Infrastructure de Communications – CR 4107 Chapter 4 1

The Medium Access ControlSublayer

Chapter 4

Infrastructure de Communications – CR 4107 Chapter 4 2

The Channel Allocation Problem

Static Channel Allocation in LANs and MANs

Dynamic Channel Allocation in LANs and MANs

Infrastructure de Communications – CR 4107 Chapter 4 3

Dynamic Channel Allocation in LANs and MANs

Station Model.

Single Channel Assumption.

Collision Assumption.

(a) Continuous Time.(b) Slotted Time.

(a) Carrier Sense.(b) No Carrier Sense.

Infrastructure de Communications – CR 4107 Chapter 4 4

Multiple Access Protocols

ALOHA

Carrier Sense Multiple Access Protocols

Collision-Free Protocols

Limited-Contention Protocols

Wavelength Division Multiple Access Protocols

Wireless LAN Protocols

Infrastructure de Communications – CR 4107 Chapter 4 5

Pure ALOHA

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

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Pure ALOHA (2)

Vulnerable period for the shaded frame.

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Pure ALOHA (3)

Throughput versus offered traffic for ALOHA systems.

Infrastructure de Communications – CR 4107 Chapter 4 8

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various random access protocols.

Infrastructure de Communications – CR 4107 Chapter 4 9

CSMA with Collision Detection

CSMA/CD can be in one of three states: contention, transmission, or idle.

Infrastructure de Communications – CR 4107 Chapter 4 10

Collision-Free Protocols

The basic bit-map protocol.

Infrastructure de Communications – CR 4107 Chapter 4 11

Wireless LAN Protocols

A wireless LAN. (a) A transmitting. (b) B transmitting.

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Ethernet

Invented at XEROX by Dr. R. Metcalfe in 1975Formal specifications published in 1980 by DEC-Intel-

XeroxBecame IEEE 802.3 standard in 1985 - adopted by ISO10 MbpsCoaxial cableUpdated to include new technologiesTwisted pair100 Mbps Fast Ethernet - IEEE 802.3UGigabit Ethernet - IEEE 802.3ZWireless versions

Infrastructure de Communications – CR 4107 Chapter 4 13

Elements of the Ethernet System Physical Medium Medium Access Control (MAC) Ethernet Frame

Operation of Ethernet No central control Stations are connected to a shared medium Ethernet signals are transmitted serially, one bit at a time,

to every connected station To send data a station listens to the channel and if it is idle

it transmits its data in the form of an Ethernet frame. Access to the medium is determined by the MAC

• CSMA/CD protocol

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CSMA/CD Protocol

The Ethernet MAC is based on the Carrier Sense Multiple Access / Collision Detection protocol

A station that wants to transmit• Listens to the channel

If the channel is idle, it transmits its data If the channel is busy it waits until it is free and then

transmits its data.

• If two stations transmit at the same time we have a collision ---- Jam signal

Truncated binary exponential backoff algorithm• nth re-transmission will be attempted with a delay

between 0 and 2n-1 time units (ex. 53s)• maximum of 16 attempts to re-transmit

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Collision detection

Collision detections can take as long as 2t

Infrastructure de Communications – CR 4107 Chapter 4 16

Ethernet MAC Sublayer Protocol

Frame formats. (a) DIX Ethernet, (b) IEEE 802.3.

Infrastructure de Communications – CR 4107 Chapter 4 17

Ethernet Addresses

Ethernet addresses (48 bits) are unique and controlled by IEEE

24 bit Organizationally Unique Identifier (OUI) by the IEEE

24 bit by the organization

Multicast and Broadcast addresses

High order address bit is 1 for multicast and broadcast

A destination address of only 1s is accepted by all stations

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Higher level addresses

Ethernet is a trucking system and can operate with different higher level protocols, like TCP/IP, AppleTalk, Novel etc.

Higher level protocols have their own addressing schemes.

They must find the right Ethernet address in order to communicate with each other.

