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Chapter 7:
Data Link Control Protocols
COE 341: Data & Computer Communications (T061)Dr. Radwan E. Abdel-Aal
WK 13
2
Where are we:
Physical Layer
Transmission Medium
Data Link
Chapter 4: Transmission Media
Chapter 3: Signals and their transmission over
media, Impairments
Chapter 5: Encoding: From data to signals
Chapter 7: Data Link: Flow and Error control
Chapter 6: Data Communication: Synchronization,
Error detection and correction
Chapter 8: Improved utilization: Multiplexing
3
Contents1. Flow Control
a. Stop-and-Wait flow controlb. Sliding-Window flow control
2. Error Controla. Stop-and-Wait ARQb. Sliding-Window ARQ
- Go-Back-N ARQ- Selective-Reject ARQ
3. High-Level Data Link (HDLC) Protocola. Basic Characteristicsb. Frame Structurec. Operation
4
What is Data Link Control? The logic or procedures used to convert the raw stream
of bits provided by the physical layer into a “reliable” data link
Performed by Data Link Control Protocol (Layer) Requirements and Objectives:
Frame-level synchronization: Recognize frame start and end Flow control: Regulate sending of frames to match
the ability of RX to absorb them Error control: Retransmission of damaged or
unacknowledged frames Addressing: Identify stations on a multipoint line Allowing for control information to go with data on same link Link management: To initiate, Maintain, and terminate data
exchange
5
Flow & Error Control
The two main functions of the data-link layer Flow control Error control
Flow & Error Control together? We usually discuss Flow Control and Error Control
together as they use the same data-link protocols
6
Flow Control
Required to avoid the TX overwhelming the RX by the flow of data it sends
RX does not ‘absorb’ the received data instantly! It buffers (temporarily stores) the data it receives
in a finite-size buffer to do some processing before sending it upward to higher layers
Without flow control, the RX buffer may overflow and data gets lost…
7
Flow Control over a link (assume no error)
For now, assume: No frames lost (loss over a single link is a kind of error) No frames arrive in error Frames arrive in the same order they were sent, after a
propagation delay
8
A Model of Frame Transmission
Frame lost:(error in start flag)
Only one frame traveling on the linkat any given time Frame damaged:
(error in data)
9
Main Flow Control Protocols
10
Stop and Wait
Frames sent and acknowledged one at a time:1. Source transmits frame and waits for ACK
2. Destination receives frame and replies with an acknowledgement ACK
3. Source gets ACK and sends next frame Disadvantages:
Destination can stop the flow by not sending ACK (but we can use timeout to overcome this)
Not efficient - wastes time in waiting for short frames on a long links (frame does not ‘fill’ the link)
i.e. I would like to make frames long in time (large in size for a given data rate)
11
Data Fragmentation (smaller frames!) However, large blocks of data are often split into several
smaller frames- Why? Limited frame buffer size at RX To reduce frame error rate: remember? FER = 1-(1-BER)F
Errors are detected sooner (when frame received) On error, we need to retransmit a smaller amount of data On a shared medium, e.g. a LAN, this ensures that
transmitting station does not occupy the medium for a long time
“Stop and wait” is inadequate in such situations where frames are “short”
Link utilization depends on the frame length in time relative to the link propagation time.
12
Stop and Wait Link Utilization: The ‘a’ ratio Total number of bits in a frame = L bits, Tb = bit duration
R = Data rate, bps Frame transmission time, tf sec: Time taken by the TX to emit all the frame bits
into medium tf is large for large frames and low data rates
d = Link physical length, m V = Velocity of propagation over link, m/s Link Propagation time, tp sec: Time for a bit to traverse the link (link length in time)
Link length in bits, B bits/link: The number of bits that ‘fill’ the link if data is transmitted continuously
a = Propagation time / Transmission time = tp/tf
R
LLTt bf
][ (sec)][ (b/sec) V
dRtRB p
V
dt p
L
B
LVd
R
RLVd
t
ta
f
p ][
Smaller ‘a’ means better link utilization (with large frames)
If tf = 1, a = Propagation time tp
13
Stop and Wait Link Utilization (Efficiency) Let us define link utilization, U, as:
Will demonstrate on next slide that U is given by:
Where ‘a’ is
Utilization is high for small a: U approaches 1, i.e. 100% efficiency Shorter links, Higher propagation velocities Larger frames sizes, Lower data rates
Efficiency is poor for large a: Longer links, Lower propagation velocities Smaller frames sizes, Higher data rates
Timeon Transmissi Frame
Timen Propagatioa
timeElapsed
data ittingfor transm used islink which during Time U
12a
1
U
14
Stop and Wait Link Utilization: 2 cases:
L
B
t
ta
f
p 2a1
1
U
2a1
1
U
Link ‘empty’ most of the time ! Underutilized
Link ‘full’ most of the time Better utilized
a > 1
a < 1
Let frame transmission time tf = 1 a = Propagation time = tp
Overhead of an ACK frame in both cases !
