COMT 625 1

Performance of Data Communications Protocols

General Protocol Concepts

COMT 625 2

Outline

• General protocol principles for flow control and error handling, including:

• Examples of protocols using these schemes• Principles of interconnecting networks• Bridges, Switches, Routers, Gateways

COMT 625 3

OSI Model for Networks

7 Application Layer

6 Presentation Layer

5 Session Layer

4 Transport Layer

3 Network Layer

2 Data Link Layer

1 Physical Layer

COMT 625 4

OSI Model Implementation?

Layer N

Layer N-1

Request Respond

Confirm Indicate

COMT 625 5

Flow Control

• Synchronize devices or routines at the sender and the receiver

• Basis for error recovery• May exist in every protocol layer

– Layer 2 for device to device– Layer 4 for user to user

COMT 625 6

Flow Control Layers

Transport

Network

Data Link

Physical

Data Link

Physical

Transport

Network

Data Link

Physical

COMT 625 7

Stop-and-Wait

Sender Receiver

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COMT 625 8

Transmit

Sender Receiver

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COMT 625 9

Propagate

Sender Receiver

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COMT 625 10

Ack-Transmit

Sender Receiver

BCD

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Ack

COMT 625 11

Ack-Propagate

Sender Receiver

BCD

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Ack

COMT 625 12

Transmission Time

• Definition: the time required for the transmitter to create the electrical signals needed to completely represent the message

• Depends on the length (in bits) of the message and the circuit speed (bits/sec).

• (Transmission Time) = (Message Length)/(Circuit Speed)

COMT 625 13

Propagation Time

• Definition: the time required for the electrical signals to travel from sender to receiver

• Depends on the length of the circuit and the signal speed.

• (Propagation Time) = (Circuit Length)/(Signal Speed)

• Speed of Light– In Vacuum: 300,000 km/sec– In a typical wire: 200,000 km/sec

COMT 625 14

Short Message orHigh Bit Rate

Sender Receiver

COMT 625 15

Long Message orLow Bit Rate

Sender Receiver

COMT 625 16

Examples

COMT 625 17

Stop and Wait Time Line

TimePacket ready

length=W bits

Last bittransmitted

Transmission Time ti

Last bit received

Ack returned

Propagation Time tp

Propagation Time tp

Note:

We ignore the length of the acknowledgement

Next packet ready

COMT 625 18

What is the transmission rate?

• In the diagram, “W” bits were transmitted (Message Length)

• The elapsed time was ti+2*tp

• Define Beff as the effective bit rate, B as the circuit speed, and Eff as the efficiency

Beff =W

ti + 2 ∗tpEff =

BeffB

COMT 625 19

How do we get high efficiency?

• Note that ti is related to W and Bti =

WB orW =ti ∗B

Eff =BeffB

=1B(

Wti + 2∗tp

)

=ti

ti + 2 ∗tpEff(ti + 2∗tp) =ti

ti −Eff∗ti =Eff∗2∗tp

ti =Eff

1−Eff∗2∗tp orW=B∗ Eff

1−Eff∗2∗tp

COMT 625 20

Why are very large packets impractical?

• Intermediate routers may not be able to store the packet

• Sharing a circuit is impractical because long packets tie up the circuit for long times

• Error checks are performed on a per-packet basis; we may need to re-transmit the entire (large) packet after an error

COMT 625 21

An aside …

• The time in the denominator is the total time elapsed from the start of transmission to the receipt of a “reaction” from the other side

• This time is more generally referred to as the Round Trip Time (RTT), and may include queue delays as well as multiple transmission and propagation times

Beff =W

ti + 2 ∗tp

Beff =W

RTT

COMT 625 22

What is the answer?

• On fast circuits (or on circuits with high RTT) I need a large amount of data “in flight”

• We don’t want large packets• Therefore, we have to allow transmission of

additional packets before the first packet is acknowledged

COMT 625 23

Sliding WindowSender Receiver

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COMT 625 24

Sliding WindowSender Receiver

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COMT 625 25

Sliding WindowSender Receiver

AB

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COMT 625 26

Sliding WindowSender Receiver

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COMT 625 27

Sliding WindowSender Receiver

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Ack

COMT 625 28

Sliding WindowSender Receiver

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Ack

COMT 625 29

Sliding WindowSender Receiver

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AckAck