1. Chapter 15 & 16 LAN (Local Area Network)

2. Topologies Tree Bus Ring Star Special case of tree
One trunk, no branches
Ring
Star

3. LAN Topologies

4. Bus and Tree Multipoint medium Heard by all stations
Need to identify target station
Each station has unique address
Full duplex connection between station and tap
Allows for transmission and reception
Terminator absorbs frames at end of medium

5. Frame Transmission on Bus LAN

6. Ring Topology Repeaters joined by point to point links in closed loop
Receive data on one link and retransmit on another
Links unidirectional
Stations attach to repeaters
Data in frames
Circulate past all stations
Destination recognizes address and copies frame
Frame circulates back to source where it is removed
Media access control determines when station can insert frame

7. Frame Transmission Ring LAN

8. Star Topology Each station connected directly to central node
Usually via two point to point links
Central node can broadcast
Physical star, logical bus
Only one station can transmit at a time
Central node can act as frame switch

9. LAN Protocol Architecture
Lower layers of OSI model
IEEE 802 reference model
Physical
Data link
Logical link control (LLC)
Media access control (MAC)

10. Logical Link Control Flow and error control
Based on HDLC
Three types
Unacknowledged connectionless service
Connection mode service
Acknowledged connectionless service
Addressing (De/Multiplexing)
Specifying source and destination LLC users
Referred to as service access points (SAP)
Typically higher level protocol

11. Media Access Control Govern access to transmission medium
Resource sharing methods
Centralized & Distributed
Centralized
Greater control
Simple access logic at station
Distributed
Synchronous & Asynchronous
Synchronous
Specific capacity dedicated to connection
Asynchronous
In response to demand

12. MAC Methods Round robin Reservation Contention
Order stations and allocate the right to use resources in turn
Reservation
First get exclusive use of resources before access
Contention
Access without explicit co-ordination
Retransmit in case of collision
Good for bursty traffic

13. LAN Protocols

14. Generic MAC Frame Format

15. Ethernet Contention Aloha Slotted Aloha
CSMA(Carrier Sense Multiple Access)
CSMA/CD(Collision Detection)
Splitting

16. ALOHA When station has frame, it sends
Station listens (for max round trip time)plus small increment
Frame may be damaged by noise or by another station transmitting at the same time (collision)
Any overlap of frames causes collision
If frame OK and address matches receiver, send ACK
No Ack in TO, retransmit
Max utilization 18%
Time t
t+1
t-1

17. Simple Analysis of ALOHA
Assumptions
Packet transmission attempt process - Poisson process with rate G
Fixed packet size = 1
: Packet transmission time of i-th packet
Condition for success
i-th packet is success if
No attempt in &&
No attempt in
Prob. of Success ?
Throughput ?

18. Slotted ALOHA Time in uniform slots equal to frame transmission time
Need central clock (or other sync mechanism)
Transmission begins at slot boundary
Frames either miss or overlap totally
Max utilization 37%
Time
Collision Idle Success Collision

19. Analysis of Slotted ALOHA
Prob. of success ?
Throughput ?
Throughput
Slotted
ALOHA
ALOHA
Attempt
Rate, G

20. CSMA Problems of slotted ALOHA Carrier Sense
Probe if the carrier is busy before transmit
If idle, transmit with some probability
Effects ?
Collision ?
If Propagation time << Transmission time, CSMA improves performance
Three CSMA schemes
Nonpersistent CSMA
1-persistent CSMA
p-persistent CSMA

21. Nonpersistent CSMA If medium is idle, transmit; otherwise, go to 2
If medium is busy, wait amount of time drawn from probability distribution (retransmission delay) and repeat 1
 Random delays reduces probability of collisions
Consider two stations become ready to transmit at same time
If both stations delay same time before retrying, then collision
Capacity is wasted because medium will remain idle following end of transmission
Even if one or more stations waiting

22. 1-persistent CSMA If medium idle, transmit; otherwise, go to step 2
If medium busy, listen until idle; then transmit immediately
If two or more stations waiting, collision guaranteed

23. P-persistent CSMA Compromise that attempts to reduce collisions
Like nonpersistent
And reduce idle time
Like1-persistent
Rules:
If medium idle, transmit with probability p, and delay one time unit with probability (1 – p)
Time unit typically maximum propagation delay
If medium busy, listen until idle and repeat step 1
If transmission is delayed one time unit, repeat step 1
What is an effective value of p?

