Computer Network and Infrastructure Communication Interface and Flow Control

1
Computer Network and InfrastructureCommunication
Interface and Flow Control

• Dr. E.C. Kulasekere

2
Section Objectives

• To understand the necessity of Interfacing and
  Flow Control in Data Communications.
• Synchronous and Asynchronous transmission
  techniques
• Line configurations and topologies
• Data link control and flow control techniques
• Overview of Error detection and control
• Now we shift to data communication from data
  transmission.

3
Interfacing and Flow Control

• To communicate any device should interface with
  the transmission system.
• The interfacing has to be designed so that it has
  characteristics in the following areas
• Mechanical
• Electrical
• Functional
• Procedural
• Flow control is required to assure that the
  source thus interfaced does not overwhelm the
  destination by sending data faster than they can
  be processed and absorbed.

4
Synchronization

• For the sender and receiver to be able to
  communicate successfully they must know
• What a BIT is
• What a BYTE is
• What a FRAME is
• that is they must synchronize at the Bit, Byte,
  and Frame level

5
Synchronization

• Without Synchronization the bits/bytes/frames
  cannot be identified at the proper sampling time.
  
• If no transmission impairments exists then the
  problem is less.
• There are two ways in which these bits can be
  identified.
• To identify the transmitter clock.
• To identify the line state change.

6
Synchronous Communication

• Receiver clock operates in synchronism with the
  received signal
• The clocking signal is embedded into the
  transmitted bit stream and subsequently extracted
  by the receiver (SYN characters)
• Example implementation The carrier frequency can
  be used to synchronize the receiver based on the
  phase of the carrier.

7
Synchronous - Bit Level

• Block of data transmitted without start or stop
  bits
• Clocks must be synchronized
• Can use separate clock line
• Good over short distances
• Subject to impairments
• Embed clock signal in data
• Manchester encoding
• Carrier frequency (analog)

8
Synchronous - Block Level

• Need to indicate start and end of block
• Use preamble and postamble
• e.g. series of SYN (hex 16) characters
• e.g. block of 11111111 patterns ending in
  11111110
• More efficient (lower overhead) than asynchronous
  transmission.
• For a sizable block of data synchronous
  transmission is more efficient than asynchronous
  transmission. Because of low overhead.

9
Synchronous (diagram)
10
Asynchronous Transmission

• The receivers clock runs asynchronously with
  respect to incoming signal
• To avoid timing problems character at a time is
  transmitted rather than a block of data.
• A Byte is framed by a START and STOP bit.
• The START bit changes the signal to Non-Idle
  state, this warns receiver a byte is
  arriving.Then 8 data bits follow
• The STOP bit follows the data bits and returns
  the signal to the Idle state.

11
Asynchronous (diagram)
12
Asynchronous - Behavior

• In a steady stream, interval between characters
  is uniform (length of stop element)
• In idle state, receiver looks for transition 1 to
  0
• Then samples next seven intervals (char length)
• Then looks for next 1 to 0 for next char
• Simple
• Cheap
• Overhead of 2 or 3 bits per char (20)
• Good for data with large gaps (keyboard)

13
Line ConfigurationTopology

• Topology of a data link refers to the physical
  arrangement of stations on transmission medium.
• Point-to-Point
• Two communication stations
• Eg. Two computers communicating through serial
  link
• Multi-point
• More than two stations in transmission link
• This configuration is possible only when the
  terminals are transmitting a fraction of the
  time.

14
Traditional Line Configurations
15
Line ConfigurationData Exchange

• Simplex transmission
• One sided transmission, does not fall into data
  exchange.
• Half-duplex transmission
• Requires one data path for data exchange.
• Full-duplex transmission
• Requires two separate data paths. Eg. Two twisted
  pairs
• If Rx and Tx are two different frequencies, one
  path is sufficient.

16
Interfacing

• Data processing devices (or data terminal
  equipment, DTE) do not (usually) include data
  transmission facilities
• Need an interface called data circuit terminating
  equipment (DCE)
• e.g. modem, NIC
• DCE transmits bits on medium
• DCE communicates data and control info with DTE
• Done over interchange circuits
• Clear interface standards required

17
Data Communications Interfacing
18
Interface Characteristics

• Mechanical
• connectors, pins, cables, etc.
• Electrical
• voltage levels, timing, etc.
• Functional
• data, control, synchronization, etc.
• Procedural
• the sequence various circuits are used

19
Data Link Control

• Synchronization and interfacing alone may not be
  enough to regulate the rate at which the data
  arrives at the receiving DTE.
• Flow control will add this additional
  functionality to the data link control layer.
• The data link control protocols are added above
  the physical interfacing discussed previously.
• When such as layer is used the transmission
  medium is then referred to as a data link.

The need, pp 208
20
Flow Control

• Ensuring the sending entity does not overwhelm
  the receiving entity
• Preventing buffer overflow
• Propagation Delay
• Time for a bit to traverse the link
• Transmission Delay
• Time taken to emit all bits into medium

21
Model of Frame Transmission
22
Stop and Wait Flow Control

• Source transmits frame
• Destination receives frame and replies with
  acknowledgement
• Source waits for ACK before sending next frame
• Destination can stop flow by not sending ACK
• This procedure works well for a few large frames
  and needs no improvement.
• Only one frame can be in transit in the link at
  one time since to transmit the next frame an ACK
  should be received

23
Fragmentation of Data

• In most cases the source will split large blocks
  of data and transmit in several frames.
• The reasons for having data fragmented to small
  frames is due to
• Limited buffer size
• Errors detected sooner (when whole frame
  received)
• On error, retransmission of smaller frames is
  needed
• Prevents one station occupying medium for long
  periods
• For very high data rates, for very long distances
  between sender and receiver, the Stop and wait
  provides inefficient line utilization.

