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S Chen
Simplex Stop and Wait Protocol
•^
Flow control
deals with problem that
sender transmits frames faster thanreceiver can accept,
and solution is
to limit sender into sending no fasterthan receiver can handle
-^
Consider the
simplex
case:
data is
transmitted
in
one
direction
(Note
although data frames are transmittedin one direction, frames are going inboth directions, i.e. link is
duplex
•^
Stop
and
wait
:^
sender
sends
one
data
frame
,^
waits
for
acknowledgement
(ACK)
from
receiver before proceeding to transmitnext frame^ –^
This simple flow control will breakdown if ACK gets lost or errorsoccur
sender may wait for ACK
that never arrives
S Chen
Simplex Stop and Wait with ARQ
•^
For noisy link, pure stop and wait protocol will break down, and solution is to incorporate some error control
mechanism
•^
Stop
and
wait
with
ARQ
:^ Automatic Repeat reQuest
(ARQ), an error control method,
is
incorporated with stop and wait flow control protocol^ –^
If error is detected by receiver, it discards the frame and send a
negative
ACK (NAK), causing
sender to re-send the frame
-^
In case a frame never got to receiver, sender has a
timer
: each time a frame is sent, timer is set
If no ACK or NAK is received during timeout period, it re-sends the frame
-^
Timer introduces a problem:
Suppose timeout and sender retransmits a frame but receiver
actually received the previous transmission
receiver has
duplicated
copies
–^
To avoid receiving and accepting two copies of same frame, frames and ACKs are alternatively labeled
0 or 1: ACK0 for frame 1, ACK1 for frame 0
•^
An important
link parameter
is defined by
a^ =
propagation time
frame time
=^
R d V L
where
R
is data rate (bps),
d^
is link distance (m),
V^
is propagation velocity (m/s) and
L
frame
length (bits)
S Chen
Sliding Window Protocol
•^
For large link parameter
a, stop and wait protocol is inefficient
•^
A universally accepted flow control procedure is the
sliding window protocol
–^
Frames and acknowledgements are numbered using
sequence numbers
–^
Sender maintains a list of sequence numbers (frames) it is allowed to transmit, called
sending
window
-^
Receiver maintains a list of sequence numbers it is prepared to receive, called
receiving window
–^
A sending window of size
N
means that sender can send up to
N
frames without the need for
an ACK
-^
A window size of
N
implies buffer space for
N
frames
–^
For
n-bit sequence number, we have
n 2 numbers:
,^2
n^ −
1 , but the maximum window
size
N
n^ −
(not
n 2
–^
ACK3 means that receiver has received frame 0 to frame 2 correctly, ready to receive frame 3(and rest of
N
frames within window)
•^
In error-free case, efficiency or maximum
link utilisation
of sliding window protocol is:
U^
^
1 ,^
N >
2 a
N 1+2a
,^
N <
2 a
Thus it is able to maintain efficiency for large
link parameter
a: just use large widow size
N
S Chen
Sliding Window (continue)
•^
Note that
U
means that link has no idle time: there are always something in it, either data
frames or ACKs
-^
Consider the case of 3-bit sequencenumber with maximum window size N^
•^
This illustration shows thatSending and receiving windows canshrink or grow during operationThe
receiver
do
not
need
to
acknowledge every frames
-^
If^
both
sending
and
receiving
window
sizes
are
N
,^
the
sliding window protocol reduces tothe stop-and-wait
3
5
0
0 1
2
4
1
3 4
6 0
1
0 1
2 3
4 5
(^10)
6
6
2 3
4 5
6 0
1
F1 F (^10) (^65) (^43) 2 (^0 ) (^54) (^32) (^10)
0 1
2 3
4 5
6 0
1 2
3 4
5 6
0
0 1
3 2 3 4 5
6
1 2
3
5 6
0
0
4
1 1
F0 ACK
0 1
2 3
4 5
6 0
1 2
3 4
5 6
0 1
F3 F^
0 1
2 3
4 5
6 0
1 2
3 4
5 6
0 1
F5 F (^10) 6 (^4 ) (^21) (^06) (^54) (^32) (^10)
ACK
A:^
B:^0
1
2 3
4 5
6 0
1 2
3 4
5 6
0 1
0 1
2 3
4 5
6 0
1 2
3 4
5 6
0 1
2
5
•^
In practice, error control must be incorporated with flow control, and we next discuss two commonerror control mechanisms