24. Value of p Avoid instability under heavy load
n stations waiting to send (backlogged station)
End of transmission, expected number of stations attempting to transmit is number of stations ready times probability of transmitting np
If np > 1 on average there will be a collision
Repeated attempts to transmit almost guaranteeing more collisions
Retries compete with new transmissions
Eventually, all stations trying to send
Continuous collisions; zero throughput
So np < 1 for expected peaks of n
If heavy load expected, p small
However, as p made smaller, stations wait longer
At low loads, this gives very long delays

25. Splitting Algorithms Total attempt rate in contention methods
Too small => Idle, Resource waste
Too large => Too many collisions
In case of collision
Should reduce attempt rate
Serve contention involved stations first
Contention resolution
Then restart with new arrivals
CRP
New arrivals
Time
Collision
Contention Resolution Restart

26. Tree Splitting
Collision => Split into two subsets, Try the first subset
Idle / Success => Try next subset
Time
All R L LR LRR LRL LL R
LR R LRL LL

27. Tree Splitting Slot Xmit set Waiting sets Result Success Success
S C
L R C
LL LR, R S
LR R C
LRL LRR, R I
LRR R C
LRRL LRRR, R S
LRRR R S
R I
LRRL LRRR
Idle
Collision
LRL LRR
Success
Collision
LL LR
Idle
Collision
L R
Collision
How to maintain the stack for the waiting sets ? S

28. Improvements of Tree Algorithm
Arrival estimation
Spilt into k subsets
Collision -> Idle sequence
Collision -> Collision sequence

29. CSMA/CD
With CSMA, collision occupies medium for duration of transmission
Stations listen whilst transmitting (Listen while talk) to detect collision
On baseband bus, collision produces much higher signal voltage than non-collided signal
Since signal attenuated over distance, Limit the bus length to 500m (10Base5) or 200m (10Base2)
If medium idle, transmit, otherwise, step 2
If busy, listen for idle, then transmit
If collision detected, jam then cease transmission
After jam, wait random time then start from step 1

30. CSMA/CD - For baseband CSMA/CD, worst-case “wasted-time”
due to a collision = 2xTprop
Packet length should be at least twice the propagation delay
(a  0.5) t0 t1 t2 t3

31. Ethernet MAC Frame Format
Preamble: A 7-octet pattern of alternating 0s and 1s used by the receiver to establish bit synchronization (establishes the rate at which bit are sampled)
Start frame delimiter (SFD): Special pattern indicating the start of a frame
Length: Length of the LLC data field
LLC data
Pad: Octets added to ensure that the frame is long enough for proper CD operation
FCS: Error checking using 32-bit CRC

32. IEEE BEB
IEEE and Ethernet use BEB(Binary Exponential Backoff)
If the medium is idle, send immediately,
1-persistent can yield higher utilization than non/p-persistent
If the medium is busy, pause for a random interval after the end of busy period
Backoff delay
Because the offered load fluctuates dynamically, it is best to adjust the attempt rate dynamically to optimize the system utilization
How do you guess the offered load?
How do you adjust the total attempt rate?

33. BEB Guessing the number of backlogged stations
More backlogged station => More collisions
Adjustment of attempt rate
Change the backoff delay based on guessed offered load
BEB(Binary Exponential Backoff)
After each successive collision, double the backoff delay
Randomly select backoff delay (0, 2^n –1) where n is number of trials
1-persistent algorithm with BEB is efficient
At low loads, 1-persistence guarantees station can seize channel once idle
At high loads, at least as stable as other techniques
Backoff algorithm gives last-in, first-out effect
Stations with few collisions transmit first

34. 10Mbps Specification (Ethernet)
<data rate><Signaling method><Max segment length>
10Base5 10Base2 10Base-T 10Base-F
Medium Coaxial Coaxial UTP 850nm fiber
Signaling Baseband Baseband Baseband Manchester
Manchester Manchester Manchester On/Off
Topology Bus Bus Star Star
Nodes

35. Repeater
A repeater connects two or more bus segments to build a longer bus
A signal received from one bus segment is repeated to all other segments

36. Bus and Star Configurations
Use hub for star wiring
Transmission from any station received by hub and retransmitted on all outgoing lines

37. Hubs Active central element of star layout
Each station connected to hub by two lines
Transmit and receive
Hub acts as a repeater
When single station transmits, hub repeats signal on outgoing line to each station
Line consists of two unshielded twisted pairs
Limited to about 100 m
High data rate and poor transmission qualities of UTP
Optical fiber may be used
Max about 500 m
Physically star, logically bus
Transmission from any station received by all other stations
If two stations transmit at the same time, collision

38. Hub Layouts Multiple levels of hubs cascaded
Each hub may have a mixture of stations and other hubs attached to from below
Fits well with building wiring practices
Wiring closet on each floor
Hub can be placed in each one
Each hub services stations on its floor

39. Two Level Star Topology

40. Hub and Layer 2 Switch

41. 100Mbps (Fast Ethernet) 100Base-TX 100Base-FX 100Base-T4
2 pair, STP 2 pair, Cat 5 UTP 2 optical fiber 4 pair, cat 3,4,5
MLT-3 MLT-3 4B5B,NRZI 8B6T,NRZ

