Winter 2005

1
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

2
LAN Topologies

• Basic topologies revisited

3
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?)
• ContentionTry to access the channel without
  prior arrangement

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

5
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
C1
C1
C
6
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

7
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

8
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

9
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

10
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

11
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 t
• At t0, station 1 starts sending
• At t0 t , station 2 sends its first bit, causes
  collision, stops sending
• Station 1 detects collision at t02t
• Hence, the contention period is 2t

12
Project 802
13
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

14
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

15
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 2t ? 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

16
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

17
Ethernet (802.3)

• Frame format
• Preamble used for sender/receiver clock
  synchronization
• MSB of destination address marks single (0) or
  group communication (1)

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

19
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

20
Token Ring (802.5)

• Token format
• Frame format

SD/ED Starting/Ending Delimiter AC Access
control 1 byte each
SD
AC
ED
FCS Coverage
End of FrameSequence
Start of FrameSequence
FC Frame Control FCS Frame Check Sequence FS
Frame Status
21
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?

22
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
• AC00 Destination not powered up or not present
• AC10 Destination present, but frame not
  accepted
• AC11 Destination present and frame accepted
• Automatic acknowledgment of frames