Chapter 8-10

Local Area Networks (LANs) 1

Comparison 4e and 5e 

Ch 7, 4e



Ch 8, 4e



Ch 9, 4e



Ch 10, 4e Ch 8, 5e Ch 10, Forouzan Ch 14, 5e Ch 13, Forouzan Ch 13, 5e Ch 14, Forouzan Ch 15, 5e Ch 14, Forouzan 2

Classification Terminology 

Network technologies classified into three broad categories



Local Area Network (LAN)



Metropolitan Area Network (MAN)



Wide Area Network (WAN)



LAN and WAN most widely deployed 3

The Local Area Network (LAN) 

Engineering classification



Extremely popular (most networks are LANs)



Many LAN technologies exist 4

Key Features of a LAN 

High throughput



Relatively low cost



Limited to short distance



Often rely on shared media 5

Scientific Justification For Local Area Networks

A computer is more likely to communicate with computers that are nearby than with computers that are distant



Known as the locality principle 6

Topology 

Mathematical term



Roughly interpreted as “geometry for curved surfaces” 7

Network Topology 

Specifies general “shape” of a network



Handful of broad categories



Often applied to LAN



Primarily refers to interconnections



Hides details of actual devices 8

Fully connected mesh topology (for five devices)

Bus Topology 

Shared medium forms main interconnect



Each computer has a connection to the medium 10

Ring Topology 

No central facility



Connections go directly from one computer to another 11

Star Topology 

Central component of network known as hub



Each computer has separate connection to hub 12

Example Bus Network: Ethernet 

Most popular LAN



Widely used



IEEE standard 802.3



Several generations



Same frame format



Different data rates



Different wiring schemes 13

Shared Medium in a LAN 

Shared medium used for all transmissions



Only one station transmits at any time



Stations “take turns” using medium



Media Access Control (MAC) policy ensures fairness 14

Illustration of Ethernet Transmission 

Only one station transmits at any time



Signal propagates across entire cable



All stations receive transmission



CSMA/CD media access scheme 15

CSMA/CD Paradigm 

Multiple Access (MA)



Multiple computers attach to shared media



Each uses same access algorithm



Carrier Sense (CS)



Wait until medium idle



Begin to transmit frame



Simultaneous transmission possible 16

CSMA/CD Paradigm (continued) 

Two simultaneous transmissions



Interfere with one another



Called collision



CSMA plus Collision Detection (CD)



Listen to medium during transmission



Detect whether another station’s signal interferes



Back off from interference and try again 17

Backoff After Collision 

When collision occurs



Wait random time t1, 0 < t1 < d



Use CSMA and try again



If second collision occurs



Wait random time t2, 0 < t2 < 2*d



Double range for each successive collision



Called exponential backoff 18

Media Access on a Wireless Net 

Limited range



Not all stations receive all transmissions



Cannot use CSMA/CD



Example in diagram



Maximum transmission distance is d



Stations 1 and 3 do not receive each other’s transmissions 19

CSMA/CA 

Used on wireless networks



Both sides send small message followed by data transmission



“X is about to send to Y”



“Y is about to receive from X”



Data from sent from X to Y



Purpose: inform all stations in range of X or Y before transmission



Known as Collision Avoidance (CA) 20

Identifying a Destination 

All stations on shared-media LAN receive all transmissions



To allow sender to specify destination



Each station assigned unique number



Known as station’s address



Each frame contains address of intended recipient 21

Assignment of physical address 

The stations may get their address in different ways:



Static



Configurable



Dynamic 22

Ethernet Addressing 

Standardized by IEEE



Each station assigned by unique 48-bit address e.g. 00:30:65:52:2E:96 in hexadecimal form



Address assigned when network interface card (NIC) manufactured (In most cases) 23

Ethernet Address Recognition 

Each frame contains destination address



All stations receive a transmission



Station discards any frame addresses to another station



Important: interface hardware, not software, checks address 24

Possible Destinations 

Packet can be sent to:



Single destination (unicast)



All stations on network (broadcast)



Subset of stations (multicast)



