ethernet lans

Ethernet LANs

Chapter 4

4-2

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)

4-3


• 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

4-4


• 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

4-6

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

4-7

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).

4-8

Figure 4-2: Ethernet Physical Layer Standards

UTP PhysicalLayerStandards

MediumRequired

MaximumRun

Length

Speed

100BASE-TX 4-pair Category 5 or higher100 meters100 Mbps

1000BASE-T(GigabitEthernet)

4-pair Category 5 or higher100 meters1,000 Mbps

10BASE-T 4-pair Category 3 or higher100 meters10 Mbps

100BASE-TX dominates access links today,

Although 1000BASE-T is growing in access links today

4-9

Fiber PhysicalLayerStandards

Medium850 nm light (inexpensive)Multimode fiber

MaximumRun

Length

Speed

1000BASE-SX 275 m1 Gbps




Figure 4-2: Ethernet Physical Layer Standards, Continued

62.5microns

160MHz-km

62.5 200

50 400

50 500

The 1000BASE-SX standard dominates trunk links today.

Carriers use 1310 and 1550 nm light and single-mode fiber.

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10 Gbps Ethernet

• 10 Gbps Ethernet usage is small but growing

• Several 10 Gbps fiber standards are defined, but none is dominant

Revised

4-11

10 Gbps Ethernet

• 10 Gbps Ethernet usage is small but growing

• Several 10 Gbps 10GBASE-x fiber standards are defined, but none is dominant

• Copper is cheaper than fiber but cannot go as far– 10GBASE-CX4 (shielded Infiniband cable) up to 15 m

– UTP

• Category 6: 55 meters maximum (UTP)

• Category 6A: 100 meters (UTP)

• Category 7: 100 meters (shielded twisted pair, STP, which has metal shielding around each pair and around the cord)

Revised

4-12

100 Gbps Ethernet

• 100 Gbps has been selected as the next Ethernet speed

– Chosen over 40 Gbps

• 100 Gbps Ethernet standards development is just getting underway

NewInformation

4-13

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

1 GbpsCord1 Gbps

Cord

1000BASE-SX SwitchWe have been looking at single cords

Link aggregation or bondingallows you to bond two or more

cords between two switches

In this example, if you need 1.6 Gbps,two bonded 1 Gbps links willmeet your need at lower cost

than moving to a 10 Gbps switch.

Link aggregation allowsincremental growthin speed and cost

1000BASE-SX Switch

4-14

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.

4-15

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

4-16


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

4-17


Station-to-station data link does not have a maximum distance(420 m maximum distance 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

4-19

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

4-20

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)

4-21

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

4-22

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.

4-23

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

4-24

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

4-25

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

4-26

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

4-27


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.

4-28


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.

4-29

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)

4-30

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

4-31


Switch 2

Switch 1 Switch 3






On Switch 2

4-32


Switch 2

Switch 3



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




On Switch 3

4-33

Figure 4-10: Hierarchical Ethernet LAN

Client PC 1

EthernetSwitch F

Server YServer X

SinglePossible Path

BetweenClient PC 1

and Server Y

EthernetSwitch E

EthernetSwitch D

EthernetSwitch B

EthernetSwitch A

EthernetSwitch C

4-34

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

4-35

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.

4-36

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

4-37

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

4-38

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

4-39

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

Virtual LANs (VLANs)

4-41

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

4-42

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

4-43

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

• VLANs primarily reduce congestion due to latency– They can also be used for security

• Only people on a server’s VLAN can reach it– This provides some degree of security

– Not sufficient by itself, but it can help

• Wireless LANs– In wireless LANs, wireless clients may be initially placed

in a VLAN that only has a single server—a server that authenticates the clients

– After authentication, clients are allowed beyond the initial VLAN

4-44

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)

4-45

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)

4-46

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.

4-47

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)

4-48

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.

ethernet lans

Documents

gbps fiber standards

ethernet speedchosen

ethernet lanschapter

finishes standards

standards iso

ethernet standardsthe

lanman standards committee

gbasex fiber standards