Protocol Efficiency and HDLC

1
Protocol Efficiency and HDLC

• In this section
• Protocol efficiency
• Effective data rates
• Utilization
• Stop and wait flow control efficiency
• ARQ flow control efficiency
• The High-level data-link control (HDLC) protocol
• HDLC data frames
• HDLC operation

2
Protocol Efficiency

• Can be measured in various ways
• One measure effective data rate (EDR)
• Parameters
• R bit rate, in bits per second
• S signal speed in transmission medium, in metres
  per second
• D distance to send, in metres
• T time to create one frame, in seconds
• TF frame, TA acknowledgement
• F frame size, in bits
• N number of user data bits in a frame, in bits
• A number of bits in an acknowledgement, in bits

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Effective Data Rate calculation

• For an unrestricted protocol (i.e. no flow
  control, or acknowledgements), the effective data
  rate (EDR) is
• For a stop and wait protocol

time needed for one frame
4
Example, for Stop-and-Wait

• Suppose
• N 160 bits, D 200 ,A 40 bits, TF 1.5x10-6
  s, TA 0.5x10-6 s, S 2x108 m/s, F 200 bits,
  R 10 Mbps 1x107 bits/s
• Note that this is about 57 of the bit rate

5
Maximum Efficiency of Sliding Window

• Adjustments to stop and wait formula
• Instead of sending 1 frame, we could send W
  frames ? replace N with N x W
• Acknowledgements could be piggy-backed on to data
  frames ? replace F A with 2F, and replace TF
  TA with 2TF
• Actual efficiency depends on error rate number
  of frames re-transmitted, etc.

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Utilization

• Objective obtain a measure of efficiency that
  is independent of the transmission speed of the
  medium.
• Utilization fraction of time (1.0 ? best case)
  that transmitter can send bits, as opposed to
  waiting for acknowledgements or flow control
• Simplifying assumptions
• TF, TA are negligible
• A is much smaller than F, so that FA ? F

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Maximum Utilization (1)

• Time to send one frame and receive an
  acknowledgement is
• If the window size is W, the time to send W
  frames is
• Actual time spent transmitting bits is
• Utilization (U) is the ratio of the actual time
  transmitting, over the time needed to send and
  receive an acknowledgement. The maximum
  utilization is

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Maximum Utilization (2)

• Simplify by using the ratio of propagation time
  (D/S) to transmission time (F/R)
• Let
• This is a pure ratio (i.e. no units).
• Another way of viewing the value a is that if one
  normalizes the frame transmission time to 1, the
  length of the link in bits is a frames.
• Therefore, the maximum utilization can be
  expressed as

A
B
Frame 1
Frame 2
Frame a
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Actual Utilization

• Example Error free sliding window protocol.
• Send W frames, receive one acknowledgement.
• Two cases
• Case 1 W 2a 1
• The acknowledgement for frame 1 reaches A before
  the sending window is exhausted.
• In this case, the sender can transmit
  continuously with no pause, and ratio of the
  actual utilization to maximum utilization is 1.0

A
B
Frame a2
Frame a3
Frame 2a1
Ack
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Actual Utilization

• Example Error free sliding window protocol.
• Send W frames, receive one acknowledgement.
• Case 2 W lt 2a 1
• A exhausts the window at time W, and cannot send
  frames until time 2a1.

A
B
W a 1
W a 2
Frame W
A
B
W a 1
W a 2
Frame W
Ack
Ack
A
B
a 2
Frame W
Ack
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Normalized Utilization

• For error-free sliding window

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Utilization in the presence of errors

• Suppose that the probability of an error in a
  frame is P.
• Stop and wait
• Selective Reject
• Go-back-N

13
Utilization for P 0.001
U
a
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Logical Link Control (LLC)

• In the IEEE 802 series of standards for local
  area networks (LANs), LLC is above the medium
  access control layer (MAC)
• For Asynchronous Transfer Mode (ATM), LLC is
  combined with network layer functions.
• In other standards, LLC comprises all of layer 2
• High-Level Data Link Control (HDLC), ISO 3009 /
  4305
• Link Access Procedure, Balanced (LAPB),ITU-T
  protocol for X.25 systems
• Link Access Procedure, D-Channel (LAPD), ITU-T
  protocol for ISDN (Integrated Services Data
  Network) systems
• Link Access Procedure for Frame-Mode Bearer
  Services (LAPF), data link protocol for Frame
  Relay
• Point-to-Point protocol (PPP) used between home
  computers and internet service providers (RFC
  1661)

