Ch3_nw4mgr.ppt

141-623 Communication networks and networking for managers

Suntorn WitosurapotSuntorn Witosurapot

Contact Address: Phone: 074 287369 or

Email: [email protected]

November 2003

141-623 communiction networks and networking for managers

Chapter 3 A small Ethernet PC Network

2

Chapter 3

A Small Ethernet PC Network



3

Review:Review: Architecture Reference

Presentation

OSI TCP/IP

Application

Application

Session

Transport

Network

Transport

Internet

Data Link

Physical

Data Link

Physical

GeneralFunction

ApplicationCommunication

Internetworking

Single NetworkTransmission



4

Review:Review: a View of Horizontal Layered Message Communication in TCP/IP

PC

Ethernet Switches

Routers (IP)

MessageMessage

Web Server

MessageMessagePC Web Server



5

Review:Review: a View of Logical Message Communication in TCP/IP

HTTP MsgApplicationLayer Proc

Proc = Process

ApplicationLayer Proc

TransportLayer Proc

TCP Msg TransportLayer Proc

NetworkLayer Proc

IP Packet NetworkLayer Proc

NetworkLayer Proc

Client PC ServerEthernet Switch Router

Data LinkLayer Proc

EthFrame

Data LinkLayer Proc

Data LinkLayer Proc

Data LinkLayer Proc

PhysicalLayer Proc

PhysicalLayer Proc

PhysicalLayer Proc

PhysicalLayer Proc



6

Review:Review: Communication at All Layers on the Source Host

HTTPMsg

ApplicationProcess

TCPHdr

HTTPMsg

TransportProcess

TCPHdr

IPHdr

HTTPMsg

InternetProcess

DLHdr

TCPHdr

IPHdr

DLHdr

HTTPMsg

Data LinkProcess



7

Preview: An Ethernet PC network

Our goals:

• Understand Ethernet architecture and its components

– Able to differentiate between hubs and switches and

reasons for the growing dominance of switches

– Understand the division of data link into the medium access

control and logical link control layers



8

Elements of a Small Ethernet PC Network

Access Router

A1-44-D5-1F-AA-4C

D4-47-55-C4-B6-9F

To the Internet Ethernet Switchor Hub

B2-CD-13-5B-E4-65C3-2D-55-3B-A9-4F

UTP

UTPUTP

UTP

UTP

BroadbandModem

ClientClient Server

Server



9

Network Interface Card (NIC)

• Each station must have a network interface card (NIC)

• This is true for both clients and servers

• The NIC implements LAN protocols

PhysicalData Link



10

Ethernet Standards

• Ethernet standards dominate in small LANs

– Governs switches, wiring and NIC (network interface cards)

• IEEE (Institute of Electrical and Electronics Engineers) Standardization

– 802 LAN/MAN Standards Committee (802 Committee) creates LAN standards

• 802.3 Working Group creates Ethernet standards

• So Ethernet standards are also known as 802.3 standards.

PhysicalData Link



11

Ethernet Standards (cont.)

• Physical Layer Ethernet Standards Using UTP

– 802.3 1010Base-T• 10 Mbps

– 802.3 100Base-TX, 100Base-FX• 100 Mbps• 10/100 operation (auto-sensing)

– 802.3 1000Base-T• Gigabit Ethernet (?? Mbps)

PhysicalData Link

T = twisted pair;

F = fiber optics;

T = twisted pair;

F = fiber optics;



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Purchasing and Installing UTP

• Wiring Quality Categories

– Governed by the TIA/EIA-568 standardGoverned by the TIA/EIA-568 standard

– Categories 3, 4, 5, 5e (enhanced) and 6 (released in 2002)

• Higher numbers indicate better quality

– Category 5e is recommended for new buildings

– Category 5 or 5e is required for 100Base-TX

– Category 5 or 5e is required for 1000Base-TX (Gigabit Ethernet), but 5 is marginal

PhysicalData Link



13

Hubs and Switches:

• Available either in a form of• Desktop box, or

•19’’ Stackable/Rack-Mountable module



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Hub Operation

A1-44-D5-1F-AA-4C

D4-47-55-C4-B6-9F

Ethernet Hub

B2-CD-13-5B-E4-65C3-2D-55-3B-A9-4F

UTPUTP

UTP

UTP

ClientClient Server

Server

Station A1-44-D5-1F-AA-4Ctransmits a bit.