tp > tf tp < tf
Link is longer than frame: a > 1 Link is shorter than frame: a < 1
2a1
1
U
tf = 1tp = a
15
Stop and Wait Efficiency: Example Compare the efficiency of stop-and-wait flow control for two
links using the parameter ‘a’: Fame size, L = 1000 characters of 8 bits each, = 8000 bits
200-m optical fiber link Data rate, R = 1 Gbps Typical wave velocity, V = 2 x 108 m/s
Frame TX time, tf = L/R = 8000/(1x109) = 8 s
Propagation time, tp = d/V
= 200/(2x108) = 1 s
End of first frame reaches RX after 8+1 = 9 s from start
ACK takes 1 s more to reach TX, i.e. it starts sending 2nd frame after 10 s
Utilization = 8/10 = 80% (=1/(2a+1))
125.0)102(8000
)200(108
9
xLV
Rd
L
B
t
ta
f
p
Satellite link between 2 ground stations d = 2 x 36,000 km, Data rate, R = 1 Mbps Typical wave velocity, V = 3 x 108 m/s
Frame TX time, tf = L/R = 8000/(1x106) = 8 ms
Propagation time, tp = d/V
= 2x36x106/(3x108) = 240 ms
End of first frame reaches RX after 8+240 = 248 s from start
ACK takes 240 ms more to reach TX, i.e. it starts sending 2nd frame after 488 ms
Utilization = 8/488 = 1.6% (=1/(2a+1))
30)103(8000
)10362(108
66
x
xx
LV
Rd
L
B
t
ta
f
p
Link is longer than frame: a > 1 Link is shorter than frame: a < 1
16
Sliding Windows Flow Control Avoids the low efficiency of Stop-and-wait when a > 1 Allows multiple frames to be “in transit” simultaneously on
the link RX keeps a buffer store (memory) for W frames So, TX can send up to W frames without waiting for ACK Each frame carries a sequence number ACK from RX shows the number of next expected frame TX keeps a list of frames it can send RX keeps a list of frames it expects to receive These lists form sliding windows at TX and RX that
shrink/expand as frames are sent, and ACKs are sent/received
Hence, Sliding Windows Flow Control
17
Frame Sequence Numbering Frame sequence number is limited by the size of a
corresponding field in the frame, e.g. k bits Frames are numbered modulo 2k
E.g. for k = 3, frame sequence # is modulo 23 = 8, i.e.= 0,1,..., 7, 0,1,…
Window size (W) is limited to a maximum of 2k-1i.e. Wmax = 7 in the above example
k = 3 bits
Frame Sequence Number
FCS HDLC Frame
18
Sliding Window Send/Receive CycleTX RX
Now you want to ACK some frames:• Delete ACKed frames from buffer• Expand Window to receive more
3. Send Acknowledgement
Receive Frames
Shrink window past received framesNow you want to send more:
Send frames
1. Delete ACKed frames from buffer2. Expand Window to send more
Frames
ACKs
Window covers frames to be received
Window covers frames to be sent
Shrink window past sent frames
Acknowledging frames is a separate issue from receiving them
Flexible- (Not rigid) Windows
19
Sliding-Window Diagramk = 3 bits, W = 7
Window covers frames to be sent
Remove ACKed frames from buffer
W = 7Max # of frames TXed without ACKed
DeletionMarker
Send/Receive Frames
Receive/Send ACKs
Shrink
ExpandDelete
FCS
Window covers frames to be received
At TX
At RX
20
Example Sliding WindowMax window size of 7
Received up to 2Ready to receive 7 frames starting with 3
RR = Receiver ReadyDelete
Received up to 3Ready to receive 7 frames starting with 4
TX RX
Expand
Shrink
21
Sliding Window Enhancements
Receiver can acknowledge receiving frames without permitting further transmission:
(Receive Not Ready RNR) Example RNR 5: “Received frames up to 4, but not
ready for 5 and beyond yet” When it becomes ready, RX must send a normal
acknowledge (RR 5) later for TX to resume sending frames
22
Sliding Window in a Duplex System In a Duplex System, destination also transmits data back
to source Piggybacking: Utilizing data frames from destination to
carry ACK signals back to source to improve channel utilization Additional field in the data frame for use only by +ive (RR) ACK If you have no data to send now or your ACK is not RR, use a
normal (dedicated) ACK frame (e.g. RR or RNR) If data is to be sent but no acknowledgement needed, insert the
last acknowledgement number to prevent RX from using the number existing in the ACK field of the data frame.
