1. Computer Interfacing and Protocols
Local Area Networks
Need for high performance communications for physically close devices (e.g. office environment)
Why “local”?
Volume of locally created data
Increasing local computation power
Advantage of locality: High Speed!
Design goals:
High speed and high bandwidth
Simple, maintainable, flexible, extendable
Low cost
Winter 2005
ECE
ECE 766
Computer Interfacing and Protocols

2. Computer Interfacing and Protocols
LAN Topologies
Basic topologies revisited:
Hub
Winter 2005
ECE
ECE 766
Computer Interfacing and Protocols

3. Computer Interfacing and Protocols
Channel Access
Line discipline is important factor of overall performance
Static allocation of resources results in poor performance
From queuing theory: dividing resources in N equal parts multiplies the mean waiting time by N
Polling vs. contention techniques
Polling: Asking everyone if they have something to send
Can be centralized or distributed (how?)
Contention: Try to access the channel without prior arrangement
Winter 2005
ECE
ECE 766
Computer Interfacing and Protocols

4. Multiple Access Protocols
Several different kinds of multiple access protocols exist:
Aloha
Pure, slotted
Carrier Sense Multiple Access (CSMA)
Persistent, non-persistent, p-persistent, CD
Collision-Free Protocols
Bit map, binary count…
Limited Contention Protocols
Adaptive tree walk… …
Winter 2005
ECE
ECE 766
Computer Interfacing and Protocols

5. Computer Interfacing and Protocols
Pure Aloha
Users send their frames as soon as they are available
Collisions will occur, but wait for a random amount of time and send the frame once again A A1 A1 A2 A2 B B1 B1 B2 C C1 C1
Winter 2005
ECE
ECE 766
Computer Interfacing and Protocols

6. Computer Interfacing and Protocols
Pure Aloha
Performance of Pure Aloha
When sending a frame, we hope that no one else is transmitting from 1 frame time before we start transmission until our transmission is over t0
t0+t
t0+2t
t0+3t
Vulnerable period
Winter 2005
ECE
ECE 766
Computer Interfacing and Protocols

7. Computer Interfacing and Protocols
Slotted Aloha
Different from Pure Aloha in the timing of channel access
Time is partitioned into slots
When a host receives a frame, it waits until the beginning of the next slot to transmit
The vulnerable period is reduced to half of Pure Aloha
A maximum of one slot waiting time is possible
Winter 2005
ECE
ECE 766
Computer Interfacing and Protocols

8. Computer Interfacing and Protocols
CSMA Protocols
Based on sensing the channel before sending the frame
Send the frame if channel is free
Behavior after detecting a busy channel determines the kind of CSMA protocol
1-persistent: Send the frame if channel is available. If busy, transmit the frame with probability 1 as soon as the channel is free. If collision occurs, wait a random amount of time and start over
Nonpersistent: Send the frame if channel is available. If busy, wait a random amount of time and try sending once again
Winter 2005
ECE
ECE 766
Computer Interfacing and Protocols

9. Computer Interfacing and Protocols
CSMA Protocols
P-persistent: Used in slotted channels. Send the frame with probability p if channel is available, defer to the next slot with probability 1-p. If busy, wait until the next slot and repeat the algorithm.
CSMA protocols have higher throughput than Aloha protocols
Nonpersistent protocol has higher throughput and delay than 1-persistent
Performance of p-persistent depends on the value of p
Winter 2005
ECE
ECE 766
Computer Interfacing and Protocols

10. Computer Interfacing and Protocols
CSMA/CD
Carrier Sense Multiple Access / Collision Detection
Sense the channel before sending
If collision is detected, stop the transmission (frame is damaged anyway)
Wait for a random amount of time before the next attempt
Collision detection is done by comparing the transmitted power to the received one
Winter 2005
ECE
ECE 766
Computer Interfacing and Protocols

11. Computer Interfacing and Protocols
CSMA/CD
How long does it take for a station to conclude that it seized the channel, i.e., what is the contention period?
Consider the worst case scenario
Largest propagation delay = τ
At t0, station 1 starts sending
At t0+ τ , station 2 sends its first bit, causes collision, stops sending
Station 1 detects collision at t0+2τ
Hence, the contention period is 2τ
Winter 2005
ECE
ECE 766
Computer Interfacing and Protocols

12. Computer Interfacing and Protocols
Project 802
802.2 Logical Link Control (LLC)
802.1 Internetworking
802.3 CSMA/CD
802.4 Token Bus
802.5 Token Ring
...
Project 802
Network
Data Link Layer
Physical
OSI Model
Winter 2005
ECE
ECE 766
Computer Interfacing and Protocols