Hub broadcasts the bitout all other ports.



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Hub Operation

A1-44-D5-1F-AA-4C

D4-47-55-C4-B6-9F

Ethernet Hub

B2-CD-13-5B-E4-65C3-2D-55-3B-A9-4F

UTPUTP

UTP

UTP

ClientClient Server

Server

While stationA1-44-D5-1F-AA-4C

is transmitting.

If stationB2-CD-13-5B-E4-65

wishes to transmit, it must wait.

WaitingTransmitting



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Hub Operation: Waiting to Transmit with a Hub

• Waiting creates latency (delay)– Becomes intolerable with many stations

• Not a problem for very small networksNot a problem for very small networks

• Speed of hub is shared by all stations—each gets only a fraction



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Switch Operation

A1-44-D5-1F-AA-4Con switch port 10

D4-47-55-C4-B6-9Fon switch port 16

Ethernet Switch

B2-CD-13-5B-E4-65on switch port 13

C3-2D-55-3B-A9-4Fon switch port 15

UTP UTP UTPUTP

ClientClient

Server

Server

A switchswitch sends a frameout a single port —the one to the receiver

FrameFrame Frame



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Switch Operation



Ethernet Switch



UTP UTP UTPUTP

ClientClient

Server

Server

Multiple simultaneoussimultaneousconversations arepossible!

FrameFrame Frame



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Ethernet Switching Table



Ethernet Switch



UTP UTP UTPUTP

ClientClient

Server

Server

FrameFrame Frame

Switching TablePort Station

10 A1-44-D5-1F-AA-4C13 B2-CD-13-5B-E4-6515 C3-2D-55-3B-A9-4F16 D4-47-55-C4-B6-9F

15 C3-2D-55-3B-A9-4F



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Ethernet Switches

• They are link-layer devices: operate on Ethernet frames, examining frame header and selectively forwarding frame based on its destination

• Switches isolates collision domains since it buffers frames

• When frame is to be forwarded on segment, bridge uses CSMA/CD to access segment and transmit



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Ethernet Switches: Advantages

• Isolates collision domains resulting in higher total max throughput, and does not limit the number of nodes nor geographical coverage

• Can connect different type Ethernet since it is a store and forward device

• Transparent: no need for any change to hosts LAN

adapters



22

802 Standards Layering

Internet Layer

MediaAccessControlLayer

Data Link

Layer

Logical Link

ControlLayer

TCP/IP Internet LayerStandards

Other Internet LayerStandards

802.2

Other MACStandards

(802.5, etc.)

Ethernet 802.3 MAC LayerStandards

Physical Layer10

Base-T100

Base-TX1000

Base-TX

Other PhysicalLayer Standards

(802.5, etc.)



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Octet

• A collection of 8 bits

• Called a byte in computer science

• The term “Octet” is widely used in networking

Octet = group of 8 bits



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Ethernet Frame

Preamble (7 Octets)10101010 …

Begins synchronization

Start of Frame Delimiter (1 Octet)

10101011

Ends synchronizationSignals start of

content of frame

Field Description

1010101010101010101010101010101010…Synchronizes the receiver’s clock with the sender’s clock

Like a boat racing calling out “Hui, Ley, Hui”



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Ethernet Frame (cont.)

