1. Data-link Layer
Computer Networks

2. Where are we?

3. The Data Link Interface

4. The Local Area Network Popular (most data links are LANs)
High Throughput
Low Cost
Short Distances
Often shared medium access
Most new installations usually "switched"

5. Shared Medium Access A Shared Medium Used by All
Only One Station Transmits at a Time
Stations "Take Turns”
MAC Protocol defines fairness policy

6. Topology Review

7. Data Link Bit Encoding

8. Example Bus: Ethernet Most Popular LAN IEEE Standardized as 802.3
Several Generations
Same frame format (mostly)
Changing data rates
Different physical layer requirements
The book: Gigabit Ethernet, Rich Seifert

9. Ethernet Transmission
Only one station transmits at a time
Signal propagates entire cable length
All stations receive all transmissions
CSMA/CD medium access control scheme

10. CSMA/CD Carrier Sense (CS) Multiple Access (MA)
Wait until medium is idle
Begin to transmit frame
Multiple Access (MA)
Multiple stations attached to shared media
Each station uses the same access algorithm
Simultaneous Transmission is Possible

11. CSMA/CD [continued] Simultaneous Transmission:
Interfere with each other
Known as a collision
CSMA with Collision Detect (CD)
Listen to media during transmission
Detect whether another station’s signal interferes
Back off from interference and try again

12. Transmission Logic 1. If media is idle, transmit.
2. Else, continue to listen to the media and when it is available, transmit.
3. Listen to media while transmitting.
4. If collision is detected while transmitting, send jam and back-off
5. Go to step 1 until max-try counter is reached.

13. Exponential Back-off Algorithm
Let 1 Slot Time = 512 bit times
Upon 1st collision, randomly choose among {0,1} slot delay
Upon 2nd collision, randomly choose among {0,1,2,3} slot delay
Up to a maximum of 16 transmission attempts with a range of delay from {0 to 1024} bit times
0 <= r < 2k-1
Where r is the random number generated, where k = MIN(n,10) and where n is the n-th retransmission attempt

14. The Collision Domain
Minimum Length Frame Must Be >= Maximum RTT of the Ethernet segment
Minimum Frame is 512 bits
Requires 46 bytes of data whether the upper layer has them or not
Distances decrease as speed increases
Full-duplex mode eliminates the collision domain

15. An Aside - Collisions They are NOT bad, unless they’re late
Collision statistics are mostly meaningless
Monitor utilization
Distance Matters
Becoming irrelevant with switching
The name "Collision” is misleading

16. Ethernet Addressing Standardized by IEEE
Each station assigned a unique 48-bit address
First 24-bits are the OUI
Second 24-bits are vendor assigned
Usually set when NIC is manufactured
Canonical address format

17. Ethernet Address Recognition
Each Frame Contains a Destination Address
All Stations Receive All Transmissions
Station Discards Any Frame Not Destined for It
Important: interface hardware, not software, checks address

18. Possible Destinations
1. Single destination (unicast)
2. All stations on the Ethernet (broadcast)
3. Subset of stations on the Ethernet (multicast)
MAC address is used to distinguish between the destinations

19. Ethernet Destination Addresses

20. Promiscuous Mode Designed for testing/debugging
Allows interface to accept all frames
Available on most Ethernet hardware

21. IEEE 802.3 Frame Format Sender fills in: Sender’s source address
Recipient’s destination address
Type of data in the frame type field
Cyclic Redundancy in FCS field

22. Demultiplexing on Frame Type Field
Network Interface Hardware
Receives a copy of each transmitted frame
Examines address and either accepts or discards
Passes accepted frame to system software
Network device software
Examines frame type
Passes frame to correct software module

23. Ethernet Wiring - 10BASE5 Thick Ethernet (Thicknet)
Heavy coaxial cable

24. Ethernet Wiring - 10BASE2 Thin Ethernet (Thinnet)
Smaller coaxial cable

25. Ethernet Wiring - 10BASE-T
Uses a hub
Twisted-pair wiring

26. Ethernet Office Wiring

27. High-speed Ethernet Fast Ethernet Gigabit Ethernet
Operates at 100 Mb/s
Standardized in IEEE as 100BASE-T and 100BASE-F standards
10/100 Devices available
Gigabit Ethernet
Operates at 1 Gb/s
Mostly fiber systems using switches
Even higher speeds coming!

