Communication System Chapter 9

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2007 Cisco Systems, Inc. All rights reserved. Cisco Public 1Version 4.0

Ethernet

Network Fundamentals

Chapter 9


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2 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Objectives Identify the basic characteristics of network media used in

Ethernet.

Describe the physical and data link features of Ethernet.

Describe the function and characteristics of the media accesscontrol method used by Ethernet protocol.

Explain the importance of Layer 2 addressing used for datatransmission and determine how the different types ofaddressing impacts network operation and performance.

Compare and contrast the application and benefits of using

Ethernet switches in a LAN as apposed to using hubs.

Explain the ARP process.


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Physical and Data Link Features of Ethernet


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Ethernet: Layer 1 and Layer 2

Ethernet defines these Layer 1 elements:

Signals

Bit streams

Physical component

Various topologies

At data link layer addresses these requirements:

Provides interface

Provides address

Uses frames to organize bits into meaningful groups

Controls transmission of data from sources


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Physical and Data Link Features of Ethernet Logic Link Control Connecting the Upper Layers


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Physical and Data Link Features of Ethernet Media Access Control (MAC)




Physical Implementations of the Ethernet



Why Ethernet is successful?

Simple and easy to maintain

Ability to incorporate to new technology

Reliable

Low cost



Characteristics of Network Media used in Ethernet

Historic Ethernet:1. Coaxial cable2. Bus topology3. CSMA/CD4. Thicknet (10BASE5) and

Thinnet (10BASE2)

Legacy Ethernet:1. 10BASET2. Star with hub as central

point

3. CSMA/CD4. Half duplex



Current Ethernet

10Mbps and 100Mbps

Physical star with switch in the middle or P2P



Moving to 1Gbps and beyond

UTP full duplex

Increased cabling distance



Layer 2 Addressing and Its Impact on NetworkOperation and Performance




Hexadecimal Numbering and Addressing



Another layer of addressing

Layer 2

Enables packet tobe carried by thelocal media

across eachsegment

Physical address

Layer 3

Enables packet tobe forwardedtoward itsdestination

Logical address




The Ethernet MAC Address



Ethernet Unicast, Multicast and Broadcast



Carrier Sense Multiple Access with Collision Detection

Collision happens in shared medium



Carrier Sense Multiple Access with Collision Detection

Steps in CSMA/CD: Listen before sending

Detecting a collision

Jam signal and random backoff



Hubs and collision domains

Conditions that cause collisions:

More devices connected to the network

More frequent access to media

Increased cable distances between devices

Hubs does not operate in higher level network, socollision occurs



Ethernet Timing

Preamble: timing synchronization information

Timing information is not required in synchronizecommunication



Ethernet timing

Bit time = period of time required for a bit to be placedand sensed on media

Slot time

How many devices can share a network

Used to establish minimum size of Ethernet frame and limiton maximum size of networks segment

Includes time it takes to travel through cables and hubs



Interframe spacing and backoff

Interframe spacing

Minimum spacing between 2 noncolliding frames

To give media time to stabilize after transmission of

previous frame for the device to process frame

Jam signal length

32 bit jam signal that will enforce collision

Repeating 1010 pattern

Backoff timing

Waiting period after collision

Random so that 2 stations do not delay for same amounntof time before transmitting


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4 different data rates 10Mbps

10BASE-T Ethernet

Physical star

Manchester encoding

100Mbps

Fast Ethernet

Implemented using UTP or fiber

100BASE-TX AND 100BASE-FX

1000Mbps

Gigabit Ethernet Signals occurring in less time

1000BASE-T, 1000BASE-SX and 1000BASE-LX

10Gbps

10Gigabit Ethernet

CSMA/CD not necessary


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Compare and Contrast the Use of Ethernet Switchesversus Hubs in a LAN

Using Hubs Does not perform filtering

Share bandwidth of media

High level of collisions

Issues: Lack of scalability

Increase latency

More network failure

More collisions


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Using Switches Allow segmentation of LAN into separate collision domains

Throughput increases because:

Dedicated bandwidth to each port

Collision free environment Full duplex environment


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Describe how a switch can eliminate collisions,backoffs and retransmissions, the leading factors inreduced throughput on a hub-based Ethernet network


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Why not using switches instead of hubs?

Availability

Economics

Requirements


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Switch

MAC table: maps destination MAC address with portsused to connect to each node

MAC table = switch table = bridge table

Bridge: device used in early days to connect to physicalnetwork segments.

Operations:

Learning

Aging

Flooding

Selective forwarding

Filtering


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Address Resolution Protocol (ARP)

2 functions

Resolving IPv4 address to MAC

Maintaining a cache of mappings


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Explain the Address Resolution Protocol (ARP) Process

Mapping IP to MAC Addresses


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Maintaining a cache of mappings

Different process is required to provide IP to MACaddress:

Adding entries to ARP table

Getting the map for a frame

Mapping destination outside local network

Removing address mappings


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Explain the Address Resolution Protocol (ARP) ProcessARP Destinations Outside the Local Network


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Explain the Address Resolution Protocol (ARP) ProcessARP Removing Address Mappings


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Explain the Address Resolution Protocol (ARP) ProcessARP Broadcasts - Issues


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Summary


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Communication System Chapter 9