Destination Address (6 Octets)Of Receiving NIC

48 BitsExpressed for humans in

HexadecimalExample:Example: A1-34-CD-7B-DF-47Unique NIC value set at factory

Field Description

Source Address (6 Octets)Of Sending NIC

48 BitsExpressed for humans in

HexadecimalExample:Example: 34-D5-E6-6A-BD-B0Unique NIC value set at factory



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Ethernet Frame: MAC Addresses

• Ethernet addressesEthernet addresses are called MAC addressesMAC addresses

• They are station (NIC) address at the MAC layer

• Usually expressed for humans in hexadecimal

– Base 16: Each symbol represents 4 bits (for example,

0101=5 hex)

– Convert each four bits to a hex symbol between 0 and F

– Put dashes between each pair

– Example: A1-34-CD-7B-DF-89

PhysicalMAC



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Ethernet Frame: Hexadecimal Notation

0

0001

0010

0011

0100

0101

1

2

3

4

5

4 Bits(Base 2*)

Decimal(Base 10)

Hexadecimal(Base 16)

0 hex

1 hex

2 hex

3 hex

4 hex

5 hex

0000

*24 =16 combinations

For example, A1-34-CD-7B-DF hex begins with 1010 0001



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Ethernet Frame: Hexadecimal Notation (cont.)

4 Bits(Base 2)

Decimal(Base 10)

Hexadecimal(Base 16

8100110101011

91011

8 hex9 hexA hexB hex

1000

01100111

67

6 hex7 hex

12110111101111

131415

C hexD hexE hexF hex

1100

Example: 110011011101111011111111 = CD-EF



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Ethernet 802.3MAC Header

LLC 802.2Header

IP PacketEthernetTrailer

Ethernet Data Field:LLC Frame

Preamble (7 octets)Start of Frame Delimiter (1 octet)

Dest. Address (6 octets)Src. Address (6 octets)

Length (2 octets)

FrameCheck

Sequence(4 octets)

LLC Data Field:IP Packet

PhysicalMAC

Direction



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Field Description

Length (2 Octets)Length of Data Field in octets

1,500 (decimal) maximum1,500 (decimal) maximum

LLC FrameContaining IP Packet

PADNeeded if data field < 46 octet min.,

(so that Data Field + PAD will be 46 Octets)

PhysicalMAC

Data Field (variable)



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Field Description

Frame Check Sequence(4 Octets)

Error detection field;If error is detected, frame is

discarded

• The FCS field is for error checking.

• Both sender and receiver compute it from the other bits in the frame.

• If the two computed values are different, there has been an error.

• In case of error, the receiver discards the frame; there is no request for retransmission.

PhysicalMAC



32

MAC Protocols

• We consider broadcast channels

• Problem: how to determine who gets to use the

channel when there is competition for it

• Two basic schemes: Static channel allocation and

Dynamic channel allocation

PhysicalMAC

•Frequency Division Multiplexing (FDM)•Time Division Multiplexing (TDM)

• Aloha• Carrier Sense Multiple Access (CSMA)



33

Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

• With Hubs, NICs must engage in CSMA/CD– CSMA/CD is not used with switches

• Two processes

1. Carrier Sense Multiple Access (CSMA)Carrier Sense Multiple Access (CSMA)

– If a NIC wishes to transmit, it must listen for traffic

• If there is no traffic, the NIC may transmit

• If there is traffic, the NIC must wait to transmit until no traffic is being transmitted; then it may send



34

Carrier Sense Multiple Access with Collision Detection (CSMA/CD) (cont.)

2. Collision Detection (CD)Collision Detection (CD)

• If there is a collision (by 2 or more stations transmitting at the same time),

– All NICs stop transmitting and wait for a random random amount of timeamount of time

– The first NIC that finishes its wait may transmit• but only if there is no traffic!

• If there is traffic, the NIC must wait until there is no traffic



35

Carrier Sense Multiple Access with Collision Detection (CSMA/CD) (cont.)

3. Collision Detection (CD)Collision Detection (CD)

– If there are multiple collisions,

• The random wait is increased each time

• After 16 collisions, the sending NIC discards the frame



36

CSMA/CD: a few details

time

A

B

C

data

interframe gap (IFG): 96 bits

collision enforcement jam signal: 32 bits

collision

backOff time: multiples of the slot time: 512 bits



37

CSMA/CD: Why collision occurs?