(When RX station receives a duplicate ACK, it ignores it)
23
Sliding Window Protocol: Efficiency Much more efficient than Stop and wait for a>1
Treats link as a pipeline to be filled with frames in transit simultaneously- not just one by one
With window size W and assuming no error, link utilization, U, is given by (Appendix 7A)
where a = Propagation time/Frame transmission time = tp/tf i.e. Sliding window protocol can achieve 100% utilization
for W (2a + 1). The smaller the W needed for this the better! (Why?). This
requires a small a (so small a is still advantageous!)
1 (2 1)
(2 1)2 1
W aU W
W aa
24
Sliding Window Efficiency: Example Compare the efficiency of Sliding Window flow control for two
links using the parameter ‘a’: Fame size, L = 1000 characters of 8 bits each, = 8000 bits
200-m optical fiber link Data rate, R = 1 Gbps Typical wave velocity, V = 2 x 108 m/s Frame TX time, tf = L/R = 8 s
Propagation time, tp = d/V = 1 s
a = tp / tf = 0.125 100 % link utilization is achieved with
window size W:
W (2 a+1) (2 x 0.125 +1) 1.25
i.e. W = 2 (A window of just 2 frames!)
- easily achieved in practice!
Satellite link between 2 ground stations d = 2 x 36,000 km, Data rate, R = 1 Mbps Typical wave velocity, V = 3 x 108 m/s Frame TX time, tf = L/R = 8 ms
Propagation time, tp = d/V = 240 ms
a = tp / tf = 30 100 % link utilization is achieved with
window size W:
W (2 a+1) (2 x 30 +1) 61
W = 61, k = 6 bit (Large window, large buffers at TX, RX)
For k = 3 bits, W = 7:
Utilization U = W/(2a+1) = 7/(61) = 11.5% > 1.6% for Stop and wait.
Shorter links are better (small a)
25
Error Control
Use of retransmission to handle errors detected in frames (Backward Error Handling)
This process is called Automatic Repeat Request (ARQ)
Types of Problems: Damaged frames
(Frame arrives at RX but in error) Lost frames
(Noise burst damages frame beyond recognition- so not recognized by RX)
Frames arriving too late e.g. due to network congestion- Will be ignored or dropped by time-out.
WK 14
26
Error Control Techniques:
Error detection: Chapter 6 Positive acknowledgment: (for one or more frames)
From RX for Error-free frames, e.g. RR i Negative acknowledgement requesting retransmission of
a lost or damaged frame:
RX sends negative ACK for damaged or lost frames, requesting retransmission, e.g. REJ i How does RX detect a “lost” frame? Through receiving the next
frame “out of sequence” – Unexpected (frames are numbered!) Retransmission after timeout:
TX automatically retransmits a frame that has not been acknowledged following a predetermined time-out interval
27
Categories of Error Control MechanismsMain types of ARQ-based standard error control mechanisms
28
Stop and Wait ARQ: Possible Scenarios Scenario for Damaged/Lost Frame TX transmits a single frame (keeping a copy)
Then waits for ACK from RX If frame reaches RX damaged (in error):
RX discovers this through error detection It then discards the frame, and does not send ACK TX “times out” on waiting for ACK … and then retransmits the frame again automatically RX thus receives only one correct copy of the frame
Same scenario applies if frame was lost
29
Stop and Wait ARQ: Scenario for Lost ACK
ACK from RX for a correct frame is lost (reaches TX damaged beyond recognition): TX will timeout and retransmit the same frame again! RX gets two copies of that frame Without labeling frames, RX will consider both copies as
two different valid frames (data duplication is not good) To avoid this, TX labels frames alternately as 0, 1
(enough for Stop and Wait) RX uses ACK0 & ACK1, Similar to sliding window RRn:
ACK0: Received 1 and ready for 0 (better named RR 0) ACK1: Received 0 and ready for 1 (better named RR 1)
30
Stop and Wait ARQ
ACK
RX gets two good copies of F1.Labeling frames allows RX torealize this and discard one of them.