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High-level Data Link Control (HDLC)

• Original source IBMs synchronous data link
  control (SDLC)
• Related protocols ITU-Ts link access procedure
  standards (LAPB, ), PPP
• ISO Standards 3009, 4305

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HDLC Basics

• Stations
• Primary sends data, controls the link with
  commands
• Secondary receives data, responds to control
  messages
• Combined can issue both commands and responses
• Link configuration
• Unbalanced one primary station, one or more
  secondary stations
• Balanced two combined stations

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HDLC Basics

• Data transfer modes (not a complete set these
  are most common)
• Normal response mode (NRM)
• Used with unbalanced configuration
• Primary initiates data transfer secondary can
  only reply
• Asynchronous balanced mode (ABM)
• Used with balanced configurations
• Either side may send data at any time
• Address modes
• Regular sequence numbers have 3 bits
• Extended sequence numbers have 7 bits

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HDLC overall frame format
octets
data
CRC
Address
Control
01111110
FS
1
variable
2 or 4
variable
1 or 2
ITU-T versions of the CRC are used
frame separator (FS) bit stuffing used for
all fields between separators
19
HDLC address fields
bits
1
7
F
address

• F bit
• if 1, this is the final octet of the address
• if 0, another address octet follows
• If the link is strictly point-to-point, the value
  of the field will be 10000000, as the address is
  not relevant
• An address of 11111111 represents all stations

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HDLC control field types

• Information (I-frames)
• Carries upper level data
• Also includes ARQ sequence numbers for sending
  and receiving
• Supervisory messages (S-frames)
• Used for flow control
• 4 types
• Includes receiving sequence number
• Un-numbered messages (U-frames)
• Used for link setup and disconnection
• 15 types

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I-frame control field
bits
1
3
1
3
regularmode
0
NS
P/F
NR
1
7
1
7
bits
extendedmode
0
NS
P/F
NS

• NS sending sequence number
• NR receiving sequence number
• P/F poll or final bit
• Command frame asks for response (P poll)
• Response frame indicates response (F final)

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S-Frame Control field
1
3
1
bits
2
1
regular mode
0
1
S
P/F
NR

• S field
• RR receive ready (bits 00)
• Positive acknowledgement, ready for I frame
• Used when no reverse data otherwise NR sent in
  an I-frame
• RNR receive not ready (bits 10)
• Positive acknowledgement, not ready to receive
• REJ reject (bits 01)
• Negative acknowledgement, go-back-N ARQ method
• SREJ selective reject (bits 11)
• Negative acknowledgement, selective reject ARQ
  method

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S-Frame Control field
1
7
1
bits
2
1
4
extended mode
0
1
S
P/F
NR
0000

• The S field is the same as the regular mode
• The four zeros are needed to pad the field to
  two full octets, 16 bits.

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U-frame control field
1
3
1
bits
2
1
1
M1
P/F
M2

• SNRM set normal response mode (M1 00, M2
  001)
• SABM set asynchronous balanced mode (M1 11, M2
  100)
• SABME set asynchronous balanced mode, extended
  (M1 11, M2 110)
• DISC disconnect (M100, M2010)
• UA un-numbered acknowledgement (M1 00, M2
  110)
• RSET resets send and receive sequence numbers
  (M1 11, M2001)
• FRMR frame reject (M1 10, M2001)
• (see Forouzan, Table 11.1, p. 286)

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HDLC operation

• One of the messages SNRM, SABM, SABME, is used
  to set up the link initially.
• Sets the mode, and the length of sequence numbers
• UA is used as a positive acknowledgment for
  U-frames
• After setting up the link, data transfer can
  occur.
• The DISC message is used to terminate the
  connection.
• If a damaged U-frame is received, FRMR is sent as
  a reply.

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Connection
A
B
27
Sample Two-Way Data Exchange
A
B
NR Sequence numbers next message expected
28
Go-Back-N ARQ
A
B
29
Selective Reject ARQ
A
B
30
Receiver Busy
A
B
31
Timeout
A
B