Cross Connection
85
Pin Assignments for V.24/EIA-232
Reserved for testing
Secn. CTS
Unassigned
Shield
Clear to Send
GND
Rx Data
Secn. Recv. Line Signal Detector
DCE Ready
Carrier Detect
Tx Data
Reg to Send
Reserved for testing
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
21
16
20
17
19
18
22
23
24
25
Transmitter signal element timing
Secondary RTS
Test Mode
Transmitter signal element timing
Remote Loopback
Data Signal Rate Select
Transmit signal element timing
Local Loopback
DTE Ready
Secondary received data
Ring Indicator
Secondary Tx Data
86
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

87

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

• 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

88
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

89
The Evolution of RS-232-C
RS-449 signals RS-422/423 electrical
(1977) RS-442 balanced circuits RS-443 unbalanced
circuits
RS-232-C

• RS-530 (1987)
• Balanced Circuits

• V.35
• Balanced Circuits

• EIA-232-D (1987)
• Unbalanced circuits

90
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

1
0
Start
End
Characters
91
RS-423/CCITT V.10Single Ended Interchange
Circuit
Noise
Error
Recvr
Trans
Signal return
Note V.10 is the same as X.26 or RS-423-A
(unbalanced)
92
RS-422/CCITT V.11 Differential Interchange
Circuit
Noise
Sensitive to differential signal
Recvr
Trans
Noise was rejected
Termination resistor
Note V.11 is the same as X.27 or RS-422-A
(balanced)
93
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

94
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

95
CCITT X.21 Interface (Cont.)
4
Switched 64 kbit/s
X.21
DSU
Bridge
96
CCITT X.21 Interface (Cont.)
Circuit Name Direction To DCE /
To DTE G 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
97
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

98
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

99
ISDN Interface
Terminal Equipment (TE)
Network Equipment (NE)
a
a
Power Source 3
b
b
c
c
d
d
Transmit
Transmit
Receive
e
Receive
e
f
f
g
Power Source 2
g
Power Sink2
h
h
100
LAN Cables and Interfaces

• Primary Cable Types
• Coaxial
• Twisted Pair
• Unshielded Twisted Pair
• Shielded Twisted Pair
• Fiber Optic

101
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

102
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

103
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

104
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

105
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

106
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.

107
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.

108
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.

109
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-T44 TP of cat3, 4, or 5 UTP
• Compatible with 10Base-T systems.

110
LAN Interfaces and Cables RJ-45 Connector
Specifications

• EIA/TIA 568B or ATT 258A RJ-45

• 1
• 2
• 3
• 4
• 5
• 6
• 7
• 8

• T2 White/Orange
• R2 Orange/White
• T3 White/Green
• R1
• T1
• R3 Green/White
• T4
• R4

Pin 1 Pin 8
111

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.

112
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

113
LAN Interfaces and Cables Cable Types
(10/100BASE-T)

• Straight Through
• MDIlt-gt 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

114
LAN Interfaces and Cables Cable Types
(10/100BASE-T) (contd.)

• Cross Over
• MDIlt-gtMDI, MDI-Xlt-gtMDI-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
115

• Multiplexing

116
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.

117
Multiplexing of Communications Links
Modem
Modem
MUX
MUX
CPU
Remote terminals
118

Multiplexing (Cont.)

• 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.

119
Multiplexing (Cont.)

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

120
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 gt 100 Mbps.

121
Time Division Muxes
Muxing
Ethernet
.
Token Ring
.
MTEMTE
Ethernet
.
Mainframe
Token Ring
MTEMTE
Mainframe
Aggregate pathway
De-Muxing
122
Time Division Multiplexing

• Each user gets the channels full capacity for a
  period of time
• Each user gets a time slot in each frame

Start
User N
User1
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

123
TDM Strengths

• Dedicated bandwidth partitions
• gt Guaranteed throughput no loss.
• Versatile scaleable.
• Low cost compared to Stat. TDM.
• Proven Reliable data transport.

124
TDM Weaknesses

• -- Bandwidth of idle sources is lost.
• -- Minimal internetworking capability.

125
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

126
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).

127
Statistical TDM (Cont.)

• Example

Data field
Control field
Port no.
Character
(5)
(8)
Frame of tagged characters
128
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
• gtMore complex front-end management.
• Greater degree of intelligence.
• Greater computer power.

129
Statistical TDM (Cont.)

• Statistical multiplexing can be generalized to
  produce packet switching
• More control information
• Multiple characters of data

130
Statistical TDM strengths

• Supports more data than available bandwidth
• gt better bandwidth utilization.
• Critical data can be given higher priority.

131
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)

132

• Data Link Control

133
Role of Data Link Layer

• Provide reliable communication between adjacent
  nodes

134
DLL Design Issues

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

135
Link Management
receiver
sender
synchronize
Negotiate connection
synchronize
Connection established
Data transfer (send segments)
136
Link Management
transmit
Buffer full process segments Buffer OK
not ready
ready
Resume Transmission
137
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

138
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

139
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.

140
An Acknowledgement Technique

• send 1
• send 2
• send 3
• Ack 4
• send 4
• send 5
• send 6
• Ack 5
• send 5
• Ack 7

X
141
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.

142
Examples of DLL Protocols

• Binary Synchronous Communication (BSC, also known
  as bisync.
• Character-oriented link protocol from the 60s
• 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
DLE
SYN
SYN
STX
DLE
Data
ETX
CRC
143
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
144
LAN Data-Link Sublayers

• Network LLC
• Data Link MAC
• Physical

Logical Link Control
Media Access Control
MAC Frame 802.2 LLC Packet or datagram