Address used to distinguish 25

Advantages of Address Alternatives 

Unicast



Efficient for interaction between two computers



Broadcast



Efficient for transmitting to all computers



Multicast



Efficient for transmitting to a subset of computers 26

Broadcast on Ethernet 

All 1s address specifies broadcast (FF:FF:FF:FF:FF:FF in hexcode)



Sender



Places broadcast address in frame



Transmits one copy on shared network



All stations receive copy



Receiver always accepts frame that contains this address 27

Multicast 

Half of addresses reserved for multicast



Network interface card



Always accepts unicast and broadcast



Can accept zero or more multicast addresses



Software



Determines multicast address to accept



Informs network interface card 28

Promiscuous Mode 

Designed for testing / debugging



Allows interface to accept all packets



Available on most interface hardware 29

Identifying Frame Contents 

Integer type field tells recipient the type of data being carried



Two possibilities



Self-identifying or explicit type (hardware record type)



Implicit type (application sending data must handle type) 30

Conceptual Frame Format 

Header



Contains address and type information



Layout fixed



Payload



Contains data being sent 31

Illustration Of Ethernet Frame 

Sender places



Sender’s address in source



Recipient’s address in destination



Type of data in frame type



Cyclic redundancy check in CRC 32

Figure 14.3

length Minimum and maximum

Example Ethernet Types

34

When Network Hardware Does Not Include Types 

Sending and receiving computers must agree



To only send one type of data



To put type information in first few octets of payload



Most systems need type information 35

Illustration of Type Information Added to Data 

In practice



Type information small compared to data carried



Format of type information standardized 36

A Standard For Type Information 

Defined by IEEE



Used when hardware does not include type field



Called LLC / SNAP header



Logical Link Control



SubNetwork Attachment Point

37

Demultiplexing On Type 

Network interface hardware



Receives copy of each transmitted frame



Examines address and either discards or accepts



Passes accepted frame to system software



Network device software



Examines frame type



Passes frame to correct software module 38

Network Analyzer 

Device used for testing and maintenance



Listens in promiscuous mode



Produces



Summaries (e.g., % of broadcast frames)



Specific items (e.g., frames from a given address) 39

Bus Topology 

Any user with a Network Analyzer can read all packets!

40

Star Topology 

Star Topology and Bus Topology are equal fom security point!

41

Ethernet Wiring 

Three schemes



Correspond to three generations



All use same frame format 42

Original Ethernet Wiring 

Used heavy coaxial cable



Formal name 10Base5



Called thicknet 43

Second Generation Ethernet Wiring 

Used thinner coaxial cable



Formal name 10Base2



Called thinnet 44

Modern Ethernet Wiring 

Uses a hub



Formal name 10Base-T



Called twisted pair Ethernet 45

Ethernet Wiring In An Office

46

A Note About Ethernet Topology 

Apparently



Original Ethernet used bus topology



Modern Ethernet uses star topology



In fact, modern Ethernet is



Physical star



Logical bus



Called star-shaped bus 47

Higher Speed Ethernets 

Fast Ethernet



Operates at 100 Mbps



Formally 100Base-T



Two wiring standards



10/100 Ethernet devices available



Gigabit Ethernet



Operates at 1000 Mbps (1 Gbps)



Slightly more expensive 48

Another LAN Using Bus Topology 

LocalTalk



Developed by Apple Corp.

1984



Simple to use



Slow by current standards



(230,4 kbps) 49

Illustration Of LocalTalk 

Transceiver required per station



Transceiver terminates cable 50

Ring Topology 

Second most popular LAN topology



Bits flow in single direction



Several technologies exist 51

Token Passing 

Used with ring topology



Guarantees fair access



Token



Special (reserved) message



Small (a few bits) 52

Token Passing Paradigm 

Station



Waits for the token to arrive



Transmits one packet around ring



Transmits token around ring



When no station has data to send



Token circulates continuously 53

Token Passing Ring Transmission 

Station waits for token before sending



Signal travels around entire ring



Sender receives its own transmission 54

Strengths of Token Ring Approach 

Easy detection of



Broken ring



Hardware failures



Interference 55

Weaknesses of Token Ring Approach 

Broken wire disables entire ring



Point-to-point wiring



Awkward in office environment



Difficult to add / move stations 56

Token Passing Ring Technologies 

ProNet-10



Operated at 10 Mbps



IBM Token Ring



Originally operated at 4 Mbps



Later version operated at 16 Mbps



Fiber Distributed Data Interconnect (FDDI)



