1. More on LANs
Module C
Copyright 2001 Prentice Hall

2. More on LANs More on Ethernet More on 802.5 Token-Ring Networks
Base5 and 10Base2
100Base-TX signaling
Ethernet II versus 802.3
802.2 Logical Link Control Layer Frame
More on Token-Ring Networks
Wireless LANs
Bridges versus Switches

3. Ethernet Recap
Created by Digital Equipment Corporation, Intel, and Xerox
Taken over by the 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 MAC layer standard for framing and CSMA/CD

4. 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

5. 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)
Drop
Cable
(AUI)
Trunk
Cable
Up to 500 meters

6. (Medium Attachment Unit)
Ethernet 10Base5
Drop Cable
(Attachment Unit
Interface or AUI)
15 wires
50 m maximum
NIC
15-pin
AUI Connector
N-Connector
Trunk Cable
Coaxial Cable
500 m maximum
Transceiver
(Medium Attachment Unit)

7. 1. Sending NIC Transmits a bit
Ethernet 10Base5
1. Sending NIC Transmits a bit
2. Transceiver
Broadcasts the bit
3. Each Transceiver
Sends the bit to
Its NIC

8. Ethernet 10Base5 Broadcast (Bus) Topology Station B transmits
Signal travels down trunk to all other stations A B C

9. Ethernet 10Base-5 Up to 5 segments connected by repeaters
2,500 meters maximum distance between farthest stations
Four repeaters maximum
No loops
Repeater
Repeater
Drop cable
Segment of Trunk Cable
AUI Port

10. 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
To next
NIC
To next
NIC
BNC
T-connector
NIC

11. 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
Terminator
NIC
NIC
NIC

12. Repeaters can Mix 802.3 PHY Standards
Repeaters must have the correct ports
Repeater
Repeater
10Base2
RJ-45
AUI
Connectors
BNC
NIC
10Base-T
UTP
Drop
Cables
10Base5 Trunk Cable

13. 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 LLC layer; Full data link layer protocol
Ethernet II Frame
Preamble
SFD DA SA
Ethertype
Data
FCS
802.3 Frame
Preamble
SFD DA SA
Length
LLC frame
FCS

14. 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)
Ethernet II Frame
Preamble
SFD DA SA
Ethertype
Data
FCS
802.3 Frame
Preamble
SFD DA SA
Length
LLC Frame
FCS

15. Ethernet II vs 802.3 Frames Telling them Apart
NIC must recognize incoming frame’s type
If 2 bytes after SA <= 1500, must be because of maximum length limitation in 802.3
If 2 bytes after SA >1500, it’s an Ethertype
Ethernet II Frame
Preamble
SFD DA SA
Ethertype
Data
FCS
802.3 Frame
Preamble
SFD DA SA
Length
LLC Frame
FCS

16. 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
Internet Layer
Internet Layer
802.2 LLC
802.2 LLC
802.2 LLC
802.3 MAC
802.3 MAC
802.5 MAC
Base-T
Base-T
Mbps

17. 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

18. 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
Data Field
Control
SSAP
DSAP

19. 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)
Data Field
Control
SSAP
DSAP

20. 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
Data Field
Ether- type
Org
Code
Control
=03
SSAP
=AA
DSAP =AA
SNAP message

21. 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
Data Field
Ether- type
Org
Code
Control
=03
SSAP
=AA
DSAP =AA
SNAP message

22. 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
Data Field
Ether- type
Org
Code
Control
=03
SSAP
=AA
DSAP =AA
SNAP message