A B C Dt0

t1

t2

t3

A B C D

A B C D

A B C D



38

Recap:Recap: Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

• Hubs do not implement it when talking to stations

• NICs implement it when talking to hub

CSMA/CD with Hubs



39

RecapRecap: Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

• Three basic elements for describing CSMA/CD

– CSMACSMA—transmit only if line is clear; waiting otherwise

– CD Part 1CD Part 1—handling a single collision

– CD Part 2CD Part 2—handling of multiple collisions



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NIC-Switch Communication

• No possibility of collisions with switches

• With switches, NICs operate in full-duplex mode

– Turn off CSMA/CD

– Can transmit and receive any time — called Full DuplexFull Duplex

operation



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Network Interface Cards (NICs)

PC Card NICsPC Card NICssnap into external PC card slots on notebooks.

RJ-45Jack

Internal NICs Internal NICs fit into slots on the motherboard.Must open PC to install.



42

RecapRecap: Ethernet

• 802.3 Physical Layer Standards– 10Base-T, 100Base-TX, gigabit Ethernet, etc.

– Media and signaling

• 802.3 MAC Layer Standards– Frame Organization

– Hub and Switch Operation

– CSMA/CD versus Full-Duplex

• NICs Handle Both Layers, Plus LLC



43

802 Standards Layering

Internet Layer

MediaAccessControlLayer

Data Link

Layer

Logical Link

ControlLayer

TCP/IP Internet LayerStandards

Other Internet LayerStandards

802.2

Other MACStandards

(802.5, etc.)

Ethernet 802.3 MAC LayerStandards

Physical Layer10

Base-T100

Base-TX1000

Base-TX

Other PhysicalLayer Standards

(802.5, etc.)



44

Logical Link Control (LLC) Layer

• 802.2 LLC Standard is used in all 802 LANs

– All MAC layer standards interact only with 802.2 at the layer aboveabove them

– All internet layer standards interact only with 802.2 at the layer belowbelow them

LLC 802.2

Ethernet MAC Wireless MAC

IP IP

MACLLC

Physical



45

Logical Link Control (LLC) Layer(cont.)

• Optional Error Correction

– Error detection, and retransmission

– Almost never used

– NICs rarely allow user to control LLC functionality

MACLLC

Physical



46

Ethernet Frame OrganizationIncluding LLC

• As noted earlier, here is how Ethernet frames are organized, including the LLC header

EthernetHeader

802.2LLC

Header

IP or OtherPacket

EthernetTrailer

Ethernet Data Field: LLC Frame

LLC Data Field

Ethernet Frame

MACLLC

Physical



47

Internetworking• To extend / connect networks using the same or different tech.

– LAN LAN

– LAN WAN

– WAN WAN

– LAN WAN LAN



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Why not just one big LAN?

• Limited amount of supportable traffic: on single LAN,

all stations must share bandwidth

• Large “collision domain” (can collide with many stations)

• Limited length and number of stations, e.g.

• 802.3 (Ethernet) specifies maximum cable length



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Ethernet Switching Hubs• Hubs can be arranged in a

hierarchy (or multi-tier design), with backbone hub at its top.

• Each connected LAN referred to as LAN segment

• Hub Advantages:– simple, inexpensive device

– Multi-tier provides graceful degradation: portions of the LAN continue to operate if one hub malfunctions

– extends maximum distance between node pairs (100m per Hub)



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Ethernet Switching Hubs: Backbone• Interconnection without a backbone is not recommended due to

- single point of failure at Computer Science hub- all traffic between EE and SE must path over CS segment

Backbonehub



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Example: 100Mbps Ethernet Backbone strategy



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Internet Access

• Desktop access routeraccess router for very small group

– Usually has built-in switch to connect several PCs

– The access router/switch usually has a built-in DHCP server

so that all PCs can share a single IP address given to the user

by the ISP

18 cm (7 in) wide



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Using an Access Router

BroadbandModem PC

PC

WAN Portto ISP

UTPUTP

UTP

Access router has built-in switch, DHCP server for PCs and NAT Firewall (will be studied later)

Auto-configuredby Access Router



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Linux/Windows Allows PCs to Share a Connection

BroadbandModem

PC With2 NICs

Other PCOr Switch

ISP

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