Ethernet LANs Chapter 4 Panko’s Business Data Networks and Telecommunications, 6th edition Copyright 2007 Prentice-Hall May only be used by adopters of.

Ethernet LANs

Chapter 4

Panko’sBusiness Data Networks and Telecommunications, 6th edition

Copyright 2007 Prentice-HallMay only be used by adopters of the book

4-2

Orientation

• Chapters 2 and 3 Looked at Standards

– Chapter 2: Layered standards (data link to application)

– Chapter 3: Physical layer standards

• Chapters 4-7 Deal With Single Networks

– Chapter 4: Ethernet LANs• Chapter 4a deals with obsolete Token-Ring Networks

– Chapter 5: Wireless LANs

– Chapters 6 and 7: WANs

– Flow is from LANs to WANs

4-3

Figure 4-1: A Short History of Ethernet Standards

• Ethernet

– The dominant wired LAN technology today

– Only “competitor” is wireless LANs (which actually are supplementary)

• The IEEE 802 Committee

– LAN standards development is done primarily by the Institute for Electrical and Electronics Engineers (IEEE)

– IEEE created the 802 LAN/MAN Standards Committee for LAN standards (the 802 Committee)

4-4


• The 802 Committee creates working groups for specific types of standards

– 802.1 for general standards

– 802.3 for Ethernet standards

• The terms 802.3 and Ethernet are interchangeable

– 802.11 for wireless LAN standards

– 802.16 for WiMax wireless metropolitan area network standards

4-5

IEEE 802 Standards

IEEE 802.1 Higher layer LAN protocols

IEEE 802.2 Logical link control

IEEE 802.3 Ethernet

IEEE 802.4 Token bus

IEEE 802.5 Token Ring

IEEE 802.6 Metropolitan Area Networks

IEEE 802.7 Broadband LAN using Coaxial Cable

IEEE 802.8 Fiber Optic TAG

IEEE 802.9 Integrated Services LAN

IEEE 802.10 Interoperable LAN Security

IEEE 802.11 Wireless LAN (Wi-Fi certification)

IEEE 802.12 demand priority

IEEE 802.14 Cable modems

IEEE 802.15 Wireless PAN

IEEE 802.15.1 (Bluetooth certification)

IEEE 802.16 Broadband Wireless Access (WiMAX certification)

IEEE 802.16e (Mobile) Broadband Wireless Access

IEEE 802.17 Resilient packet ring

IEEE 802.18 Radio Regulatory TAG

IEEE 802.19 Coexistence TAG

IEEE 802.20 Mobile Broadband Wireless Access

IEEE 802.21 Media Independent Handoff

IEEE 802.22 Wireless Regional Area Network

4-6


• Ethernet Standards are OSI Standards

– Single networks, including LANs, are governed by physical and data link layer standards

– Layer 1 and Layer 2 standards are almost universally OSI standards

– Ethernet is no exception

– The IEEE makes 802.3 standards; ISO ratifies them

– In practice, when 802.3 finishes standards, vendors begin building compliant products

Ethernet Physical Layer Standards

4-8

Figure 4-3: Baseband Versus Broadband Transmission

Baseband Transmission

Source

Signal Transmitted Signal (Same)

Transmission Medium

Signal is injected directly into the transmission medium(wire, optical fiber)

Inexpensive, so dominates wired LAN transmission technology

BASE in standard names means baseband

4-9

Figure 4-3: Baseband Versus Broadband Transmission, Continued

Broadband Transmission

SourceRadioTuner

Modulated Signal

Radio Channel

The radio tuner modulates the signal to a higher frequency. The transceiver then sends the signal in a radio channel.

Expensive but needed for radio-based networks.

Not used in Ethernet, but is used in wireless LANs (discussed in Chapter 5).