Example in TCP/IP : Address Resolution Protocol (ARP)• Station A sends a broadcast requesting the Ethernet address the

station B that has the specific IP address• All stations receive the message• Only the station with the requested IP address reply

Infrastructure de Communications – CR 4107 Chapter 4 19

Ethernet Topology

The total size of an Ethernet LAN is defined by the round trip propagation delay.

• All stations should by able to respond to signals within a specified amount of time.

• Ethernet segments can be connected at any physical topology (star, tree, bus ...) as long as the timing restrictions are fulfilled.

Repeaters• A repeater is simply re-transmits the signal extending the overall

size of the LAN

Switching hubs• Divides a set of Ethernet segments to multiple LANs

Packet switching

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Ethernet Cabling

The most common kinds of Ethernet cabling.

Infrastructure de Communications – CR 4107 Chapter 4 21

Ethernet Cabling (2)

Three kinds of Ethernet cabling.

(a) 10Base5, (b) 10Base2, (c) 10Base-T.

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Ethernet Cabling (3)

Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.

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Ethernet Cabling (4)

(a) Binary encoding, (b) Manchester encoding, (c) Differential Manchester encoding.

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Ethernet Performance

Efficiency of Ethernet at 10 Mbps with 512-bit slot times.

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Switched Ethernet

A simple example of switched Ethernet.

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Fast Ethernet

The original fast Ethernet cabling.

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Gigabit Ethernet

(a) A two-station Ethernet. (b) A multistation Ethernet.

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Gigabit Ethernet (2)

Gigabit Ethernet cabling.

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Ethernet problems

Capture effectA station with high traffic load can capture the network for a considerable amount of time

Security Packets are received by all stations

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IEEE 802.2: Logical Link Control

(a) Position of LLC. (b) Protocol formats.

Infrastructure de Communications – CR 4107 Chapter 4 32

The 802.11 (WLAN) Protocol Stack

Part of the 802.11 protocol stack.

Infrastructure de Communications – CR 4107 Chapter 4 33

The 802.11 MAC Sublayer Protocol

(a) The hidden station problem.

(b) The exposed station problem.

Infrastructure de Communications – CR 4107 Chapter 4 47

Bluetooth Architecture

Two piconets can be connected to form a scatternet.

Infrastructure de Communications – CR 4107 Chapter 4 48

Bluetooth Applications

The Bluetooth profiles.

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Data Link Layer Switching

Bridges from 802.x to 802.y

Local Internetworking

Spanning Tree Bridges

Remote Bridges

Repeaters, Hubs, Bridges, Switches, Routers, Gateways

Virtual LANs

Infrastructure de Communications – CR 4107 Chapter 4 52

Data Link Layer Switching

Multiple LANs connected by a backbone to handle a total load higher than the capacity of a single LAN.

Infrastructure de Communications – CR 4107 Chapter 4 53

Bridges from 802.x to 802.y

Operation of a LAN bridge from 802.11 to 802.3.

Infrastructure de Communications – CR 4107 Chapter 4 54

Bridges from 802.x to 802.y (2)

The IEEE 802 frame formats. The drawing is not to scale.

Infrastructure de Communications – CR 4107 Chapter 4 59

Repeaters, Hubs, Bridges, Switches, Routers and Gateways

(a) Which device is in which layer.

(b) Frames, packets, and headers.

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Repeaters, Hubs, Bridges, Switches, Routers and Gateways (2)(a) A hub. (b) A bridge. (c) a switch.

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Virtual LANs

A building with centralized wiring using hubs and a switch.

Infrastructure de Communications – CR 4107 Chapter 4 62

Virtual LANs (2)

(a) Four physical LANs organized into two VLANs, gray and white, by two bridges. (b) The same 15 machines organized into two VLANs by switches.

Infrastructure de Communications – CR 4107 Chapter 4 63

The IEEE 802.1Q Standard

Transition from legacy Ethernet to VLAN-aware Ethernet. The shaded symbols are VLAN aware. The empty ones are

not.

Infrastructure de Communications – CR 4107 Chapter 4 64

The IEEE 802.1Q Standard (2)

The 802.3 (legacy) and 802.1Q Ethernet frame formats.