Basic Data Communications Technology

1
Chapter 4

• Basic Data Communications Technology

2
Objectives

• Modem Operation
• Modem Comparative Features
• Communications Software

3
Business Requirements for Modems (1)

• Faster
• New modem standards
• Digital transmission services
• More Efficient
• Data compression
• Compatibility with older modems (Backward
  Compatibility)
• More reliable
• Error prevention
• Error detection
• Error correction
• Line Failure Backup
• Flow Control
• Error Control Standards
• More secure
• Unauthorized access prevention
• Encryption

4
V.34 Features
5
V.32 Features
6
V.34

• ITU (CCITT) V.34 standard
• provides 28.8Kbps over standard VG dialup line.
• Worldwide standard.
• Fast retrain
• the ability to adjust data rates on the fly
  depending on line conditions. This is done
  automatically by the modem.

7
Figure 4-4 Quantization Noise
8
Figure 4-5 V.90 Implementation
9
Data Compression

• V.42bis data compression
• This compression scheme can increase the
  effective throughput of a V.34 modem to 115.2Kbps
  (41).
• Actual throughput is still 28.8Kbps over the
  line.
• Compression works by finding repetitive patterns
  of characters and abbreviating them.

10
Data Compression Transmission Rate vs.
Throughput
11
V 42.bis

• Compression Lempel-Ziv algorithm.
• Principle Looks for repetitive pattern of up to
  32 characters.
• Both modems store this pattern with an 11-bit key
  in an updated library.
• The following similar patterns are replaced by
  the key.
• The more repetitive pattern, the higher the
  compression ratio.

12
Cheaper (2)

• Compatibility with other modems.
• As newer technologies enter the market, the
  ability to communicate with other (older) modem
  technologies will save money since it isn't
  necessary to replace all of your equipment at
  once.
• Hayes AT command set is the industry standard.
  It refers to the modem's ability to respond to a
  basic set of commands, defined by Hayes.

13
Reliability (1)

• Goal reduce the error rate.
• Improved reliability implies faster data
  transmission. Fewer retransmission increase the
  throughput.
• Error prevention
• Error detection
• Error correction

14
Reliability (2)

• Error prevention.
• Line conditioning is available for VG leased
  circuits. It helps eliminate noise and
  interference.
• Repeaters and amplifiers help assure signal
  quality over longer distances. Signals tend to
  lose their strength over great distances (this is
  known as attenuation).
• Adaptive protocols which dynamically adjust
  packet sizes.

15
Error Prevention

• Errors occur when data is misinterpreted due to
  noise or interference on transmission lines.
• Reducing the amount of noise or interference.
• Employing modulation techniques able to adapt to
  and overcome noisy lines.

16
Line conditioning

• Value added service provided by phone companies.
• Additional equipment is installed to guarantee
  signal quality.
• A repeater is used to overcome attenuation.
• A repeater regenerates the digital signal.
• A repeater on an analog circuit is called an
  amplifier.
• An amplifier does not distinguish between voice
  data signal and the background noise.

17
Adaptive Protocols

• Adapt transmission session parameters in response
  to various line conditions.
• Techniques play on one of two things
• Amount of data per packet
• Transmission rate is varied according to line
  conditions.
• Adaptive size packet assembly
• Dynamic speed shifts Transmission rate

18
Adaptive size packet assembly

• MNP class 4 protocol
• Increase and decrease amount of data per packet
  based on circuit condition.
• Protocol optimizes amount of data per packet by
  building packets containing the greatest amount
  of data that can be transmitted reliably without
  requiring retransmission.

19
Dynamic speed shifts

• MNP class 10 adaptive protocol.
• Allows two modems changing their speed up or down
  during the transmission session in response to
  varying line conditions.
• Assures highest data rate used most of the time.
• Useful in cellular phone environments where line
  quality varies significantly over short periods
  of time.

20
Error Detection Process
21
Error Detection

• Additional bits added by transmitter for error
  detection code

22
Error Detection Techniques

• Parity can be used (even, odd, space, mark,
  none).
• Longitudinal Redundancy Checks (LRC).
• Checksums.
• Cyclic Redundancy Check (CRC), can be CRC-16 or
  CRC-32 (16-bit or 32-bit).