4-10

Figure 4-2: Ethernet Physical Layer Standards

PhysicalLayerStandard

MediumRequired

MaximumRun

Length

Speed

UTP

100BASE-TX 4-pair Category 5 orhigher

100 meters100 Mbps

1000BASE-T(GigabitEthernet)

4-pair Category 5 orhigher

100 meters1,000 Mbps

10BASE-T 4-pair Category 3 orhigher

100 meters10 Mbps

UTP dominates the Ethernet access line market

T: twisted-pair wire X: 2 pair (transmit, receive)

4-11


Medium850 nm light (inexpensive)Multimode fiber

MaximumRun

Length

Speed

1000BASE-SX 275 m1 Gbps



Gigabit Ethernet, 850 nm, various core sizes and modal bandwidthsDominates switch-to-switch trunk lines for gigabit Ethernet


Figure 4-2: Ethernet Physical Layer Standards, Continued

62.5microns

160MHz-km

62.5 200

50 400

50 500

S: 850 nm (short wavelength)

4-12

Perspective

• Access links to client stations today are dominated by 100BASE-TX100BASE-TX

– But 1000BASE-T usage is growing

• Trunk links today are dominated by 1000BASE-SX1000BASE-SX

– Sufficient for most LAN trunk line distances and speeds

– Short trunk links, however, use UTP

– Longer and faster trunk links use other fiber standards

4-13


MediumMaximumRun

Length

Speed

10GBASE-SR/SW 62.5/125 micronmultimode, 850 nm.

65 m10 Gbps

10 Gbps Ethernet, multimode S = 850 nm R=LAN (10 Gbps) W=WAN (9.95328 Gbps)


10GBASE-SRis used primarily fortrunk lines within equipment rooms

4-14


MediumMaximumRun

Length

Speed

10GBASE-LR/LW 8.3/125 micron singlemode, 1,310 nm.

10 km10 Gbps

10GBASE-ER/EW 8.3/125 micron singlemode, 1,550 nm.

40 km10 Gbps

10 Gbps Ethernet, for wide area networks L = 1,310 nm,

E = 1,550 nm R = LAN (10 Gbps)

W = WAN (9.95328 Gbps) for SONET/SDH


4-15



MediumMaximumRun

Length

Speed

40 GbpsEthernet

8.3/125 micronsingle mode.

UnderDevelopment

40 Gbps

4-16

Figure 4-4: Link Aggregation (Trunking or Bonding)

FiberCordFiber

Cord

1000BASE-SX Switch

Two links double trunk capacity between the switches

(1 Gbps to 2 Gbps)

No need to buy a more expensive10 Gbps Ethernet port or switch

Switch must support link aggregation(also called trunking or bonding)

1000BASE-SX Switch

4-17

Figure 4-5: Data Link Using Multiple Switches

OriginalSignal

ReceivedSignal

RegeneratedSignal

Switches regenerate signals before sending them out;this removes propagation effects.

It therefore allows signals to travel farther.

4-18

Figure 4-5: Data Link Using Multiple Switches, Continued

OriginalSignal

ReceivedSignal

ReceivedSignal

ReceivedSignalRegenerated

Signal RegeneratedSignal

Thanks to regeneration, signals can travel far acrossa series of switches

4-19


OriginalSignal

ReceivedSignal

ReceivedSignal


SignalRegenerated

Signal

UTP UTP62.5/125Multimode Fiber

100BASE-TX(100 m maximum)

Physical Link


Physical Link

1000BASE-SX(220 m maximum)

Physical Link

Each trunk line along the way has a distance limit

4-20


Station-to-station data link does not have a maximum distance(420 m distance spanned in this example)

OriginalSignal

ReceivedSignal

ReceivedSignal


Signal RegeneratedSignal

UTP UTP62.5/125Multimode Fiber


Physical Link


Physical Link

1000BASE-SX(220 m maximum)

Physical Link

Ethernet Data Link (MAC) Layer Standards

802 Layering

Frame Syntax

Switch Operation

4-22

Figure 4-6: Layering in 802 Networks, Continued

TCP/IP InternetLayer Standards(IP, ARP, etc.)

Other InternetLayer Standards

(IPX, etc.)

802.2

Ethernet 802.3 MAC LayerStandard

Physical Layer

MediaAccessControlLayer

Non-EthernetMAC Standards

(802.5,802.11, etc.)

