1CMPE 150- Introduction to Computer Networks

CMPE 150

Fall 2005Lecture 15

Introduction to Computer Networks

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Announcements

• Midterm moved to 11.04.– In class, closed books/notes.

• Homework 3 is up.– Due on 11,07.05.

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Today

• MAC.

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Example DLL Protocols

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Example DLL Protocols

• High-Level Data Link Control (HDLC).• Point-to-Point Protocol (PPP).

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HDLC

Flag

Identifieshost.

ACK,seq. #

CRC

ISO standard.

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PPP

• Internet’s DLL.• Router-to-router.• Home user-to-ISP.• RFC 1661, etc.

• PPP is a multi-protocol framing mechanism that can be used over multiple PHYs (dial-up, dedicated point-to-point connections).

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The Data Link Layer in the Internet

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PPP Frame

Defaultvalue; noneed for addresses.

Default value:unumbered frame;No reXmissions.

Type of protocolin the payload

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Medium Access Control

• MAC.• Tanenbaum, Chapter 4.

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Why MAC?

• Point-to-point versus shared-medium networks.

• Shared-medium networks use “broadcast” channels.– A.k.a. multi-access or random access

channels.

MAC layer protocols regulate access to medium in shared-medium networks.

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Where is the MAC Sub-Layer?

MAC

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Where is the MAC Sub-Layer?

MAC

PHY

DLL Link Control

Network

Transport

Application

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MAC and LANs

• LANs typically use shared-medium.• Examples?• MAC layer critical!– BTW, in wireless networks also!

• WANs typically use point-to-point connections.

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Channel Allocation Problem

• How to allocate single shared, broadcast channel among several stations/users.

• If no arbitration, several stations/users may transmit at the same time: COLLISIONS!

• A.k.a., “floor control”.

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Multiplexing

• Sharing a link/channel among multiple source-destination pairs.

• Example: high-capacity long-distance trunks (fiber, microwave links) carry multiple connections at the same time.

MU

X

...

DE

MU

X ...

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Multiplexing Techniques

• 3 basic types:– Frequency-Division Multiplexing (FDM).– Time-Division Multiplexing (TDM).

– Statistical Time-Division Multiplexing (STDM).

Static

Dynamic

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FDM

Time

Frequency

1 2 N

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FDM

• Simple.

• But:– What if number of users is large?

– What if number of users changes over time?

– What if traffic is bursty?

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TDM

Time

Frequency

12

N

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TDM (Cont’d)

• Time divided into time slots.• One or more slots assigned to a data source.

• But, also inefficient…

1 2 N... 1 2 ... N

frame Time

U1 U2 ... UN

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Dynamic Multiplexing

• Dynamic allocation.

• In particular, statistical TDM.–Dynamically allocates time slots on

demand.

• Increased channel utilization.

• But…

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Multiple Access Protocols

• Centralized approaches:– Controller grants access to medium.

– Simple, greater control: priorities, QoS.

– But, single point of failure and performance bottleneck.

• Decentralized schemes:– All stations collectively run MAC to decide

when to transmit.

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Round-Robin MAC

• Each station is allowed to transmit; station may decline or transmit (bounded by some maximum transmit time).

• Centralized (e.g., polling) or distributed (e.g., token ring) control of who is next to transmit.

• When done, station relinquishes and right to transmit goes to next station.

• Efficient when many stations have data to transmit over extended period (stream).

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Scheduled-Access MAC

• Time divided into slots.• Station reserves slots in the future.• Multiple slots for extended transmissions.• Suited to stream traffic.

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Contention-Based MAC

• No control.• Stations try to acquire the medium.• Distributed in nature.• Perform well for bursty traffic.• Can get very inefficient under heavy load.

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Contention-Based MACs

• ALOHA family.• CSMA family.

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Pure ALOHA

• In pure ALOHA, frames are transmitted at completely arbitrary times.

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Pure ALOHA: Performance

• Vulnerable period for the shaded frame.

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Pure ALOHA

• Throughput versus offered traffic for ALOHA systems.