= RR 1
Lost FrameScenario
Lost ACKScenario
31
Stop and Wait - Pros and Cons
Simple Inefficient (As seen with flow control)
For improved efficiency, we use sliding-window based ARQ (Continuous ARQ)
32
Sliding Window ARQ Improves line utilization by sending up to W frames
before worrying about ACK A form of Pipelining (several tasks started before 1st
task is finished) TX uses a window to mark frames to be transmitted
until they are sent and acknowledged The window size W should be ≤ 2k – 1, k is the size of the
frame sequence field in the frame header, Frame are given sequence numbers modulo 2k, i.e. for k = 3:
0,1,2,3,4,5,6,7,0,1,2,….. W is fixed in this protocol, but may be variable in others
Each time a proper ACK is received for a number of frames the TX window slides past them, hence the name Sliding Window. This: Releases those frames for deletion from TX buffer memory Introduces new frames for transmission
33
TX Sliding Window
In (b), frames 0 and 1 were properly acknowledged
S Pointer to next frame to be sent
Frames that can be sent
F0, F1Positively ACKed
Frames already sent but not yet ACKed
- Window now has a fixed width (W) and slides rigidly as one pieceupon receiving ACKs. - Within the window, frame sending is handled using a send (S) pointer
Slide window as a whole
(Important: Pointer is not pushed with window)
34
RX Sliding Window Error Control
Size of the RX window for error control is always 1 The RX window contains the sequence number for
the frame expected to be received next If a different frame arrives (i.e. out of sequence
arrival), it is immediately discarded and the window does not slide
Once the expected frame arrives correctly, the window slides one step to point to the next expected frame
35
RX Sliding Window
F0 Received Correctly
F1 now expected
F0 now expected
36
Sliding Window ARQ: Summary Uses sliding windows (now rigid- moves as a
whole) at TX and RX to track frame movement TX uses timeout on waiting for ACK If no error: RX acknowledges with RR i, where i is
number of the next frame expected
Frames received correctly and have been or will soon be acknowledgedFrames received correctly.Have or will soon be acknowledged
Frames waiting to be received
Next frame to be sentExpected next frame to be received
As you transmit
As you receive ACKs
As you receive expected frame without error
Size = 1Size ≤ 2k-1
k
37
Sliding Window ARQ: Two main standard approaches:
Go Back N Selective Reject
38
Sliding Window ARQ: Go Back N Error Scenarios:
Will consider the following error scenarios Damaged Frame Lost Frame Lost ACK
Lost Positive ACK (RR) Lost Negative ACK (REJ)
39
Go Back N ARQ: Error Scenarios
Damaged Frame:
RX received frame i damaged RX discards frame i and all subsequent frames until
frame i is received correctly RX can either:
Scenario 1.A: Send a negative ACK (REJ i ) TX must go back (hence the name go back N) and
retransmit that frame and all subsequent frames that were transmitted in the mean time
Scenario 1.B: Does not send REJ (relies on TX time out) Handled as a lost frame (next) (and as lost negative ACK)
40
Go Back N ARQ: Error Scenarios
Lost Frame: RX expects frame i but does not get it, So TX does not get any ACK for it Scenario 2.A:
TX carries on sending subsequent frames, i+1, … RX gets frame (i+1) out of sequence (as it did not get frame i).