S Chen
Selective-Reject ARQ
•^
In^
selective-reject
ARQ error control, the only frames retransmitted are those
receive a NAK or which time outAn illustrative example:
•^
Selective-reject would
appear
to
be
more efficient than go-back-n, but itis harder to implement and less used
E^
ACK
ACK
NAK error 1
ACK
ACK
ACK
ACK
buffered
F2-5 released
•^
The window size is also more restrictive: for
n
-bit sequence number, the maximum
window size is
N
n 2 2
to avoid possible confusion
•^
Go-back-n and selective-reject can be seen as trade-offs between
link bandwidth
(data rate) and data link layer
buffer space
–^
If link bandwidth is large but buffer space is scarce, go-back-n is preferred
–^
If link bandwidth is small but buffer space is pretty, selective-reject is preferred
S Chen
From Simplex to Duplex
•^
So far, we consider data transmission in one direction (simplex), although the link is duplex
-^
If two sides exchange data (duplex), each needs to maintain two windows: one for transmitting andone for receiving
-^
In^
duplex
communication, frames transmitted from either side can be data, ACKs and NAKs
the need to distinguish them
-^
Frame type
: Recall in frame header there is a control field, and part of it is typically used as frame
type field to tell what type the frame is
-^
Piggybacking
: In duplex situations, piggybacking is often used
If one has data and an ACK to
send, it sends both in one frame
-^
Discussion so far: data link layer is primarily concerned with making point-to-point link reliable^ –^
It is responsible for transmitting frames from sender to receiver (service to network layer), andcan only uses physical layer to do the job
-^
It has to take into account that transmission error may occur and sender/receiver may operate atdifferent speeds
error control/flow control (ACKs, NAKs, CRC, windows, sequence numbers)
–^
Next lecture will see how all these fit into some data link layer protocols
S Chen
Protocol Verification
•^
How to know a protocol really works
specify and verify protocol using, e.g. finite state machine
–^
Each protocol machine (sender or receiver) is at a specific state at every time instant
-^
Each state has zero or more possible transitions to other states
-^
One particular state is initial state: from initial state, some or possibly all other states may bereachable by a sequence of transitions
-^
Simplex stop and wait ARQ protocol:^ –^
State
SRC
:^
S^
=^
0 ,^
which
frame sender is sending;
R
,^1
which frame receiver is expecting; C^
=^
0 ,^
1 , A
(ACK)
,^ −
(empty)
channel state, i.e. what is in channel
-^
There are 9 transitions
Transition^012345678
Whoruns?–RSRSRRSS
Frameaccepted^0 A^1 A^01 (timeout)(timeout)
FrameemittedA^1 A^0 AA^01
Tonetworklayer–Yes–Yes–NoNo––
0 0 –
0 1 –
0 1 0
0 0 0
0 1 A 1 1 1 1 1 –
1 0 A
1 0 – (^0) 1 0 1 0 8
0 7
7
0
8
4
2
6
0
0
5
(^13) (a)^
(frame lost) (b)
–^
Initial state
: sender has just sent frame 0, receiver is expecting frame 0, and frame 0 is
currently in channel
-^
Transition 0 consists of channel losing its contents, transition 1 consists of channel correctlydelivering frame 0 to receiver, and so on
-^
During normal operation, transitions 1,2,3,4 are repeated in order over and over: in each cycle,two frames are delivered, bringing sender back to initial state
S Chen
Summary
•^
Flow control and error control techniques for data link layer:Stop and wait ARQ, sliding window, go-back-n, selective-reject (repeat)
•^
Data link layer (part I) discussed so far:It is concerned with making a point-to-point link reliable, and is responsible fortransmitting frames from sender to receiver, can only use physical layer to do job
•^
Error control and flow control (ACKs, NAKs, CRC, sliding windows, sequencenumbers, go-back-n etc.):How these are included in a data link layer protocol will be discussed in next lecture
