Physical%20Interconnection%20Requirements

1
Physical Interconnection Requirements

• 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
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.

3
Communication Requirements (Cont.)

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

4
Communication Requirements (Cont.)

• The protocol allows each party to show the other
  end that it has something to send, it is ready to
  accept messages, a message has been received, and
  the reception has been successful.
• If any of the communication steps fails, the
  protocol should indicate this, and each party
  follows a predefined set of rules too handle the
  exception.

5
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.

6
Purpose of Physical Layer Connections (Cont.)

• The communications circuit may need to be
• Established (initially)
• Controlled or maintained
• Released when no longer needed
• The Physical Layer may also be responsible for
  sharing (multiplexing) the communications circuit.

7
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

8
Inter- vs. Intracomputer Communications(Cont.)
Host Internal Bus
External Communications Line
9
Serial vs. Parallel Transmission

• Internal computer buses transfer many bits in
  parallel.

Data
Address
Timing Control
10
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?

11
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
12
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
13
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
14
Error Rates

• The physically lines have inherently different
  error properties.
• The average error rate the fraction of bits
  delivered with errors e.g.,one in 105 for
  telephone channels
• For lengthy transmissions, this error rate is
  often unsatisfactory
• It must be improved by higher level protocol
  mechanisms

15
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

16
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)

17
Switched Voice-Grade Telephone Channels (Cont.)
Analog
Analog
Digital
Digital
Modem
Modem
Switched telephone network
18
Switched Voice-Grade Telephone Channels (Cont.)
Home PC
PSTN
PAD
19
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

20
Leased Voice-Grade Telephone Channels (Cont.)
Two one-way analog circuits
4-wire modem
4-wire modem
Router
Router
Modem
Modem
Modem
PSTN
21
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

22
Analog Communications Channels (Cont.)
Pass
100
3 kH bandwidth
Frequency, Hz
300
3300
kHz1000 hertz
23
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
24
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
25
Digitized Voice Channels (Cont.)

• The frames for digitized voice have two different
  forms
• T1 has 24 slots per frame
• 24 slots at 56 kbit/s (or 64 kbit/s)
• A total of 1.544 Mbit/s
• E1 or CEPT
• 32 slots at 64 kbit/s
• 2.048 Mbit/s

26
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

27
Digital Telephone Channels (Cont.)

• Instead of modem, Data Service Unit / Channel
  Service Unit (DSU/CSU) adapter devices are
  needed.
• The DSU adapts the digital signal (transmit and
  receive voltages and timing)
• The CSU normalizes voltage levels, provides
  maintenance capabilities, and protect the public
  network.

28
Digital Telephone Channels (Cont.)
DSU/CSU
DSU/CSU
Inter-central office/exchange links (high data
rates)
DSU/CSU
DSU/CSU
Central office or exchange
Central office or exchange
29
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
30
Direction of Data Flow

• Simplex

• Half Duplex

• Duplex (or Full duplex)

31

• ANALOG AND DIGITAL PHYSICAL INTERFACES

COMPUTER NETWORK
32
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
33
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

34
The RS-232/CCITT V.24 and V.28 Interface (Cont.)

• In EIA, known as RS-232-C (the third -C version
  of RS-232)
• More recent version of RS-232-D (now EIA-232-D)
• Sometimes TIA-232-D (Telecommunications Industry
  Association)

35
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

36
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
37
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
38
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

39

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

40
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

41
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

42
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
43
V.24/EIA-232 dial-up operation
44
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)
45
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)
46
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

47
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

48
CCITT X.21 Interface (Cont.)
4
Switched 64 kbit/s
X.21
DSU
Bridge
49
CCITT X.21 Interface (Cont.)
50
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

51
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

52
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
53
Synchronous /Asynchronous Transmission
COMPUTER NETWORK
54
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
55
Asynchronous Timing (Cont.)