42. 100BASE-X Bidirectional data rate 100 Mbps over two links
Encoding scheme same as FDDI
4B/5B-NRZI
Modified for each option
100BASE-TX
Two pairs of twisted-pair cable
STP and Category 5 UTP allowed
The MTL-3 signaling scheme is used
100BASE-FX
Two optical fiber cables
Intensity modulation used to convert 4B/5B-NRZI code group stream into optical signals
1 represented by pulse of light
0 by either absence of pulse or very low intensity pulse

43. 100BASE-T4 100-Mbps over lower-quality Cat 3 UTP
Taking advantage of large installed base
Cat 5 optional
Does not transmit continuous signal between packets
Useful in battery-powered applications
Can not get 100 Mbps on single twisted pair
Data stream split into three separate streams
Each with an effective data rate of Mbps
Four twisted pairs used
Data transmitted and received using three pairs
Two pairs configured for bidirectional transmission
NRZ encoding not used
Would require signaling rate of 33 Mbps on each pair
Does not provide synchronization
Ternary signaling scheme (8B6T)

44. Full Duplex Operation Traditional Ethernet half duplex
Either transmit or receive but not both simultaneously
With full-duplex, station can transmit and receive simultaneously
100-Mbps Ethernet in full-duplex mode, theoretical transfer rate 200 Mbps
Attached stations must have full-duplex adapter cards
Must use switching hub
Each station constitutes separate collision domain
In fact, no collisions
CSMA/CD algorithm no longer needed
Ethernet MAC frame format used
Attached stations can continue CSMA/CD

45. Gigabit Ethernet - Differences
Carrier extension
At least 4096 bit-times long (512 for 10/100)
Frame bursting

46. Gigabit Ethernet – Physical
1000Base-SX
Short wavelength, multimode fiber
1000Base-LX
Long wavelength, Multi or single mode fiber
1000Base-CX
Copper jumpers <25m, shielded twisted pair
1000Base-T
4 pairs, cat 5 UTP
Signaling - 8B/10B

47. Gbit Ethernet Medium Options (log scale)

48. Gigabit Ethernet Configuration

49. 10Gbps Ethernet - Uses
High-speed, local backbone interconnection between large-capacity switches
Server farm
Campus wide connectivity
Enables Internet service providers (ISPs) and network service providers (NSPs) to create very high-speed links at very low cost
Allows construction of (MANs) and WANs
Connect geographically dispersed LANs between campuses or points of presence (PoPs)
Ethernet competes with ATM and other WAN technologies
10-Gbps Ethernet provides substantial value over ATM

50. 10Gbps Ethernet - Advantages
No expensive, bandwidth-consuming conversion between Ethernet packets and ATM cells
Network is Ethernet, end to end
IP and Ethernet together offers QoS and traffic policing approach ATM
Advanced traffic engineering technologies available to users and providers
Variety of standard optical interfaces (wavelengths and link distances) specified for 10 Gb Ethernet
Optimizing operation and cost for LAN, MAN, or WAN 

51. 10Gbps Ethernet - Advantages
Maximum link distances cover 300 m to 40 km
Full-duplex mode only
10GBASE-S (short):
850 nm on multimode fiber
Up to 300 m
10GBASE-L (long)
1310 nm on single-mode fiber
Up to 10 km
10GBASE-E (extended)
1550 nm on single-mode fiber
Up to 40 km
10GBASE-LX4:
1310 nm on single-mode or multimode fiber
Wavelength-division multiplexing (WDM) bit stream across four light waves

52. 10Gbps Ethernet Distance Options (log scale)

53. Token Ring (802.5) Developed from IBM's commercial token ring
Because of IBM's presence, token ring has gained broad acceptance
Never achieved popularity of Ethernet
Market share likely to decline

54. Ring Operation
Each repeater connects to two others via unidirectional transmission links
Single closed path
Data transferred bit by bit from one repeater to the next
Repeater regenerates and retransmits each bit
Repeater performs data insertion, data reception, data removal
Repeater acts as attachment point
Packet removed by transmitter after one trip round ring

55. Listen State Functions
Scan passing bit stream for patterns
Address of attached station
Token permission to transmit
Copy incoming bit and send to attached station
Whilst forwarding each bit
Modify bit as it passes
e.g. to indicate a packet has been copied (ACK)

56. Transmit State Functions
Station has data
Repeater has permission
May receive incoming bits
If ring bit length shorter than packet
Pass back to station for checking (ACK)
May be more than one packet on ring
Buffer for retransmission later

57. Bypass State
Signals propagate past repeater with no delay (other than propagation delay)
Partial solution to reliability problem (see later)
Improved performance

58. Ring Repeater States

59. 802.5 MAC Protocol Small frame (token) circulates when idle
Station waits for token
Changes one bit in token to make it SOF for data frame
Append rest of data frame
Frame makes round trip and is absorbed by transmitting station
Station then inserts new token when transmission has finished and leading edge of returning frame arrives
Under light loads, some inefficiency
Under heavy loads, round robin

60. Token Ring Operation

61. Dedicated Token Ring Central hub Acts as switch
Full duplex point to point link
Concentrator acts as frame level repeater
No token passing