100BASE-TX

1000Base-

SX…

LogicalLink

ControlLayer

Non-EthernetPhysical

LayerStandards

(802.11, etc.)

DataLink

Layer

Internet LayerThe 802 LAN/MAN Standards Committee

subdivided the data link layer

The media access control (MAC) layerhandles details specific to a

particular technology (Ethernet 802.3,802.11 for wireless LANs, etc.)

The logical link control layerhandles some general functions:

Connection to the internet layer, etc.;Not important to corporatenetworking professionals

4-23

Figure 4-6: Layering in 802 Networks, Continued

TCP/IP InternetLayer Standards(IP, ARP, etc.)

Other InternetLayer Standards

(IPX, etc.)

802.2

Ethernet 802.3 MAC LayerStandard

Physical Layer

MediaAccessControlLayer

Non-EthernetMAC Standards

(802.5,802.11, etc.)

100BASE-TX

1000BASE-

SX…

LogicalLink

ControlLayer

Non-EthernetPhysical

LayerStandards

(802.11, etc.)

DataLink

Layer

Internet LayerEthernet only has a single MAC standard(The 802.3 MAC Layer Standard)

Ethernet has many physical layer standards (Fig. 4-2)

4-24

Figure 4-7: The Ethernet MAC Layer Frame

Preamble (7 Octets)10101010 …

Start of Frame Delimiter (1 Octet)10101011

Destination MAC Address (48 bits)

Source MAC Address (48 bits)

Field

Preamble and Start ofFrame Delimiter

Strong repeating 10…pattern. Synchronizesreceiver’s clock withsender’s clock

Like quarterbackcalling out “Hut 1,Hut 2, Hut 3 …” tosynchronize the team

4-25

Figure 4-7: The Ethernet MAC-Layer Frame, Continued

Preamble (7 Octets)10101010 …

Start of Frame Delimiter (1 Octet)10101011

Destination MAC Address (48 bits)

Source MAC Address (48 bits)

Field

Computers use raw48-bit MAC addresses;Humans useHexadecimal notation(A1-23-9C-AB-33-53),which is discussednext.

4-26

Figure 4-8: Hexadecimal Notation

4 Bits(Base 2)*

Decimal(Base 10)

Hexadecimal(Base 16)Symbol

0000 0 0 hex

0001 1 1 hex

0010 2 2 hex

•With 4 bits, there are 24=16 possible symbols.•For example, 01-34-CD-7B-DF hex begins with 00000001 for 01.

0011 3 3 hex

0100 4 4 hex

0101 5 5 hex

0110 6 6 hex

0111 7 7 hex

BeginCounting atZero

4-27

Figure 4-8: Hexadecimal Notation, Continued

4 Bits(Base 2)

Decimal(Base 10)

Hexadecimal(Base 16)Symbol

1000 8 8 hex

1001 9 9 hex

1010 10 A hex

1011 11 B hex

1100 12 C hex

1101 13 D hex

1110 14 E hex

1111 15 F hex

After 9,Count A

Through F

4-28

Figure 4-8: Hexadecimal Notation, Continued

• Converting 48-Bit MAC Addresses to Hex– Start with the 48-bit MAC Address

• 1010000110111011 …

– Break the MAC address into twelve 4-bit “nibbles”• 1010 0001 1101 1101 …

– Convert each nibble to a hex symbol• A 1 D D

– Write the hex symbols in pairs (each pair is an octet) and put a dash between each pair

• A1-DD-3C-D7-23-FF

4-29

Figure 4-7: The Ethernet MAC Layer Frame, Continued

Length (2 Octets)

PAD

Field

Packet(VariableLength)

LLC Subheader(Usually 8

Octets)Data Field

(VariableLength)

Frame Check Sequence (4 Octets)

Data field containsA packet of variablelength

Packet is preceded inthe data field by anLLC subheader thatdescribes the typeof packet (IP, IPX, etc.)