13. Computer Interfacing and Protocols
Ethernet (802.3)
Xerox, DEC, Intel
Properties:
Simple, low cost, low delay
High speed (10, 100, 1000 Mbps)
Aims data exchange at data link level
Fairness in channel access
Single node, group, broadcast addressing
No unused fields, no variants
Stability: increase in offered traffic should not choke the system
Winter 2005
ECE
ECE 766
Computer Interfacing and Protocols

14. Computer Interfacing and Protocols
Ethernet (802.3)
Properties (not so attractive ones):
Not full duplex
Limited error control
Detection of and recovery from collision
Error detection using CRC, retransmissions left to higher level
No security integrated
Best effort service
No measures against malicious users
Winter 2005
ECE
ECE 766
Computer Interfacing and Protocols

15. Computer Interfacing and Protocols
Ethernet (802.3)
Limit on cable length
Minimum frame size is 64 bytes
At 10Mbps, it takes 51.2μsec to transmit the shortest frame
51.2μsec = 2τ  2500 meter cable length
To achieve 1Gbps:
Keep cable length at 2500m, minimum frame size becomes 6400 bytes
Keep minimum frame size at 64 bytes, maximum cable length becomes 25m
Winter 2005
ECE
ECE 766
Computer Interfacing and Protocols

16. Computer Interfacing and Protocols
Ethernet (802.3)
Binary Exponential Backoff Algorithm
Slot time = 51.2μsec
When collision occurs, wait 0 or 1 slot time
If another collision occurs, wait a random number of slot times between 0 and 3
After kth collision, randomly wait 0-(2k-1) slot times
Maximum slot time to be waited is 1023
Give up after 16 consecutive collisions
Winter 2005
ECE
ECE 766
Computer Interfacing and Protocols

17. Computer Interfacing and Protocols
Ethernet (802.3)
Frame format
Preamble used for sender/receiver clock synchronization
MSB of destination address marks single (0) or group communication (1)
Preamble
Destination Address
Source Address
Data
Pad
Checksum 7 1
2-6 2
0-1500
0-46 4
Bytes
Start of frame delimiter
Length of data
Winter 2005
ECE
ECE 766
Computer Interfacing and Protocols

18. Other Ethernet Networks
Switched Ethernet:
Switch isolates communication between two stations
Medium is no longer truly broadcast medium
Fast Ethernet:
Reduce the cable size to 250m, increase the speed to 100Mbps
Gigabit Ethernet:
1Gbps speed
25m with cable, 550/5000m with multi/single mode optical fiber
Winter 2005
ECE
ECE 766
Computer Interfacing and Protocols

19. Computer Interfacing and Protocols
Token Ring (802.5)
Unidirectional ring
Stations are either active or let the frames pass
Medium access:
Station waits for token
Capture token and transmit your message instead
Wait until you get your own message, then place the token on the line
Winter 2005
ECE
ECE 766
Computer Interfacing and Protocols

20. Computer Interfacing and Protocols
Token Ring (802.5)
Token format
Frame format
SD/ED: Starting/Ending Delimiter
AC: Access control
1 byte each SD AC ED SD
Destination Address
Source Address
Data
FCS 1 6
No limit 4
Bytes AC FC ED FS
Start of Frame Sequence
FCS Coverage
End of Frame Sequence
FC: Frame Control
FCS: Frame Check Sequence
FS: Frame Status
Winter 2005
ECE
ECE 766
Computer Interfacing and Protocols

21. Computer Interfacing and Protocols
Token Ring (802.5)
Starting Delimiter: J K 0 J K 0 0 0
Violations of Differential Manchester encoding
J: Cancel both transitions
K: Cancel middle transition only
Access Control: P P P T M R R R
P: Priority bits indicating which stations are allowed to use token
T: Token bit, 1 if token or abort, 0 if data or command
M: Monitor bit, used by active monitor station to detect orphan frames
R: Reservation bits to reserve the next token, cannot be set to less than priority of the frame  How does it ever decrease?
Winter 2005
ECE
ECE 766
Computer Interfacing and Protocols

22. Computer Interfacing and Protocols
Token Ring (802.5)
Frame Control:
Used to distinguish data frames from control frames
Frame Status:
Includes A and C bits
A is set when destination passes the frame
C is set when destination copies the frame
AC=00: Destination not powered up or not present
AC=10: Destination present, but frame not accepted
AC=11: Destination present and frame accepted
Automatic acknowledgment of frames
Winter 2005
ECE
ECE 766
Computer Interfacing and Protocols