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ECE453 – Introduction to Computer Networks. Lecture 5 – Data Link Protocols (II). TBD. CRC Checksum. Elementary Data Link Protocols. An Unrestricted Simplex Protocol A Simplex Stop-and-Wait Protocol A Simplex Protocol for a Noisy Channel. Header. Data. Trailer.

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ece453 introduction to computer networks

ECE453 – Introduction to Computer Networks

Lecture 5 – Data Link Protocols (II)

elementary data link protocols

TBD

CRC Checksum

Elementary Data Link Protocols
  • An Unrestricted Simplex Protocol
  • A Simplex Stop-and-Wait Protocol
  • A Simplex Protocol for a Noisy Channel

Header

Data

Trailer

unrestricted simplex protocol the utopia
Unrestricted Simplex Protocol – the Utopia
  • Both the transmitting and receiving network layers are always ready
  • Infinite buffer space is available
  • The communication channel between the data link layers never damages or loses frames
  • Data are transmitted in one direction only
  • Processing time can be ignored
unrestricted simplex protocol
void sender1 (void)

{

frame s;

packet buffer;

while (true) {

from_network_layer(&buffer);

s.info = buffer;

to_physical_layer(&s);

}

}

Unrestricted Simplex Protocol

void receiver1 (void)

{

frame r;

event_type event;

while (true) {

wait_for_event(&event);

from_physical_layer(&r);

to_network_layer(&r.info);

}

}

simplex stop and wait protocol assumptions
Simplex Stop-and-Wait Protocol - Assumptions
  • Both the transmitting and receiving network layers are always ready
  • Infinite buffer space is available
  • The communication channel between the data link layers never damages or loses frames
  • Data are transmitted in one direction only
  • Processing time can be ignored
simplex stop and wait protocol
Simplex Stop-and-Wait Protocol

void receiver2 (void)

{

frame r, s;

event_type event;

while (true) {

wait_for_event(&event);

from_physical_layer(&r);

to_network_layer(&r.info);

to_physical_layer(&s);

}

}

void sender2 (void)

{

frame s;

packet buffer;

event_type event;

while (true) {

from_network_layer(&buffer);

s.info = buffer;

to_physical_layer(&s);

wait_for_event(&event);

}

}

simplex stop and wait protocol1

TBD

Simplex Stop-and-Wait Protocol

Header

Data

CRC

frame_kind kind

a simplex protocol for a noisy channel also a stop and wait
A Simplex Protocol for a Noisy Channel (also a Stop-and-Wait)

Both the transmitting and receiving network layers are always ready

Infinite buffer space is available

The communication channel between the data link layers never damages or loses frames

Data are transmitted in one direction only

Processing time can be ignored

a simplex protocol for a noisy channel

void sender3 (void)

{

seq_nr next_frame_to_send;

frame s;

packet buffer;

event_type event;

next_frame_to_send = 0;

from_network_layer(&buffer);

while (true) {

s.info = buffer;

s.seq = next_frame_to_send;

to_physical_layer(&s);

start_timer(s.seq);

wait_for_event(&event);

if (event == frame_arrival) {

from_physical_layer(&s);

if (s.ack == next_frame_to_send) {

stop_timer(s.ack);

from_network_layer(&buffer);

inc(next_frame_to_send);

}

}

}

A Simplex Protocol for a Noisy Channel

void receiver3 (void)

{

seq_nr frame_expected;

frame r, s;

event_type event;

frame_expected = 0;

while (true) {

wait_for_event(&event);

if (event == frame_arrival) {

from_physical_layer(&r);

if (r.seq == frame_expected) {

to_network_layer(&r.info);

inc(frame_expected);

}

s.ack = 1 – frame_expected;

to_physical_layer(&s);

}

}

}

simplex stop and wait protocol in noisy channel

TBD

Simplex Stop-and-Wait Protocol in Noisy Channel

Header

Data

CRC

frame_kind kind

seq_nr seq

seq_nr ack

sliding window protocols
Sliding Window Protocols
  • Assumptions
    • Both the transmitting and receiving network layers are always ready
    • Infinite buffer space is available
    • The communication channel between the data link layers never damages or loses frames
    • Data are transmitted in one direction only
    • Processing time can be ignored
  • Go Back N (GBN)
  • Selective Repeat (SR)
piggybacking
Piggybacking
  • Temporarily delaying outgoing acknowledgements so that they can be hooked onto the next outgoing data frame

Header

Data

CRC

frame_kind kind

seq_nr seq

seq_nr ack

slide14
GBN
  • Sender is allowed to transmit multiple packets without waiting for an acknowledgement, but is constrained to have no more than some maximum allowable number (N)
  • Use cumulative acknowledgement
  • Discard out-of-order packets, no receiver buffering
idea behind sliding window
Idea Behind Sliding Window

1

2

3

4

5

1

2

3

4

5

Initial window

Window slides

Send packet 1

Receive packet 1

Send ACK1

Send packet 2

Receive packet 2

Send ACK2

Send packet 3

Receive ACK 1

Receive packet 3

Send ACK3

Receive ACK 2

Receive ACK 3

slide17
SR
  • Out-of-order packets are buffered until any missing packets are received (receiver also has a window)
  • Re-acknowledgement is required, otherwise the sender’s window will never move.
summary reliable data transfer rdt protocols
Summary – Reliable Data Transfer (RDT) Protocols
  • Utopia
  • Stop-and-wait protocol
    • Simplex stop-and-wait
    • Simplex stop-and-wait in noisy channel
  • Sliding window protocols
    • Go-Back-N (GBN)
    • Selective Repeat (SR)