28. Ethernet - Final Notes
Data Link Layer Usually Implemented with Physical Layer
Link Beat
Interframe Gap Time
Capture Effect
Modern Ethernet is a star-shaped bus
news://comp.dcom.lans.ethernet
IETF increasing maximum frame size?

29. Example Ring: Token Ring
Popular in IBM environments
IEEE Standardized as 802.5
Operates at 4Mb/s, 16Mb/s
Quickly Being Abandoned
802.5 working group moved to "hibernation" status in July 2000
Still worth learning about!

30. Token Ring Transmission
Station waits for token before sending
Signal travels the entire ring
Sender receives its own transmission

31. Token Passing Paradigm
Frames travel in a unidirectional fashion around the ring
Stations must wait for token to transmit
Stations can reserve the token
Token will circle indefinitely until a station wants to transmit

32. MAC Frames Ring management and control frames
Beacon, Ring purge, claim token, report error
Ring Poll every 7 seconds
Active monitor present
Standby monitor present
NAUN notification process

33. Active and Standby Monitor
Only 1 Active Monitor per ring
AM is the master clock for the ring
AM inserts 24-bit delay to transmissions
AM ensures tokens/frames are present
AM removes circulating frames
SMs are ready to take over if AM fails

34. Monitor Contention Ring elects a new Active Monitor Initiated when:
Loss of signal is detected
Active monitor not detected
Time-outs of token timer, NAUN, etc.
Highest MAC address wins
Everyone else is Standby Monitor

35. Token Ring Insertion Process
Phase 0 - Media Lobe Check
Phase 1 - Physical Insertion
Phase 2 - Address Verification
Phase 3 - Participation in Ring Poll
Phase 4 - Request Initialization

36. The Token Frame
When no station is transmitting, the token frame travels continuously around the ring.

37. Token Ring Addressing Standardized by IEEE
Each station assigned a unique 48-bit address
First 24-bits are the OUI
Second 24-bits are vendor assigned
Usually set when NIC is manufactured
Non-canonical address format

38. Token Ring Address Recognition
Each Frame Contains a Destination Address
All Stations Receive and Repeat All Transmissions
Stations Copy Any Frame Destined for It
Important: interface hardware, not software, checks address

39. Token Ring Destination Addresses

40. Token Ring Frame Format
Sender fills in:
Sender’s source address
Recipient’s destination address
Cyclic Redundancy in FCS field
Other stations may change:
Frame Status

41. High-speed Token Ring HSTR Operates at 100 Mb/s
1 Gb/s was being worked on
Standardized in IEEE 802.5
Some 4/16/100 devices

42. Why Token Ring Lost
IBM was the only systems manufacturer that promoted it
Cost
Complexity
Support throughout the industry
Only one vendor left to develop product!

43. Token Ring - Final Notes
Jitter
Early Token Release
Backup Path
Token Transmission Timer
Needs LLC - we haven’t talked about it yet
news://comp.dcom.lans.token-ring

44. Example Ring: FDDI Uses Optical Fiber cabling
High reliability (dual rings)
Immune to interference
Standardized by ANSI
Transmission rate of 100 Mb/s
Similar to token ring

45. FDDI Dual Ring Operation

46. Logical Link Control Standardized by IEEE 802.2
Often used for MACs that don’t use type field

47. LLC with SNAP

48. What else? ATM Wireless (802.11) Fiber Channel HIPPI Token Bus (802.4)
IEEE 802 standards may become free!