Operated at 100 Mbps 57

FDDI Terminology 

FDDI



Uses optical fibers



High reliability



Immune to interference



CDDI



FDDI over copper



Same frame format



Same data rate



Less noise immunity 58

FDDI Hub Technology 

Part of FDDI standard



Stations attach to hub



Same frame format and data rate as FDDI



Called star-shaped ring 59

FDDI Failure Recovery 

Uses two rings



Automatic failure recovery



Terminology



Dual-attached



Counter rotating



Self healing

60

Illustration of FDDI Failure Recovery

61

Another Example of a Physical Star Topology 

Asynchronous Transfer Mode (ATM)



Designed by telephone companies



Intended to accommodate



Voice



Video



Data 62

ATM 

Building block known as ATM switch



Each station connects to switch



Switches can be interconnected 63

Details of ATM Connection 

Full-duplex connections



Two fibers required 64

ATM Characteristics 

High data rates (e.g. 155 Mbps)



Fixed size packets



Called cells



Important for voice



Cell size is 53 octets



48 octets of data



5 octets of header 65

Summary 

Local Area Networks



Designed for short distance



Use shared media



Many technologies exist



Topology refers to general shape



Bus



Ring



Star 66

Summary (continued) 

Address



Unique number assigned to station



Put in frame header



Recognized by hardware



Address forms



Unicast



Broadcast



Multicast 67

Summary (continued) 

Type information



Describes data in frame



Set by sender



Examined by receiver



Frame format



Header contains address and type information



Payload contains data being sent 68

Summary (continued) 

LAN technologies



Ethernet (bus)



IBM Token Ring



FDDI (ring)



ATM (star) 69

Summary (continued) 

Wiring and topology



Can distinguish

 Logical topology  Physical topology (wiring) 

Hub allows

 Star-shaped bus  Star-shaped ring

70

Figure 14.1

Three generations of Ethernet

Figure 14.5

addresses Unicast and multicast

Figure 14.6

Physical layer

Figure 14.7

PLS

Figure 14.8

AUI

Figure 14.9

MAU (transceiver)

Figure 14.10

Ethernet Categories of traditional

Figure 14.11

10Base5 Connection of a station to the medium using

Figure 14.12

using 10Base2 Connection of stations to the medium

Figure 14.13

using 10Base-T Connection of stations to the medium

Figure 14.14

10Base-FL Connection of stations to the medium using

Figure 14.15

Sharing bandwidth

Figure 14.16

bridge A network with and without a

Figure 14.17

Collision domains in a nonbridged and bridged network

Figure 14.18

Switched Ethernet

Figure 14.19

Ethernet Full-duplex switched

14.2 Fast Ethernet

MAC Sublayer Physical Layer Physical Layer Implementation

Figure 14.20

Fast Ethernet physical layer

Figure 14.21

MII

Figure 14.22

Fast Ethernet implementations

Figure 14.23

100Base-TX implementation

Figure 14.24

100Base-TX Encoding and decoding in

Figure 14.25

100Base-FX implementation

Figure 14.26

100Base-FX Encoding and decoding in

Figure 14.27

100Base-T4 implementation

Figure 14.28

T4 Using four wires in 100Base-

14.3 Gigabit Ethernet

MAC Sublayer Physical Layer Physical Layer Implementation

Figure 14.29

Ethernet Physical layer in Gigabit

Figure 14.30

Gigabit Ethernet implementations

Figure 14.31

1000Base-X implementation

Figure 14.32

Encoding in 1000Base-X

Figure 14.33

1000Base-T implementation