More on LANsModule C

Copyright 2001 Prentice Hall

More on LANs• More on Ethernet

– 802.3 10Base5 and 10Base2– 100Base-TX signaling– Ethernet II versus 802.3– 802.2 Logical Link Control Layer Frame

• More on 802.5 Token-Ring Networks• 802.11 Wireless LANs• Bridges versus Switches

Ethernet Recap• Created by Digital Equipment Corporation, Intel,

and Xerox• Taken over by the 802.3 Working Group of the

IEEE 802 LAN MAN Standards Committee• 802.3 has since produced many standards

– Especially at the physical layer: 10Base5, 10Base2, 10Base-T, 100Base-TX, 1000Base-X

– Only one MAC layer standard--the 802.3 MAC layer standard for framing and CSMA/CD

Original Ethernet Versus 802.3 10Base5

• Created by Digital, Intel, and Xerox– Popular version was Ethernet II

• 802.3 Working Group took over– Created the first Ethernet standard, 802.3

– Slightly changed the physical layer; became 10Base-5

– Changed the data link layer (later)

• 10Base5– 10 Mbps, baseband, 500 meters/segment

Ethernet 10Base5

• Multidrop Layout– Main trunk cable: thick coaxial cable

• Up to 500 meters long

– Drop cable: 15-wire cable• Up to 50 meters

• Also called the attachment unit interface (AUI)

TrunkCable

DropCable(AUI)

Up to 500 meters

Ethernet 10Base5

NIC

15-pinAUI Connector

Drop Cable(Attachment UnitInterface or AUI)

15 wires50 m maximum

Trunk CableCoaxial Cable

500 m maximum

N-Connector

Transceiver (Medium Attachment Unit)

Ethernet 10Base5

1. Sending NIC Transmits a bit

2. TransceiverBroadcasts the bit

3. Each TransceiverSends the bit to

Its NIC

Ethernet 10Base5

• Broadcast (Bus) Topology

– Station B transmits

– Signal travels down trunk to all other stations

A B C

Ethernet 10Base-5

• Up to 5 segments connected by repeaters– 2,500 meters maximum distance between

farthest stations– Four repeaters maximum– No loops

Repeater RepeaterDrop cable

Segment of Trunk Cable AUI Port

Ethernet 10Base2 (802.3a)• Cheaper Physical Layer Standard

– NICs have BNC connector– T-connector attaches to it– T-connector has BNC connectors for cable runs

attaching it to adjacent stations

NIC

BNC T-connectorTo next

NICTo next

NIC

Ethernet 10Base2 (802.3a)• Segments are thin coaxial cable

– Run only between NICs– Daisy chain of NICs is a segment– Terminator at end of each segment– Up to 30 stations per segment– 5 segments (4 repeaters) maximum– 10Base2: 185 meters/segment

NIC NIC NICTerminator

Repeaters can Mix 802.3 PHY Standards

Repeater Repeater

NICBNC

AUIConnectors

RJ-45

UTP DropCables

10Base5 Trunk Cable

10Base2

10Base-T

Repeaters must have the correct ports

Ethernet II vs 802.3 Frames• Ethernet II Frame

– Ethertype field (2 bytes)– Tells protocol of message in data field (e.g., IP)– No length field– No 802.2 LLC layer; Full data link layer protocol

Preamble SFD DA SA Ethertype

Preamble SFD DA SA Length LLC frame FCS

Data FCS

Ethernet II Frame

802.3 Frame

Ethernet II vs 802.3 Frames• Why the Differences

– Lack of a length field could cause problems

– 802 would create multiple standards; Giving them all the same connection to the next higher layer (802.2 seemed wise)

Preamble SFD DA SA Ethertype

Preamble SFD DA SA Length LLC Frame FCS

Data FCS

Ethernet II Frame

802.3 Frame

Ethernet II vs 802.3 Frames• Telling them Apart

– NIC must recognize incoming frame’s type

– If 2 bytes after SA <= 1500, must be 802.3 because of maximum length limitation in 802.3

– If 2 bytes after SA >1500, it’s an Ethertype

Preamble SFD DA SA Ethertype

Preamble SFD DA SA Length LLC Frame FCS

Data FCS

Ethernet II Frame

802.3 Frame

802.2 LLC Layer Functions• Logical Link Control Layer (LLC)

– Links subnet to next-higher-layer (internet)– One LLC standard (802.2) for all 802 LANs– Makes interface to internet layer simple

Internet Layer

802.2 LLC

802.3 MAC

802.3 10Base-T

Internet Layer

802.2 LLC

802.3 MAC

802.3 100Base-T

Internet Layer

802.2 LLC

802.5 MAC

802.5 16 Mbps

802.2 LLC Layer Functions

• Logical Link Control

– At the MAC layer, the destination NIC discards incorrect MAC layer frames

– 802.2 standard provides optional error correction to retransmit lost or discarded LLC frames

802.2 LLC Frame• Fields

– Destination Service Access Point (DSAP) designates the next-higher layer protocol on the receiving device (IP, IPX, etc.)

– There can be multiple internet layer programs on the destination machine; DSAP designates the specific one to get the frame’s data field

– SSAP: Source Service Access Point

DSAPSSAPControlData Field

802.2 LLC Frame

• Fields

– Control field contains instructions to the receiver’s LLC process on the NIC

– Data field contains message of the next higher layer (usually internet)

DSAPSSAPControlData Field

802.2 LLC Frame• Subnet Access Protocol (SNAP) Alternative

for IP and other TCP/IP internet layer standards

– DSAP = AA hex

– SSAP = AA hex

– Control = 03 hex

DSAP=AA

SSAP=AA

OrgCode

Data FieldEther-type

SNAP message

Control=03

802.2 LLC Frame• Subnet Access Protocol (SNAP) Alternative

for IP and other TCP/IP internet layer standards

– Org code lists the organization maintaining a list of protocol codes for next higher layer

– Ethertype (Ethernet type) code names a specific protocol for the data field

DSAP=AA

SSAP=AA

OrgCode

Data FieldEther-type

SNAP message

Control=03

802.2 LLC Frame

• Subnet Access Protocol (SNAP) Alternative for IP and other TCP/IP internet layer standards

– Data field has message of that protocol

DSAP=AA

SSAP=AA

OrgCode

Data FieldEther-type

SNAP message

Control=03