Length field givesthe length of thedata field in octets

4-30


Length (2 Octets)

PAD

Field



Octets)Data Field

(VariableLength)


A PAD is added if the data field is less than 46octets; length is set tomake the data field plusPAD field 46 octets;

A PAD field is notadded if data fieldis greater than 46octets long.

4-31


Length (2 Octets)

PAD

Field



Octets)Data Field

(VariableLength)


Sender computes theframe check sequencefield value based on thebits in the other fields.

The receiver redoes thecomputation. If it getsa different results, theframe must have atransmission error.

The receiver discardsthe frame. There isno error correction.Ethernet is not reliable.

4-32

Figure 4-9: Multiswitch Ethernet LAN

Switch 2

Switch 1 Switch 3

Port 5 on Switch 1to Port 3 on Switch 2


A1-44-D5-1F-AA-4CSwitch 1, Port 2

E5-BB-47-21-D3-56Switch 3, Port 6

D5-47-55-C4-B6-9FSwitch 3, Port 2

B2-CD-13-5B-E4-65Switch 1, Port 7

The Situation:A1… Sends to E5…

Frame must go through3 switches along the way

(1, 2, and then 3)

4-33

Figure 4-9: Multiswitch Ethernet LAN, Continued

Switching Table Switch 1Port Station

2 A1-45-D5-1F-AA-4C7 B2-CD-13-5B-E4-655 D5-47-55-C4-B6-9F5 E5-BB-47-21-D3-56

Switch 2

Switch 1



B2-CD-13-5B-E4-65Switch 1, Port 7


On Switch 1

4-34


Switch 2

Switch 1 Switch 3






On Switch 2

4-35


Switch 2

Switch 3



D5-47-55-C4-B6-9FSwitch 3, Port 2




On Switch 3

4-36

Figure 4-10: Hierarchical Ethernet LAN

EthernetSwitch F

Server YServer X

Client PC1

SinglePossible Path

BetweenClient PC 1

and Server Y

EthernetSwitch E

EthernetSwitch D

EthernetSwitch B

EthernetSwitch A

EthernetSwitch C

4-37

Figure 4-10: Hierarchical Ethernet LAN, Continued

• With only one possible path between stations…

– Therefore there is only one possible port on a switch to send the frame back out

– Therefore only one row per MAC address in switching table

– Switch can find the one row quickly

– This makes Ethernet switches inexpensive per frame

– Low cost has ledto Ethernet’sLAN dominance

Port Station 2 A1-44-D5-1F-AA-4C7 B2-CD-13-5B-E4-655 E5-BB-47-21-D3-56

4-38

Figure 4-10: Hierarchical Ethernet LAN, Continued

Workgroup EthernetSwitch F

CoreSwitches

WorkgroupEthernetSwitch E

WorkgroupEthernetSwitch D

Core EthernetSwitch B

Core EthernetSwitch A

Core EthernetSwitch C

Core

Workgroup Switch

As noted in Chapter 3, thereare workgroup and core switches.

Core switches need more capacity.

4-39

Figure 4-11: Single Point of Failure in a Switch Hierarchy

No CommunicationNo Communication

Switch 1

Switch 2

Switch 3

Switch Fails

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

B2-CD-13-5B-E4-65 D4-47-55-C4-B6-9F

E5-BB-47-21-D3-56

4-40

Figure 4-12: 802.1D Spanning Tree Protocol (STP)

Switch 1

Switch 2

Switch 3

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

B2-CD-13-5B-E4-65 D4-47-55-C4-B6-9F

E5-BB-47-21-D3-56

Activated

Activated

Deactivated

Normal OperationLoop, but Spanning Tree ProtocolDeactivates One Link

4-41

Figure 4-12: 802.1D Spanning Tree Protocol (STP), Continued

Switch 1

Switch 2

Switch 3

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

B2-CD-13-5B-E4-65

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

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

E5-BB-47-21-D3-56

Deactivated Deactivated

Reactivated

Switch 2 Fails

4-42

Figure 4-12: 802.1D (STP), Continued

• Spanning Tree Protocol (STP)

– Works but when there is a break in the hierarchy, the network converges to a new hierarchy too slowly

