This document discusses various data link layer protocols for flow control. It describes an unrestricted simplex protocol with infinite buffer capacity and no need for flow control. For a simplex stop-and-wait protocol, a sequence number is needed to distinguish frames when the channel is noisy. A simplex protocol for a noisy channel uses positive acknowledgement and retransmission. This becomes a duplex channel, requiring a type field and ability to piggyback acknowledgements. A sliding window protocol maintains a sender and receiver window of allowed sequence numbers. It provides flexibility while ensuring ordered delivery to the network layer. Different sliding window protocols are examined, including ones using go-back-N and selective repeat.

1. Data Link Layer Protocols
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Mrs.R.Sabitha

2. Flow Control: Initial Assumptions
• Simplex Channel
• Infinite buffer capacity with the receiver
• Error free transmission
• Network layer at the senders end is always
ready with data
• No need for flow control

3. Protocol Definitions

4. Protocol
Definitions

5. Unrestricted
Simplex
Protocol

6. • Infinite capacity in the buffer of the receiver.
• Need for “flow control”
• Stop-n-Wait protocol
– Sender sends a frame
– And waits for a signal in the form of a dummy frame
– No seq no. is required since the line is still error free

7. Simplex
Stop-and-
Wait
Protocol

8. • The channel is noisy, frames may be damaged or
lost
• Advantage : data frame reaches intact, ack sent
back and received, next frame sent
• Disadvantage :
– Data frame damaged or lost ..hence no ack – sender
times out and resends .. No problems
– Data frame reaches intact but Ack lost .. Times out
..resends.. Receiver receives duplicate frames..Problem

9. Problem of Duplicate frames
• Solution :
– Keep a sequence number for each frame to distinguish
between the new frame and a duplicate frame.
• How large should be the sequence number? Or
• What should be minimum number of bits required
for the sequence number?
– The only ambiguity at the receiver is between two
successive frames.. Say m and m+1 and never between
m-1 and m +1 .. Its only after the ack for m-1 reaches
back intact mth frame is sent.. And once mth frame
reaches intact at the receiver’s end .. Story of m-1th
frame is over .. However depending upon whether ack
of mth frame reaches back intact or not either mth or
m+1 th frame is sent .. hence only 1 bit is sufficient

10. A Simplex Protocol for a Noisy
Channel
A positive
acknowledgement
with retransmission
protocol.
Continued 

11. A Simplex Protocol for a Noisy Channel
(ctd.)
A positive acknowledgement with retransmission protocol.

12. • Channel is simplex ..
• So, Now communication is two way .. I.e.
Duplex channel
– Interleave data and ack .. So ..a kind field is
required to specify whether the frame contains
data or ack.
– Piggyback ack on data frame

13. Sliding Window
Protocol(Duplex)
• At any instant of time, the sender maintains a set
of sequence numbers corresponding to frames it is
permitted to send – this includes
– frames not yet sent
– Frames sent but not yet acknowledged .. May have to
be sent again
This corresponds to the sender’s window. When a packet
comes from the NL ..it is appended in the window and
when an ack arrives for a frame..it is deleted from the
buffer(and from the window)

14. Sliding Window
Protocol(Duplex)
• Similarly, the receiver also maintains a set of
sequence numbers corresponding to frames it is
permitted to accept.
– When a frame arrives, its seq number is checked..if it
falls in the window it is accepted else discarded .. Once
an ack is sent for a frame it is deleted from the window
so that duplicate frames are not accepted.
• Sliding window gives the DLL the flexibility to
accept the frames in any order but they must be
handed over to the NL in order.

15. Sliding Window Protocols (2)
A sliding window of size 1, with a 3-bit sequence number.
(a) Initially.
(b) After the first frame has been sent.
(c) After the first frame has been received.
(d) After the first acknowledgement has been received.

16. Sliding Window Protocols
• A One-Bit Sliding Window
Protocol
• A Protocol Using Go Back N
• A Protocol Using Selective Repeat

17. A One-Bit Sliding Window
Protocol
Continued 