This allows RX to detect the problem
RX then either: Sends REJ i (Note i, not i+1) Or: Takes no further action (relies on TX time out)
TX must then go back and retransmit frame i and all subsequent frames transmitted in the mean time
41
Go Back N ARQ: Error Scenarios Lost Frame, i:
TX kept sending, RX does nothing, TX times out
F3 Arriving outof sequence (Not the next expected frame ),RX ignored it, butdid nothing
RX frame pointerdoes not leave ‘2’ until F2 is correctly received
LHS of TX window: Frames sent but not yetACKed
Go back 1
S points to next frame tobe TXed
R points to next frame tobe RXed
Slide W toUncover F0, F1
On F2
42
Go Back N ARQ: Error Scenarios
Lost Frame, Contd. Scenario 2.B:
TX does not send further frames after i, and RX does not send any RR or REJ (RX does not know there is a problem!)
TX times out waiting for RX response Scenario B-1:
TX sends a polling command (RR with P bit = 1) to force RX to report its receiving status by sending RR i
When TX receives the RR I response, it retransmits frame I and all subsequently sent frames
Scenario B-2: TX retransmits frame i after timeout
43
Go Back N ARQ: Error Scenarios Lost Positive ACK (RR) from RX:
RX gets frame i OK and sends a positive ACK: RR (i+1) This RR frame is lost on its way to TX Two Scenarios:
Scenario 3.A: A later Acknowledgement from RX, e.g. RR (i+n) manages to
reach TX before TX times out. This solves the problem, since ACKs are cumulative
Scenario 3.B: TX times out before receiving any subsequent RR acks:
TX sends a polling command (RR with P bit = 1) to force RX to report its receiving status by sending RR i.
TX Repeats step above a few times, if no success it initiates a Reset procedure
44
Go Back N ARQ: Error Scenarios
Lost Negative ACK (REJ) from RX:
Similar to RX not sending REJ
i.e. Scenario 1.B for Damaged Frame.
45
Go Back N ExampleTX doesnot wait
Got 0, 1Ready for 2
Got 3Ready for 4
Got 5 not 4 REJ 4 and beyond
Frame 4lost on the way
RR 7 (+ ive ACK)lost on the way.
Transmitter Receiver
TX polls RXTo send receivingstatus
Go back 3(4, 5, 6, 7) andRetransmitthem
Go back in window untilyou meet the rejectedFrame #, and resumetransmission from there
3 4 5 6 7 0 1 22107
Any corrective actionneeded here at TX?
TX times outBefore gettingany subsequentRRs after lost one
F bit = 1
This is acommand
46
Two neighboring nodes (A and B) use a sliding-window go-back-N for error control with a 3-bit sequence number. The window size is 4. Assuming A is TX and B is RX, show the window positions at A for the following situations:
a) Before A sends any frames
a) After A sends frames 0, 1, 2 and receives RR 2 from B
b) Later, after A sends frames 3, 4, and 5 and receives RR 5 from B
Example:
Send Next
At TX
47
Window Size limit for Go-back-N ARQ
Size of the window must be W 2k – 1 i.e. W < 2k where “k” is the number of bits reserved (in the control field) for the sequence number
Let k = 2,
i.e. W should be < 4 By comparing the figures
opposite for W = 3 and W = 4, justify the need for this limit on window size in Go Back N
F0F0
F0
F0 sent twice in both cases. Mistake is detected only on the left (for the correct W size)
All happened before timeout
on F0
on F0TX goes back to send F0
TX goes back to send F0
F0
F0
W 2k – 1
ACK ACK
x
By mistakeRX gets 2 copies of F0
48
Selective Reject (Selective Repeat ARQ)
Also called selective retransmission RX requests retransmission of only the rejected
frame using SREJ i Subsequent frames received after the rejected
frame are buffered at RX (not thrown away as in Go Back N ARQ)
TX retransmits only the frame that was specifically rejected and those that timed out
Less retransmission traffic than Go Back N
49
Selective Reject: Example
5, 6
- Waiting for 4 - 4 gets lost
5 received out of sequence, Problem detected, SREJ 4 sent
Acknowledge up to 6
• Only rejected frame (4) retransmitted(not 4,5,6)• Normal transmission resumes where left (7)
TX times out before getting subsequent RRs to the lost ones
So it polls RX for its receiving status
Things turned out OK
+ ive ACK lost on the way!