• Rapid Spanning Tree Protocol (RSTP)

– Newer algorithm that converges very quickly

4-43

Figure 4-13: Virtual LAN (VLAN) with Ethernet Switches

Client A

Client B

Client C

Server D Server E

ServerBroadcast

Server Broadcasting without VLANS

Servers SometimesBroadcast; GoesTo All Stations;Latency Results

Destination MAC address: FF-FF-FF-FF-FF-FF

4-44

Figure 4-13: Virtual LAN (VLAN) with Ethernet Switches, Continued

Server Broadcasting with VLANS

Client Aon VLAN1

Client Bon VLAN2

Client Con VLAN1

Server Don VLAN2

Server Eon VLAN1

ServerBroadcast

NoNo

With VLANs,Broadcasts Only GoTo a Server’s VLAN

Clients; LessLatency

4-45

Figure 4-14: Tagged Ethernet Frame (Governed By 802.1Q)

Destination Address(6 Octets)

Destination Address(6 Octets)

Source Address (6 Octets)

Length (2 Octets)Length of Data Field in

Octets1,500 (Decimal) Maximum

Tag Protocol ID (2 Octets)1000000100000000

81-00 hex; 33,024 decimal.Larger than 1,500, So not

a Length Field

By lookingat the value

in the 2octets after

theaddresses,the switchcan tell ifthis frameis a basic

frame(value lessthan 1,500)or a tagged(value is 33,024).

Basic 802.3 MAC Frame Tagged 802.3 MAC Frame

Start-of-Frame Delimiter(1 Octet)

Preamble (7 octets)

Start-of-Frame Delimiter(1 Octet)

Preamble (7 octets)

Source Address (6 Octets)

4-46

Figure 4-14: Tagged Ethernet Frame (Governed By 802.1Q), Continued

Tag Control Information(2 Octets) Priority Level (0-7)

(3 bits); VLAN ID (12 bits)1 other bit

Basic 802.3 MAC Frame Tagged 802.3 MAC Frame

Length (2 Octets)

Data Field (variable)

Data Field (variable)

PAD (If Needed)

Frame Check Sequence(4 Octets)

PAD (If Needed)

Frame Check Sequence(4 Octets)

4-47

Figure 4-15: Handling Momentary Traffic Peaks with Overprovisioning and Priority

Traffic

Network Capacity

Momentary Traffic Peak:Congestion and Latency

Time

Momentary Traffic Peak:Congestion and Latency

Momentary traffic peaks usually last onlya fraction of a second;

They occasionally exceed the network’s capacity.When they do, frames will be delayed, even dropped.

4-48

Figure 4-15: Handling Momentary Traffic Peaks with Overprovisioning and Priority, Continued

Traffic

Overprovisioned Network Capacity Momentary Peak:No Congestion

Time

Overprovisioned Traffic Capacity in Ethernet

Overprovisioning:Build high capacity than will rarely if ever be exceeded.

This wastes capacity.But cheaper than using priority (next)

4-49

Figure 4-15: Handling Momentary Traffic Peaks with Overprovisioning and Priority, Continued

Traffic

Network Capacity

MomentaryPeak

Time

Priority in Ethernet

High-Priority Traffic GoesLow-Priority Waits

Priority:During momentary peaks, give priority to

traffic that is intolerant of latency (delay), such as voice.No need to overprovision, but expensive to implement.

Ongoing management is very expensive.

4-50

Figure 4-16: Hub Versus Switch Operation, Continued

A B C D

EthernetSwitch

Switch Sends Frame Out One PortIf A Is Transmitting to C,

B Can Transmit to DSimultaneously

4-51

Figure 4-16: Hub Versus Switch Operation

A B C D

EthernetHub

Hub Broadcasts Each BitOut All Other Ports

---If A Is Transmitting,

B Must Wait to Transmit

X

4-52

Figure 4-16: Hub Versus Switch Operation, Continued

• Hubs Need Media Access Control– This limits when a station may transmit– Ethernet hubs use CSMA/CD

• Carrier Sense Multiple Access (CSMA)– Only transmit if no other station is transmitting– Otherwise, wait