• 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
56
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

57
Clocking at the Sending End (Cont.)
Start bit
Stop bit
Serial I/O hardware
P
Character
Memory
Hardware generated

• Each character is framed with these control bits

I/O input/output
58
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
59
Modulation

• We will explore methods 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

60
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)

61
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 circuit-terminating equipment (DCE)
Data terminal equipment (DTE)
Generic interface to transmission medium
62
Data Communications Interfacing (Cont.)
EIA 232/ V.24 interface
Network
Modem
Modem
63
External Modem Connections
64
CCITT Modems
65
Typical Modern Modem Capabilities

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

66
Typical Modern Modem Capabilities (Cont.)

• Modems can automatically dial the telephone
  number
• V.25 bis sync/async autodial
• Or the non-CCITT Hayes AT command set (discussed
  later)
• Modems can perform operations previously done by
  software
• V.42 error correction (discussed later)
• V.42 bis error compression (discussed later)

67
Typical Modern Modem Capabilities (Cont.)

• Modems can fall back to a lesser data rate if
  needed for communications, and some can later
  fall forward when possible
• Leased-line modems can automatically dial a
  backup line as needed.

68
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

69
The Hayes AT Command Set (Cont.)

• The AT commands start with an escape sequence and
  AT(tention)
• An example AT command is to dial a number
• ATDT18007654321 ltcrgt
• When D is for dial, T is for tone, and
  18007654321 is the telephone number

70
CCITT V.42 and V.42 bis Modern Capabilities

• The CCITT V.42 recommendation provides a reliable
  data transfer capability (error correction)
• There are actually two forms (CCITT couldnt
  agree on only one)
• The preferred approach s Link-Access Procedure
  for Modems (LAPM)
• MNP 4 is also included (see next slide)

71
CCITT V.42 and V.42 bis Modern Capabilities
(Cont.)

• The CCITT recommendation V.42 bis builds on V.42
• V.42 bis is a data compression standard
• Uses an automatic adaptation algorithm that
  handles different degrees of randomness in the
  data
• V.42 bis achieves a data compression factor of up
  to 4X

72
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

73
Microcom Network Protocol (MNP) (Cont.)

• MNP 4 is a reliable public-domain delivery
  protocol
• MNP 4 is built into hundreds of thousands of
  modems
• MNP 4 is part of the CCITT V.42 recommendation

74
XMODEM File Transfer Protocol (1978)

• XMODEM was the first file transfer protocol for
  use with PCs
• XMODEM actually predates PCs and DOS
• XMODEM is available from many bulletin boards
• Transfers are limited in many ways
• Transfers data in small (128-byte) blocks
• Operates as a simple stop and wait ACK/NAK
  protocol
• Inefficient use of links in excess of 1200 bit/s

75
XMODEM File Transfer Protocol (Cont.)

• There are many variations YMODEM, ZMODEM, etc.
• Larger block sizes
• Better error detection
• DOS disk operating system
• ACK acknowledgement
• NAK negative acknowledgement

76
XMODEM File Transfer Protocol (Cont.)

• The operating mode is negotiated at connection
  establishment

77
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

78
Kermit (Cont.)

• All transmitted bytes are printable ASCII (except
  ASCII SOH start) 7-bit code
• Avoids problems with control characters, for
  example, which might affect PAD operation.

79
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

80
Remote-Control Software (Cont.)

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

Roving laptop
PSTN
Remotely controlled
81
Terminal Emulation

• A terminal-emulation program allows your PC to
  appear to be a terminal hat a remote host knows
  how talk to
• It may appear to be a scroll-mode terminal
  (e.g., VT100)
• It may appear to be a page-mode terminal (e.g.,
  an IBM 3270)

82
Terminal Emulation (Cont.)

• Terminal emulation is a common approach
• To log in at a host or server
• To log in at any other device to access services
• For network management
• To read and write network management objects
  (variables)

83
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

84
Fax Modem Facts (Cont.)

• Flow is unidirectional
• Support software is required
• Class 1 Minimal processing on the fax board
• Class 2 More on-board processing, less required
  by the PC

85

MULTIPLEXING
COMPUTER NETWORK
86
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.

87

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.

88
Multiplexing (Cont.)

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

89
Multiplexing of Communications Links
Modem
Modem
MUX
MUX
CPU
Remote terminals
90
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

91
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

92
Statistical Time Division Multiplexing (Cont.)

• For example

Data field
Control field
Port no.
Character
(5)
(8)
Frame of tagged characters
93
Statistical Time Division Multiplexing (Cont.)

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

94
Typical Statistical Multiplexer (STAT MUX) Example
Printer
MUX
MUX
CPU
Modem
Modem
Terminal
Supervisory Terminal