23
Parity Checkings Inability to Detect Multiple
Bit Errors
24
Longitudinal Redundancy Checks
25
Check Sum

• Block oriented error detection characters
• A checksum is calculated by
• adding the decimal face values of all characters
  in a block of data.
• divide the sum by 255
• The remainder of the operation is the checksum to
  be transmitted and verified by the receiving
  modem.

26
CRC

• Cyclic Redundancy Check
• For a block of k bits, transmitter generates an n
  bit sequence called frame check sequence (FCS)
• Transmit a kn bit number which is exactly
  divisible by some number (polynomial generator).
• Receiver divides the received block by that
  number, if no remainder, assumes no error.

27
Error Correction

• The receiving modem has detected an error and
  requests a re-transmission of the erroneous block
  of data from the sending modem.
• The transmitting modem retransmits the incorrect
  data.
• Three main issues
• How is retransmission requested ?
• How much data must be retransmitted ?
• How is retransmission time minimized ?

28
Automatic Retransmission Request

• ARQ (Automatic Retransmission Request) is a
  general term to describe this process.
• ARQ turns unreliable data link into a reliable
  one
• Request for retransmission may occur in different
  ways
• Discrete ARQ Stop and Wait
• Continuous ARQ Go Back N
• Selective ARQ Selective Reject

29
Discrete ARQ

• Also known as Stop and wait protocol.
• The receiving modem sends an ACK (positive
  acknowledgment) for every block correctly
  received.
• A negative acknowledgment or NAK for every
  erroneous block of data received.

30
Continuous ARQ

• Also known as Go-Back N Protocol
• Sliding Window Protocols
• ACK signals are sent much less frequently.
• A NAK is sent (along with the block number) if an
  error occurs.
• Transmitting modem slides its window back to the
  block number in error.

31
Selective ARQ

• Also known as Selective Reject or Selective
  retransmission
• Only rejected frames are retransmitted
• Subsequent frames are accepted by the receiver
  and buffered
• Minimizes number of retransmitted block and time.
• More complex login in transmitter receiver

32
Model of Frame Transmission
33
Go Back-N
34
Selective Reject
35
Flow Control

• Blocks of data to be sent are saved in memory a
  buffer.
• A buffer management technique is necessary.
• Flow Control prevents buffer overflow (no data
  overruns). i.e. the sending entity does not
  overwhelm the receiving entity
• A signal is sent from the receiving device to
  tell the transmitter to stop or resume the flow
  of data.
• Hardware Flow Control
• Software Flow Control

36
Hardware Flow Control

• Involves the use of additional pins on the
  interface to start or stop the flow of data.
• RS-232 uses pins 4 5 for RTS (request to send)
  and CTS (clear to send)
• CTS pin high ?Transmitter sends data
• CTS pin low ? Transmitter stops transmitting
  data

37
Software Flow Control

• Sending special characters in the data.
• The most popular flow control standard is known
  as XON/XOFF.
• When a device cannot receive data it sends a XOFF
  (Ctrl-S)
• To resume transfer of data receiving device sends
  an XON character (Ctrl-Q).

38
Forward Error Correction (1)

• Correction of the received data without the need
  for retransmission.
• Similar approach as for error detection.
• If the redundant bits calculated at the receiver
  do not match the received ones, the forward error
  correction circuitry at the receiver uses those
  bits to correct the incoming data signal.

39
Forward Error Correction (2)

• Cost More redundant data is added to the
  transmitted blocks.
• Thus Reduction in data throughput
• Compromise
• Not enough redundant data, the overall throughput
  is reduced due to retransmissions.
• Too much redundant data will also reduce the
  throughput due to time spent to send data and
  processing at the receiver.

40
Trellis Coded modulation

• Modems that employ TCM can overcome twice as much
  noise on a given circuit as QAM modems without
  TCM.
• Uses convolutional encoding.
• TCM adds a redundant bit to avoid misinterpreting
  the received sequence.