• Collision Detection (CD)– If two NICs transmit at the same time, this is a collision

– Both will stop, wait a random amount of time, and the go back to CSMA to send again

4-53

Carrier Sense Multiple Access with Collision Detection

• With CSMA, collisioncollision occupies medium for duration of transmission

• Stations listen whilst transmitting

1. If medium idle, transmit, otherwise, step 2

2. If busy, listen for idle, then transmit

3. If collision detected, send a jamming signal and then cease transmission

4. After jam, wait random time (backoff) then start from step 1

• Binary exponential backoff– Random delay is doubled (the first 10 retransmission)– After 16 unsuccessful attempts, give up

Purchasing Switches

4-55

Figure 4-17: Switch Purchasing Considerations

• Number and Speeds of Ports

– Buyers must decide on the number of ports needed and the speed of each

– Buyers often can buy a prebuilt switch with this configuration

4-56

Figure 4-18: Switching Matrix

1

2

3

4

100 Mbps

100 Mbps

100 Mbps

100 Mbps

1 2 3 4

Port 1to

Port 3400 MbpsAggregateCapacity

to BeNonblocking

InputQueue(s)

100BASE-TXInput Ports

100BASE-TXOutput Ports

Any-to-AnySwitching

Matrix

Note: Input Port 1 and Output Port 1 are the same port.Aggregate switching matrix capacity is its total switching speed.

Maximum input for this switch is 400 Mbps (4 x 100 Mbps).400 Mbps aggregate capacity is needed for switch to be nonblocking

4-57

Figure 4-17: Switch Purchasing Considerations, Continued

• Store-and-Forward Versus Cut-Through Switching (see Figure 4-19)

– Store-and-forward Ethernet switches read whole frame before passing the frame on

– Cut-through Ethernet switches read only some fields before starting to pass the frame back out

– Cut-through switches have less latency, but this is rarely important

4-58

Figure 4-19: Store-and-Forward Versus Cut-Through Switching

Preamble

Start-of-Frame Delimiter

Destination Address

Source Address

Tag Fields if Present

Length

Cyclical Redundancy Check

Data (and Perhaps PAD)

2.Cut-Through BasedOn MAC DestinationAddress (14 Octets)

3..Cut-Through forPriority or VLANs(24 Octets)

4.Cut-Through at64 Bytes (Not a Runt)

1.Store-and-ForwardProcessingEnds Here(OftenHundredsOf Bytes)

4-59

Figure 4-20: Managed Switches

Manager

Command to ChangeConfiguration(can fix many

problems remotely)

Get Data

Data Requested

Managed Switch

Managed Switch

Manager can manageall switches remotely

Managed switchescost much morethan unmanaged

switcheds

4-60

Ethernet Security

• Port-Based Access Control (802.1X)

– Attackers on site can walk up to any Ethernet port and plug in a computer, bypassing the firewall

– 802.1X standard

• Computer attaching to a port must first authenticate itself. (Details in Chapter 5)

No AccessWithout

Authentication

4-61

Ethernet Security

• MAC Security (MACsec) 802.1AE

– Switches must talk to one another for STP, VLANs, and other supervisory protocols

– An attacker on a PC can pretend to be a switch and send false supervisory messages

– 802.1AE MACsec protects supervisory communication, preventing many types of attacks

Stops FakeMessage

PC impersonatinga switch

False SupervisoryMessage

Box: Advanced Switch Purchase Considerations

Physical and Electrical Features

Box

4-63

Figure 4-21: Physical and Electrical Features

• Physical Size

– Switches fit into standard 19-in (48-cm) wide equipment racks

– Switch heights usually are multiples of 1U (1.75 in or 4.4 cm)

19 inches(48 cm)