24
Stop and Wait Link Utilization
25
Stop and Wait Link Utilization ..

• Blength of link in bits
• Lnumber of bits in the frame
• agt1 the link is under utilized and hence not
  efficient
• alt1 when the first bit is received at the
  receiving DTE, the last bit of the frame has not
  left the source DTE. Hence the acknowledge will
  not come till the entire message is received,
  till them the link is inefficiently used.

26
Problems with Stop-and-Wait

• It is inefficient for long links (agt1) the link
  is under utilized for a long time till the ack
  comes.
• It is also inefficient for high data rates (alt1)
  again the link is held up will all the data
  arrives at the receiving DTE and then it can be
  acknowledged.
• The primary problem in SAW is inefficient link
  utilization.

27
Sliding Window Flow Control

• Allow multiple frames to be in transit, hence the
  issues of stop-and-wait is taken care of.
• Receiver has buffers for W frames long
• Transmitter can send up to W frames without ACK
• Each frame is numbered
• ACK includes number of next frame expected
• Sequence number bounded by size of field (k)
• Frames are numbered modulo 2k

28
Sliding-Window Flow Control

• The sequence number occupies a field in the frame
  and hence the number is bounded. Eg. For a 3 bit
  field, the sequence number can range from 0 to 7
• The next example uses a 3-bit sequence number.
• In the example the sender can send only 5 frames
  at a time.
• The shaded are can shrink from the left and
  expand from the right when frames are sent and
  received respectively (from the senders point of
  view)

29
Sliding Window Diagram
30
Example Sliding Window
31
Sliding Windows Enhancements

• Receiver can acknowledge frames without
  permitting further transmission (Receive Not
  Ready)
• Must send a normal acknowledge to resume
• If duplex, use piggybacking
• If no data to send, use acknowledgement frame
• If data but no acknowledgement to send, send last
  acknowledgement number again, or have ACK valid
  flag (TCP)

32
Review Questions

• How is the transmission of a single character
  differentiated from the transmission of the next
  character in asynchronous transmission?
• The beginning of a character is signaled by a
  start bit but with a value of binary zero. A stop
  (binary one) follows the character.
• What is a major disadvantage of asynchronous
  transmission?
• Asynchronous transmission requires an overhead of
  two or three bits per character, and is,
  therefore, significantly less efficient than
  synchronous transmission.

33
Review Questions

• Suppose that a sender and receiver use
  asynchronous transmission and agree not to use
  any stop elements. Could this work? If so,
  explain any necessary conditions.
• No it will not work. The stop bit is needed so
  that the start bit can be recognized as such. The
  start bit is the synchronization event, but it
  must be recognizable. The start bit is always a
  0, and the stop bit is always a 1, which is also
  the idle state of the line. When a start bit
  occurs, it is guaranteed to be different from the
  current state of the line.

34
Review Questions

• List and briefly define some of the requirements
  for effective communications over a data link.
• Frame synchronization The beginning and end of
  each frame must be recognizable. Flow control
  The sending station must not send frames at a
  rate faster than the receiving station can absorb
  them. Error control Bit errors introduced by the
  transmission system should be corrected.
  Addressing On a multipoint line, such as a local
  area network (LAN), the identity of the two
  stations involved in a transmission must be
  specified. Control and data on same link The
  receiver must be able to distinguish control
  information from the data being transmitted. Link
  management The initiation, maintenance, and
  termination of a sustained data exchange require
  a fair amount of coordination and cooperation
  among stations. Procedures for the management of
  this exchange are required.

35
Review Questions

• What is the advantage of sliding-windows flow
  control compared to stop-and-wait flow control?
• The stop wait approach requires acknowledgments
  after each frame. The sliding window flow control
  technique can send multiple frames before waiting
  for an acknowledgment. Efficiency can be greatly
  improved by allowing multiple frames to be in
  transit at the same time.

36
Review Problems

• Consider a half-duplex point-to-point link using
  a stop-and-wait scheme, in which a series of
  messages is sent, with each message segmented
  into a number of frames. Ignore errors and frame
  overhead.
• What is the effect on line utilization of
  increasing the message size so that fewer
  messages will be required? Other factors remain
  constant.
• What is the effect on line utilization of
  increasing the number of frames for a constant
  message size?
• What is the effect on line utilization of
  increasing frame size?

37
Review Problems ..

• a. Because only one frame can be sent at a time,
  and transmission must stop until an
  acknowledgment is received, there is little
  effect in increasing the size of the message if
  the frame size remains the same. All that this
  would affect is connect and disconnect time.
• b. Increasing the number of frames would decrease
  frame size (number of bits/frame). This would
  lower line efficiency, because the propagation
  time is unchanged but more acknowledgments would
  be needed.
• c. For a given message size, increasing the frame
  size decreases the number of frames. This is the
  reverse of (b).