TX sends:5,6,4,7,0(complex sequencing)
F bit = 1
50
Selective Reject: Pros and Cons Minimizes retransmission Better link utilization
(Useful where link utilization is poor- sending a frame is an inefficient process) e.g. short frames on long (e.g. satellite) links
But more complex: (so, used less than Go back N) Receiver:
Must maintain large enough buffer to save post-SREJ frames Needs logic for inserting requested frame in place when it arrives later
Transmitter: Needs logic to allow sending the requested frame out of normal
sending sequence Also, more restricted window size, W:
With k bits, max window size is 2(k-1) (vs 2k-1 for Go Back N),
e.g. for k = 3: Wmax = 4 for SREJ, Wmax = 7 for GO Back N
51
How such flow and error mechanisms are implemented:High-Level Data Link Control Protocol (HDLC) HDLC is the most common data link control
protocol and forms the basis for many others Runs in the Data Link Layer (Layer 2 in OSI) Main Functions:
Flow Control: Data is transmitted by TX- only as fast as RX can absorb it.
Error Control: Objective: Pass data up to higher layer exactly as transmitted, i.e.: Without error, Without loss, Without duplication, and in the correct order
52
High-Level Data Link Control Protocol (HDLC) To satisfy various applications, HDLC defines:
Three types of communicating stations Two types of link configurations, and Three data transfer modes of operation.
53
High-Level Data Link Control Protocol (HDLC)Station types:
Primary Station (PS): (e.g. computer) Responsible for controlling operation of the link Control frames issued by the PS are called
commands Secondary Station (SS): (e.g. terminal)
Operates under the control of a primary station Control frames issued by the SS are called
responses Combined Station (CS):
Issues both commands and responses
54
High-Level Data Link Control Protocol (HDLC)
Link configurations:
Determined by the types of stations on the link Unbalanced (different status): e.g. One primary station
plus one or more secondary stations Balanced (same status): e.g. Two combined stations
55
High-Level Data Link Control Protocol (HDLC)Data Transfer modes: (Who can send?, what? and when?)
For unbalanced link configuration: (Response Modes) Normal Response Mode (NRM)
Secondary may send data only in response to a command from primarye.g. Computer (PS) connected to a number of terminals (SS) over a multi drop line. Computer polls terminals for data
Asynchronous Response Mode (ARM) Secondary may send data without explicit permission from primary. Primary still retains link control (initialization, error recovery, logical disconnection, …)(Rarely used)
56
High-Level Data Link Control Protocol (HDLC)Data Transfer modes: (Who can send?, what? and when?)
For balanced link configuration Asynchronous Balanced Mode (ABM) Either combined
stations may send data without obtaining permission from the other station
Most widely used (no polling involved)
e.g. full duplex point-to-point
57
HDLC – Frame Structure HDLC Uses Synchronous Transmission, i.e. large frames Frame consists of the following ‘fields’:
Flag fields at start and end: for frame-level sync Address field: for addressing in multi-point links Control field: for Flow and Error control Information field: (payload data or link management data) FCS field: for error detection
58
HDLC – Frame StructureFrame
Address Field
Control Field
59
HDLC Frame Format Flag:
Size: 1 Byte Special pattern 0 1 1 1 1 1 1 0 used as frame begin/end and synch. Used for Header and trailer
Address: Size: 1 Byte (or extendible to more bytes for larger networks) If primary station created the frame, the address is that of the destination
secondary station If secondary station created the frame, the address is that of the source
secondary station Networks not using “primary/secondary” (e.g. Ethernet) use 2-Byte address
(source/destination) Control:
Size: 1 or 2 Bytes Identifies frame type (I, S, U) Used for error & flow control
Information: Payload of user data (I Frames) or link management data (U) Size: Varies (as multiple bytes) from network to network. Always fixed within a
network I frames contains user data received from the higher layer (Network layer)
FCS: Size: 2 or 4 Bytes (depending on the divisor P used) Implements CRC for error detection
60
HDLC – Frame Structure: Flag field
Flag Field: unique pattern 01111110 at both start and end of frame
Used for frame-level synchronization This pattern should not exist in any other part of the frame Two ways to ensure this:
Ensure that higher layers avoid using these pattern in the data they generate (causes lack of data transparency)
TX uses bit stuffing for data (inserts a 0 after each consecutive five 1s) to ensure data transparency at higher layers
61
HDLC Bit Stuffing: At TX:
Inserts a 0 after each consecutive five 1s of non-flag data
(01111101….)