Box

4-64

Figure 4-21: Physical and Electrical Features, Continued

• Port Flexibility

– Fixed-port switches

• No flexibility: the number of ports is fixed

• 1 or 2 U tall

• Most workgroup switches are fixed-port switches

Box

4-65



– Stackable Switches

• Fixed number of ports

• 1 or 2 U tall

• High-speed interconnect bus connects stacked switches

• When demand increases, firm can simply add a new stackable switch

Box

4-66



– Modular Switches

• 1 or 2 U tall

• Contain one or a few slots for modules

• Each module usually contains 1 to 4 ports

Box

Module

4-67



– Chassis switches

• Several U tall

• Contain several expansion slots

• Each expansion board contains several slots

• Most core switches are chassis switches

Box

4-68


• Switch and NIC Ports

– Normal Ethernet RJ-45 switch ports transmit on Pins 3 and 6 and listen on Pins 1 and 2

– NICs transmit on Pins 1 and 2 and listen on Ports 3 and 6

Box

NormalPC NIC

Port

NormalSwitch

Port

Pins1 & 2 Pins

3 & 6

4-69


• Switch and NIC Ports

– If you connect two normal ports on different switches via UTP cords, BOTH will send on Pins 3 & 6 and neither will listen on Pins 3 & 6

• Communication will be impossible

Box

NormalSwitch

Port

NormalSwitch PortOn Parent

Switch

Pins3 & 6

Pins3 & 6

4-70


• Switch Uplink Ports

– On a growing number of switches, normal ports change automatically to uplink ports if used that way

Box

NormalSwitch

Port


Switch

Pins3 & 6

1.Normallytransmits

on Pins 3 & 6

2.Changes automatically

to Pins 1 & 2

4-71


• Crossover Cables

– Designed to connect ordinary ports on two switches

– Internally, connect Pins 1 & 2 on one machine to Pins 3 & 6 on the other switch

– Do NOT use to connect NICs to switches or a switch uplink port to another switch!

Box /New

NormalSwitch

Port


SwitchPins3 & 6

Pins1 & 2

CrossoverCable

4-72


• Electrical Power

– Under the 802.3af standard, switches can provide electrical power to devices over the UTP cord

– Currently limited to 12.95 watts; sufficient for most wireless access points (Chapter 5) and voice over IP telephones (Chapter 6) but not sufficient for computers

– New slightly higher-power version of the standard is being developed to be able to serve sophisticated access points; still not good enough for computers.

Box

Topics Covered

4-74

Topics Covered

• Ethernet Standards Setting– 802.3 Working Group

– Physical and data link layer standards

– OSI standards

• Physical Layer Standards– BASE means baseband

– 100BASE-TX dominates for access lines

– 10GBASE-SX dominates for trunk lines

– Link aggregation for small capacity increases

– Regeneration to carry signals across multiple switches

4-75

Topics Covered

• Ethernet MAC Layer Standards– Data link layer subdivided into the LLC and MAC layers– The Ethernet MAC Layer Frame

• Preamble and Start of Frame Delimiter fields• Destination and Source MAC addresses fields

– Hexadecimal notation• Length field• Data field

– LLC subheader– Packet– PAD if needed

• Frame Check Sequence field

4-76

Topics Covered

• Ethernet MAC Layer Standards– Switch operation

• Operation of a hierarchy of switches

– Single possible path between any two computers

– Hierarchy gives low price per frame transmitted

– Single points of failure and the Spanning Tree Protocol

• VLANs and frame tagging to reduce broadcasting

• Momentary traffic peaks: addressed by overprovisioning and priority

• Hubs and CSMA/CD

4-77

Topics Covered

• Switch Purchasing Considerations

– Number and speed of ports

– Switching matrix (nonblocking)

– Store-and-forward versus cut-through switches

– Managed switches

– Ethernet security

• 802.1X Port-Based Access Control

• 802.1AE MACsec

4-78

Topics Covered

• Advanced Switch Purchasing Considerations

– Physical size

– Fixed-Port-Speeches

– Stackable Switches

– Modular Switches

– Chassis Switches

– Pins in Switch Ports and Uplink Ports

– Electrical Power (802.3af)

Box

Ethernet LANs Chapter 4 Panko’s Business Data Networks and Telecommunications, 6th edition Copyright 2007 Prentice-Hall May only be used by adopters of.

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