41
Error Control Standards

• MNP4
• Optimize the full-duplex mode over dial-up lines
• Use adaptive size packet assembly and elimination
  of redundant or overhead information from
  transmissions.
• V.42
• Incorporates MNP 4 and the Link Access Protocol
  for Modems (LAP-M).
• LAP-M uses selective ARQ.
• Provides for negotiation during modem handshaking
  to allows modems decide which protocol to use.
• V.42 is not to be confused with CCITT V.42bis for
  data compression.

42
MNP4 and V.42

• Error control protocols implemented within
  modems.
• Modems assure error-free transmissions.
• Error control protocol, supplied by
  communications software running on PCs, is not
  needed.

43
Security

• Security is required for the transmission of
  sensitive data.
• Access control can be accomplished by password
  protection and dial-back security systems.
• Encryption can be used to alter the data so that
  it is unrecognizable.
• The DES standard is very popular, using a public
  key and private key system.

44
Security

• Granting dialup network access to only
  authorized people is done through one or both of
• Password protection
• Dial back or callback security.

45
Password protection

• Typically a modem answers a call and requires a
  password.
• If no password is entered in a given time the
  modem terminates the call.
• Some modems require an additional layer beyond
  password protection.

46
Callback unit

• Once the password is validated, the answering
  modem
• Terminates the call
• Looks up and validates the user ID and/or
  password in a directory.
• Finds the phone number of the user
• Dials back the valid user and establishes the
  communication session.

47
Callback

• Advantages
• Only valid users are allowed.
• Valid users are maintained in one central updated
  directory.
• Users can have time restricted access
• Some dial back units record all network access.
• Disadvantages
• Dial back unit always incurs any long distance
  charges.
• Not suitable for traveling users.
• Some units feature variable callback the user
  enters a password and a phone number to which the
  unit should return the call.

48
Modem Trends

• USB
• ADSL
• Cable
• Cellular/wireless

49
Modem Technology Analysis Grid
50
Part II

• Communications Software Technology Analysis

51
Communications Software

• Communications Software interprets business DC
  needs into a language understood by the
  communication hardware.
• Interactive GUI
• Commands are passed to the OS.
• The OS interfaces directly to the Hardware (Modem
  or Card)
• Communication SW is the interface between DC
  needs and the DC HW
• Comm. Software is required to turn a PC into a
  terminal or network client.

52
Hardware/Software Interface

• Several Interfaces are involved in the process
  of the communications SW interaction with a
  modem
• Use of application program interfaces (API),
  commands for the OS that are embedded in
  communications SW.
• Basic input output system (BIOS) handles
  interaction of OS with HW.
• Physical connection between PC and Modem (RS232).

53
The Role of Communication Software
54
Hayes AT Command Set

• How can the Communications SW talk to the modem.
  ?
• What language or Commands does the modem
  understand ?
• How does the Communications SW initialize the
  modem ?
• Answer Hayes AT Command Set.
• Modems that understand Hayes commands are known
  as Hayes compatible.

55
Hayes AT Command Set

• De facto standard for commands for both SW
  modems
• AT means attention (literally)
• Modem setup strings sent from SW to modem to
  initiate communication session
• Comm. SW packages contain various packaging of
  modem setup strings various levels of user
  friendliness.

56
Redirection for LAN based modems

• Many PCs can share one LAN based modem
• Cost Effective (compared to individual modems)
• Shared modems are connected to a communications
  server or remote access server.
• Communication software still resides on clients
  PC, but commands must be sent to shared modem via
  the NIC

57
Redirection for LAN-Based Modems
58
Technology Analysis

• Communications Software has undergone significant
  evolution.
• Most manufacturer often include high-quality
  Communications Software at no extra cost.
• Operating systems often include Communications
  Software functionality.
• Many of todays Communications Software are
  categorized as Remote control or remote access
  products.
• Remote control or remote access software will be
  reviewed in the chapter dedicated to Remote
  Access and Wireless Networking (Chapter 10 2nd
  Edition).

59
Popular Communications Software Packages
60
Communication Software Technology Analysis Grid