1. Chapter 14. LAN Systems Ethernet and Fast Ethernet (CSMA/CD)
Token Ring and FDDI
100VG-AnyLAN
ATM LANs
Wireless LANs

2. Ethernet (CSMA/CD) Precursors ALOHA for packet radio networks
Whenever a station has a frame to send, it does so
The station then listens for an amount of time equal to the maximum possible round-trip propagation delay on the network plus a small fixed time increment
If the station hears an acknowledgement during that time, fine; Otherwise, it resends the frame
If the station fails to receive an acknowledgement after repeated transmission, it gives up.
Maximum utilization of the channel = 18%
Slotted ALOHA
Maximum utilization = 37%

3. CSMA If a station wishing to transmit
first listens to the medium to determine if another transmission is in progress (carrier sense)
If the medium is in use, the station must wait
If the medium is idle, the station may transmit
If two or more stations attempt to transmit at about the same time, there will be a collision
To account for the collision, a station waits a reasonable amount of time, after transmitting, for an acknowledgement. If no ack., wait a random amount of time, then retransmit

4. CSMA (cont) 1-persistent used in IEEE 802.3
A station wishing to transmit listens to the medium
If the medium is idle, transmit
If the medium is busy, continue to listen until the channel is sensed idle; then transmit immediately

5. CSMA/CD Inefficiency of CSMA CSMA/CD
When two frames collide, the medium remains unusable for the duration of transmission of both damaged frames
CSMA/CD
Carrier sense for transmission
If a collision is detected during tx, transmit a brief jamming signal and then cease tx
After transmitting the jamming signal, wait a random amount of time, then attempt to transmit again (binary exponential backoff)

6. CSMA/CD (cont)
The amount of time to detect a collision is no greater than twice of the end-to-end propagation delay
Rules followed in CSMA/CD
Frames should be long enough to allow collision detection prior to the end of transmission
If shorter frames are used, then collision detection does not occur

7. CSMA/CD (ref.) Minimal frame size for 10BASE5, 10Mbps
Typical electromagnetic wave’s speed in coaxial cable = 2.5x108 m/s (table 4.1 in textbook)
Maximum segment length = 500m, a maximum of four repeaters  length of medium = 2500m
Minimal Ethernet frame size =

8. IEEE 802.3 MAC Frame Preamble: alternating 0s and 1s
to establish bit synchronization
Start frame delimiter
indicates the actual start of the frame
Pad
Octets added to ensure that the frame is long enough for proper CD operation

9. IEEE 802.3 Specification Notation 10Mbps 100Mbps
<data rate in Mbps> <signaling method> <max. segment length in hundreds of meters>
10Mbps
10BASE5, 10BASE2, 10BASE-T, 10BROAD36, 10BASE-F
100Mbps
100BASE-TX, 100BASE-FX

10. IEEE 802.3 10Mbps Physical Layer

11. IEEE 802.3 100Mbps Specification

12. IEEE 802.3 100Mbps Physical Layer

13. 4B/5B NRZ-I

14. 4B/5B NRZ-I (cont)

15. Gigabit Ethernet Media Access Layer Physical Layer
Two enhancements to CSMA/CD
1. Carrier extension
The minimum 512 bit-times (64 octets) for 10/100 Mbps  4096 bit-times (512 octets)
2. Frame bursting
Allows for multiple short frames to be transmitted consecutively
Physical Layer
1000BASE-SX, -LX, -CX, -T
Signal encoding scheme: 8B/10B

16. Token Ring Operation

17. IEEE Frame

18. IEEE 802.5 Frame (cont) Ending delimiter field Frame status field
E bit: each station checks passing frames for errors and set the E bit to 1
Frame status field
Destination nonexistent or not active: A=0, C=0
Destination exists but frame not copied: A=1, C=0
Frame received: A=1, C=1

19. Token Ring Priority SchemeD D A B A B E E C C
A is sending to B
D makes a Higher
priority level reservation
A generates a higher priority
token and remembers preempting
lower priority

20. Token Ring Priority Scheme (cont)D D A B A B E E C C
D uses the token to send data to E
D generates a token (at current priority level)

21. Token Ring Priority Scheme (cont)D D
Hi-pri
free token A B A B E E C C
A sees the high priority token
A generates a token at the preempted priority level

22. Updates to Token Ring Early Token Release (ETR)
Origin: The transmitting station must wait until the leading edge of the frame returns before issuing a token
ETR: Allows a transmitting station to release a token as soon as it completes frame transmission
Dedicated Token Ring (DTR)  Token Ring switch
Define the use of stations and concentrators in the switched mode
Each link from concentrator to station is a dedicated link with immediate access possible; token passing is not used

23. IEEE Physical Layer

24. Disadvantage of Token Ring
Requirement for token maintenance
Loss of the token prevents further utilization of the ring
Duplication of the token can disrupt ring operation
One station must be selected as a monitor to ensure that exactly one token is on the ring and to ensure that a free token is reinserted, if necessary

25. In FDDI, 802.5 priority scheme is not used.
FDDI MAC Frame
In FDDI, priority scheme is not used.

26. FDDI MAC Protocol

27. FDDI MAC Protocol (cont)

28. FDDI Capacity Allocation
Two types of traffic
Synchronous and asynchronous
TTRT: target token-rotation time
SAi: synchronous allocation
Dmax + Fmax + TokenTime + SSAi <= TTRT
Dmax: propagation time for one complete circuit of the ring
Fmax: time required to tx a max.-length frame
TokenTime: time required to tx a token