At RX:After detecting the preamble flag, RX monitors incoming bits – when a pattern of five 1s appears; the 6th and the 7th bit are checked: If 00 or 01 Bit 6 is a stuffed 0 Remove it If 10 This is the postamble flag end of frame If 11 ABORT (Error- there must be a 0 after every
five 1’s)
62
HDLC Bit Stuffing
Flag pattern existing in original user data
Not any more- with bit stuffing!
63
HDLC Bit Stuffing & Removal
Data handed in for transmission Would this data havecaused any frame sync problem?
TX may ensure that the flag patterndoes not occur in non-data fields
64
HDLC Bit Stuffing Problems: Single-bit errors could split a frame into
2 or merge two frames into 1.
01111100(Bit stuffed)
(Bit stuffed)
01111110
FrameSplitting
FrameMerging
01111110
01111100
65
HDLC Frame Types HDLC defines three types of frames Frame type determined by first bits in the control field
User Information frames (I-frames): 8 or 16 bit control field Used to transport user data In a duplex system, they can carry control messages regarding
previously received data to be carried along with data TXed (piggybacking)
Supervisory frames (S-frames): 8 or 16 bit control field Used to transport control information only- Short frame, No
Data Unnumbered frames (U-frames): 8 bit control field
Provides link management functions Does not carry a frame sequence number (N or S) Can carry Management data
66
HDLC I, S & U Frames Format
RR jRNR jREJ jSREJ j
Arrow indicates that the first transmitted field is the Header flag, then the address, then the control…
67
HDLC I-Frames Designed to carry user data received from Network layer They can include flow & error control information (piggybacking in
duplex links) Their Control field can be either 8 bits or 16 bits (Defined at link
initialization) An 8-bit Control field consists of:
First bit identifies the frame type: “0” for I-frames Next 3-bits, called N(S), indicate the sequence number of the
transmitted frame So frame sequence numbers are 0,1,2,3,4,5,6,7
Next bit is called P/F (Poll or Final) When a primary station polls other stations, sets this bit to 1 (P bit) When a secondary station responds to a poll, sets this bit to 1 (F bit) i.e. same bit has different meanings depending on source
Next 3-bits, called N(R), define the frame sequence number for the RR carried when piggybacking is used (next expected frame)
68
HDLC I-Frames
16-bit control field: Extends N(S) and N(R) to 7 bits each instead of 3 bits, allowing larger windows to be used
Note: Control field contains no ‘Function’ part - So, only the + ive ACK function (RR) can be sent as a default in piggybacking!
8-bit Control filed# of Frame sent(This frame)
# of next Frame expectedPositive ACK: = RR N(R)
With Piggybacking
Duplex Link- Source - Destination
Frame sequenceNumbering is modulo ?
Carries 2 sequence numbers
69
HDLC Supervisory (S)-Frames (S is also for short!)
RRRNRREJSREJ
• The workhorse for sending separate ACKs. Used when:- No user data to send, or - ACK is not RR, i.e. (RNR, REJ, SREJ)
• This is the only method to send ACKs other than RR
Example: RNR 6
00011011
0 1 1 1 0
First 2 bits:’10’ identifies frame
as an S-Frame
Assume:Carries only 1 sequence number
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HDLC U-Frames
Examples of Link ManagementFunctions(up to 32) As a As a
Used for Link management operations
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HDLC U-Frames
Examples of Network ManagementFunctions
As a As a
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HDLC Frame Structure – Address Field
Address field identifies the secondary destination station transmitting the frame or intended to receive the frame
Not needed for point-to-point links (only one source and one destination) - but included for uniformity
Extendable – in multiples of 7 bits The unique address (11111111) (single octet) is used by
the primary to broadcast to all secondary stations
Extended Address Field‘1’ marks last octet of extendible address
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HDLC Frame Structure – Control Field
Poll/Final (P/F) bit: In command frames (P): used to solicit response from peer entity In response frames (F): indicate response is the result of soliciting command
Frame Type:
I or not S or U
Extended frame sequence #s (7 instead of 3 bits)
Why N(R) on an I frame?
