Title: Data Communication Basics
1Data Communication Basics
- Sadiq M. Sait, Ph.D
- sadiq_at_ccse.kfupm.edu.sa
- Department of Computer Engineering
- King Fahd University of Petroleum and Minerals
- Dhahran, Saudi Arabia
2 3Electromagnetic 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)
4Transmission 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
5Guided Transmission Configurations
6Guided Transmission Configurations
Transmitter/ Receiver
Transmitter/ Receiver
Transmitter/ Receiver
Transmitter/ Receiver
Amplifier or repeater
Medium
Medium
0 or more
Multipoint
7Point-to-point transmission characteristics
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.
8Twisted-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
9Twisted-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
10Coaxial 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
11Fiber-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
12Fiber-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
13Fiber-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)
14Wireless 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)
15Satellite Links
16Satellite 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
17- Physical Interconnection Requirements
18Communication 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)
19Communication 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.
20Purpose 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.
21Purpose 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.
22Inter- 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
23Inter- vs. Intracomputer Communications(Cont.)
Host Internal Bus
External Communications Line
24Serial vs. Parallel Transmission
- Internal computer buses transfer many bits in
parallel.
Data
Address
Timing Control
25Inband Control
Bit Serial transmission line
- Which bits are data, which are address, and which
are control ? - How is timing (clocking) determined at the
receiver?
26Framing Control
- A sequence of bits on the line is called frame
- There is a known format of the serial data frame
Control Information
Data
27Framing 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
28Some 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
29Error 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
30Error 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
31Switched 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)
32Switched Voice-Grade Telephone Channels (Cont.)
Analog
Analog
Digital
Digital
Switched telephone network
Modem
Modem
33Switched Voice-Grade Telephone Channels (Cont.)
Home PC
PSTN
PAD
34Leased 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
35Leased Voice-Grade Telephone Channels (Cont.)
Two one-way analog circuits
4-wire modem
4-wire modem
Router
Router
Modem
Modem
Modem
PSTN
36Analog 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
37Digitized 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
38Digitized Voice Channels (Cont.)
- Digitized samples are placed in a slot in each
frame
Frame N
Frame N1
001..0
001..0
Slot no. 2
39Digitized 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
40Digital 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
41Digital 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.
42Digital 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
43Reason 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
44Direction of Data Flow
45- Synchronous /Asynchronous Transmission
46Asynchronous 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
47 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
48Clocking 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
49Clocking 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
50Synchronous 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
51Modulation
- 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
52Modulation (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)
53Data 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
54Data Communications Interfacing (Contd.)
EIA 232/ V.24 interface
Network
Modem
Modem
55External Modem Connections
56Typical 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
57Typical 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
58Typical 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.
59The 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
60The 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
61CCITT 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)
62CCITT 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
63Microcom 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
64Microcom 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
65XMODEM 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
66XMODEM File Transfer Protocol (Contd.)
- There are many variations YMODEM, ZMODEM, etc.
- Larger block sizes
- Better error detection
67XMODEM File Transfer Protocol (Contd.)
- The operating mode is negotiated at connection
establishment
68Kermit (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
69Kermit (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.
70Remote-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
71Remote-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
72Terminal 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)
73Terminal 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)
74Fax 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
75Fax 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
76- ANALOG AND DIGITAL PHYSICAL INTERFACES
77The 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
78The 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
79The 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)
80The 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
81The 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
82Pin 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
83RS-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
84RS-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
85Limitations 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
86The 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
- EIA-232-D (1987)
- Unbalanced circuits
87Synchronous 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
88RS-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)
89RS-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)
90CCITT 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
91CCITT 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
92CCITT X.21 Interface (Cont.)
4
Switched 64 kbit/s
X.21
DSU
Bridge
93CCITT 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
94CCITT 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
95CCITT 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
96ISDN 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
97LAN Cables and Interfaces
- Primary Cable Types
- Coaxial
- Twisted Pair
- Unshielded Twisted Pair
- Shielded Twisted Pair
- Fiber Optic
98LAN 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
99LAN 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
100LAN 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
101LAN 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
102LAN 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
103LAN 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.
104LAN 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.
105LAN 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.
106LAN 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.
107LAN Interfaces and Cables RJ-45 Connector
Specifications
- EIA/TIA 568B or ATT 258A RJ-45
- T2 White/Orange
- R2 Orange/White
- T3 White/Green
- R1
- T1
- R3 Green/White
- T4
- R4
Pin 1 Pin 8
108 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.
109LAN 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
110LAN 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
111LAN 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
112 113Multiplexing
- 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.
114Multiplexing of Communications Links
Modem
Modem
MUX
MUX
CPU
Remote terminals
115Multiplexing (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.
116Multiplexing (Cont.)
- Time Division Multiplexing
- The stations take turns, each one periodically
getting the entire bandwidth for a short interval
of time. -
117Time 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.
118Time Division Muxes
Muxing
Ethernet
.
Token Ring
.
MTEMTE
Ethernet
.
Mainframe
Token Ring
MTEMTE
Mainframe
Aggregate pathway
De-Muxing
119Time 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
120TDM Strengths
- Dedicated bandwidth partitions
- gt Guaranteed throughput no loss.
- Versatile scaleable.
- Low cost compared to Stat. TDM.
- Proven Reliable data transport.
121TDM Weaknesses
- -- Bandwidth of idle sources is lost.
- -- Minimal internetworking capability.
122Statistical 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
123Statistical 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).
124Statistical TDM (Cont.)
Data field
Control field
Port no.
Character
(5)
(8)
Frame of tagged characters
125Statistical 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
126Statistical 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)
127 128Role of Data Link Layer
- Provide reliable communication between adjacent
nodes
129DLL Design Issues
- Framing and frame synchronization
- Sequenced Delivery of Frames
- Error and Flow Control
- Addressing (multi-access link)
- Link Management
130Link Management
receiver
sender
synchronize
Negotiate connection
synchronize
Connection established
Data transfer (send segments)
131Link Management
transmit
Buffer full process segments Buffer OK
not ready
ready
Resume Transmission
132Flow 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
133Windowing (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
134Error 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.
135An Acknowledgement Technique
- send 1
- send 2
- send 3
- Ack 4
- send 4
- send 5
- send 6
- Ack 5
- send 5
- Ack 7
-
X
136Error 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.
137Examples 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
138Examples 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
139LAN Data-Link Sublayers
-
- Network LLC
- Data Link MAC
- Physical
Logical Link Control
Media Access Control
MAC Frame 802.2 LLC Packet or datagram