29. FDDI Capacity Allocation (cont)
Timers
token-rotation (TRT), token-holding timer (THT)
Each station is initialized with TRT = TTRT
When a station receives the token
if the token is early
THT  TRT, TRT  TTRT, enable TRT
It may transmit synchronous frames for a time SAi
After tx synchronous frames, THT is enabled. The station may begin tx of asynchronous frames as long as THT > 0

30. FDDI Timed Token Protocol
Timer ms
THT := TRT
Token captured 50 40
TRT := TTRT 30
THT := 0
Token released 20
Async traffic 10 10 20 30 40 50 60 70 80 ms

31. FDDI Synchronous Traffic
Timer ms
THT := TRT
Token captured 50
TRT := TTRT 40
THT := 0
Token released 30
Sync traffic
End of guaranteed
transmission time 20
Async traffic 10 10 20 30 40 50 60 70 80 ms

32. Property of Timed Token Protocol
Each station will get access at least once within the pre-configured TTRT (no SA)
Assume Tki : Tx time of station k for ith captured token
TRTki : ith observed token rotation time for station k
Prove that TRTki <= TTRT for all stations k 1 2
Station k N

33. Proof 1. Tki >= 0 for all station k and all cycles i.
2. From the timed token protocol
Tki <= TTRT - TRTki
TRTki = T1i + T2i +…+ Tk-1i + Tki-1 + Tk+1i-1 + … + TNi-1
3. Assume for station k and cycle i, TRTki > TTRT , it means
T1i + T2i +…+ Tk-1i + Tki-1 + Tk+1i-1 + … + TNi-1 > TTRT
T1i + T2i +…+ Tk-2i + TTRT - TRTk-1i + Tki-1 + … + TNi-1 > TTRT
T1i + T2i +…+ Tk-2i - (T1i + T2i +…+ Tk-2i + Tk-1i-1 + Tki-1 +… + TNi-1 ) +
Tki-1 + Tk+1i-1 + … + TNi-1 > 0
Tk-1i-1 < 0 ==> conflict !! Therefore TRTki <= TTRT
Last cycle

34. FDDI Physical Layer

35. 100VG-AnyLAN 802.12 Demand priority Topology MAC hierarchical star
round-robin scheme with two priority levels
Single-hub network
When a station wishes to transmit a frame, it first issues a request to the central hub and then awaits permission from the hub to transmit
The central hub continually scans all of its ports for a request in round-robin fashion

36. 100VG-AnyLAN: Single-Hub Network

37. 100VG-AnyLAN: Hierarchical Network
Level 1
Root repeater
1-1
1-2
1-4
1-6
1-7 A B
Level 2
repeater
Level 2
repeater 1 2 k 1 2 n
. . .
. . .
3-1
3-2
3-k
5-1
5-2
5-n

38. ATM LANs Gateway to ATM WAN Backbone ATM switch Workgroup ATM
An ATM switch acts as a router and traffic concentrator for linking a premises network complex to an ATM WAN
Backbone ATM switch
ATM switches interconnect other LANs
Workgroup ATM
End systems connect directly to an ATM switch

39. Backbone ATM LAN ATM LAN Link to other ATM LAN ***** ***** To public
FDDI
622Mbps
*****
*****
155Mbps
155Mbps
To public
ATM Network
10Mbps Ethernet
*****
*****
155Mbps
100Mbps Ethernet

40. ATM LAN Hub Configuration

41. Fibre Channel Design to combine the best features of
I/O channel
Network communications
More like traditional circuit/packet switching
Protocol architecture: 5 levels
FC-0 Physical Media
FC-1 Transmission protocol: 8B/10B encoding
FC-2 Framing protocol
FC-3 Common services: includes multicasting
FC-4 Mapping: IEEE802, ATM, IP, SCSI

42. Wireless LANs IEEE 802.11 Extended Service Set (a single logical LAN)
Server
Distribution System
(a cell)
Basic service set
Basic service set
Access point
Access point
station
station
station
station
station

43. Wireless LANs (cont) Three types of stations MAC No transition
BSS-transition
ESS-transition
MAC
DFWMAC (distributed foundation wireless MAC)
Contention-free service
Contention
service
Point coordination function (PCF)
Distributed coordination function (DCF)
CSMA-CA
Physical Layer

44. IEEE MAC Timing
Carrier Sense Multiple Access with Collision Avoidance (CSMA-CA)

45. IEEE 802.11 MAC Timing (cont) SIFS (short inter-frame space)
The shortest IFS, used for all immediate response actions
PIFS (point coordination function IFS)
A mid-length IFS, used by the centralized controller in the PCF scheme when issuing polls
DIFS (distributed coordination function IFS)
The longest IFS, used as a minimum delay for asynchronous frames contending for access

46. IEEE 802.11 MAC Timing (cont) SIFS is used for Acknowledgment (ACK)
MAC-level ACK provides for efficient collision recovery
Clear to send (CTS)
Sender sends Request to Send (RTS) frame
If receiver is ready to receive, responds with a CTS frame
All other stations defer using the medium until they see a corresponding CTS, or timeout
Poll response
For PCF

47. IEEE MAC Timing (cont)