ACKs such as:REJ N(R)
Data-carrying,Allows Piggybacking
Additional link control functions
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HDLC Frame Structure – Information/FCS Fields Information field:
Present ONLY in I-frames and some U-frames Contains an integer number of octets (bytes) Variable number of octets – up to some system
defined maximum
m bytes
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HDLC Frame Structure – Information/FCS Fields FCS field:
CRC Error detection code Calculated from ALL remaining bits in frame
(excluding the two flags) Normally 16 bits: (F is 1-bit shorter than P)
- (CRC-CCITT polynomial = X16+X12+X5+1), or
- 32-bit optional FCS using CRC-32
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HDLC – Operation HDLC Operation: Exchange of I-frames, S-
frames, and U-frames between two stations Table 7.1 lists types of Control/Response functions for
various frame types The operations of HDLC involve three phases:
Link Setup or Initialization (by either side): with U-Frames Both agree on various options
Actual Data Transfer (by the two sides): I- and S-Frames Exchange of user data and control info for flow and error control
Link Disconnect (by either side): U-Frames Indicating termination of operation
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Some U-frame commands and responsesCommand/response Meaning
SNRMSNRM Set normal response mode
SNRMESNRME Set normal response mode (extended)
SABMSABM Set asynchronous balanced mode
SABMESABME Set asynchronous balanced mode (extended)
UPUP Unnumbered poll
UIUI Unnumbered information
UAUA Unnumbered acknowledgment
RDRD Request disconnect
DISCDISC Disconnect
DMDM Disconnect mode
RIMRIM Request information mode
SIMSIM Set initialization mode
RSETRSET Reset
XIDXID Exchange ID
FRMRFRMR Frame reject
Table 7.1 (Subset)
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HDLC – Operation1. Initialization (Link Setup)
2. Data Transfer, 3. Link Disconnect
A issues one of 6 set mode commands: and sets a timer:SNRM, SARM, SABM (k = 3 bits: Modulo 8 frame
sequencing) 8-bit control fieldorSNRME, SARME, SABME (k = 7 bits: Modulo 128
frame sequencing) 16-bit control field B responds with either:
UA (Unnumbered ACK) (if it agrees), or: or DM (Disconnect mode) (if request is rejected)
A receives the UA, initializes its variables for data exchange, and data exchange takes place
After finishing: to disconnect, A sends DISC command Will get a UA from B
SABME: Set Asynchronous balanced/extended mode;7-bit frame sequence
2. Data Transfer
1. Link Setup
3. Link Disconnect
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HDLC – OperationDisconnect: Either side can issue a DISC U-frame to request
disconnect The remote entity MUST accept the request by
sending UA Any outstanding unACKed I-frames may be lost.
These can be recovered by higher layers
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HDLC – OperationNormal Data Transfer: Once initialization is complete, a logical path is
established. Both sides start sending I-frames (Full-duplex
exchange) starting with sequence number 0 N(S), N(R) are sequence numbers to support
flow and error control in the send and receive directions, respectively
N(R) is the ACK for the I-frame received; it allows the HDLC module to indicate the I-frame number it expects to receive next (Note: No control function used for this positive ACK- understood by default as RR)
when no reverse user data (I-frame) needs to be sent, a dedicated RR should be sent on an S-frame
If no new acknowledgement needs to be sent, the last N(R) value is repeated
S-Frame
Repeat N(R)if no new Fsreceived +1
+1
+1
same
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HDLC – Operation Busy condition: Note use of P/F bit
When A is unable to keep up with the speed of the transmitter “B” or buffer is full
A sends RNR, to halt transmission from B
To check the readiness of A, B periodically sends RR frame with P set
Once the busy condition is cleared at A, it responds with an RR and F=1
An RR with F set indicates it is a response to a previous polling using RR, P
+1
At last !!B sending 4 andwaiting for 0
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HDLC – Operation Reject RecoveryReject Recovery:
I-frame 4 was lost B receives I-frame 5 (out
of order) – responds with REJ 4
A resends I-frame 4 and all subsequent frames previously sent
(Go-back-N) For any – ive ACK must use
an S-Frame!
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HDLC – Operation Timeout RecoveryTimeout Recovery::
A sends I-frame 3 – but it is lost Timer expires before
acknowledgement arrives A polls B with RR, P = 1 B responds with RR, F =1, indicating it
is still waiting for frame 3 A responds by retransmitting I-frame 3 This time it gets a + ive ACK from B