LAN

1
LAN MAC (Medium Access Control) protocols

• Two basic types of networks
• Switched networks transmission lines,
  multiplexers, and switches routing, hierarchical
  address for scalability.
• Broadcast networks a single shared medium,
  simpler, no routing, messages received by all
  stations, flat address however, when users try
  to transmit messages into the medium, potential
  conflict, so MAC is needed to orchestrate the
  transmission from various users.
• LAN is a typical broadcast network.

2
Peer-to-peer protocols VS. MAC

• Both are to transfer user information despite
  transmission impairments
• For peer-to-peer
• Main concern loss, delay, resequencing
• Using control frames to coordinate their actions
• Delay-bandwidth is important
• Involved only two peer processes
• MAC
• Main concern interference from users
• Using some mechanisms to coordinate the access of
  channel
• Delay-bandwidth is important
• Need the coordination from all MAC entities, any
  one does not cooperate, the communication will
  not take place.

3
What are going to be discussed

• Introduction to broadcast networks
• Overview of LANs frame format placement in
  OSI.
• Random access ALOHA CSMA-CD (Carrier Sensing
  Multiple Access with Collision Detection ) i.e.,
  Ethernet.
• Scheduling token-ring.
• LAN standards (brief view)
• LAN bridges used to connect several LANs.

4
Multiple access communications
3
2
4
1
Shared Multiple Access Medium
5
M
?

• Any transmission from any station can be heard by
  any other stations
• If two or more stations transmit at the same
  time, collision occurs

Figure 6.1
5
Approaches to sharing transmission medium
Minimize the incidence of collision to achieve
reasonable usage of medium. Good for bursty
traffic.
Medium Sharing Techniques
Static Channelization
Dynamic Medium Access Control
Partitioned channels are dedicated to individual
users, so no collision at all. Good for steady
traffic and achieve efficient usage of channels
Scheduling
Random Access
Try and error. if no collision, that is good,
otherwise wait a random time, try again. Good
for light traffic.
Schedule a orderly access of medium. Good for
heavier traffic.
Figure 6.2
6
Satellite communication involves sharing of
uplink and downlink frequency bands
fin

• Satellite Channel

fout
Figure 6.3
7
Cellular networks radio shared by mobile users
and require MAC
BSS
BSS
MSC
SS7
STP
HLR
wireline terminal
VLR
EIR
PSTN
AC
MSC mobile switching center PSTN public
switched telephone network STP signal transfer
point VLR visitor location register
AC authentication center BSS base station
subsystem EIR equipment identity register HLR
home location register
Figure 4.52
8
Multi-drop telephone line requires access control
Multidrop telephone lines
Inbound line
Host
Outbound line
Terminals
Figure 6.4
9
Ring networks and multi-tapped buses require MAC
Ring networks
Multitapped Bus
Figure 6.5
10
Wireless LAN share wireless medium and require
MAC
Figure 6.6
11
MAC and performance

• Shared medium is the only means for stations to
  communicate
• Some kind of MAC technique is needed
• Like ARQs, which use ACK frame to coordinate the
  transmission and consume certain bandwidth, the
  MAC will need to transfer some coordination
  information which will consume certain bandwidth
  of shared medium.
• Delay-bandwidth product plays a key role in the
  performance of MAC (as in ARQs).

12
Delay-bandwidth product and performance
(suppose two station A and B want to transmit
information)
Distance d meters tprop d / ? seconds
A transmits at t 0
A
B
B transmits before t tprop and
detectscollision shortly thereafter
A
B
A detects collision at t 2 tprop
A
B
1. ? the speed of light, 3108 meters/second
2. Before A begins to transmit, A listens to
medium, if busy, wait otherwise, do it (suppose
t0) 3. If B wants to transmit after ttprop
As transmission has reached B, so B waits and A
captures medium successfully and transmits its
entire message.
4. If B wants to transmit before ttprop, it
listens and no transmission is going on, so B
begins to transmit, then collision occurs. B
detects collision shortly, but A detects
collision at t2tprop 5. Therefore, 2tprop is
required to coordinate the access for each packet
transmitted.
Figure 6.7
13
MAC efficiency

• Suppose bit rate of medium is R, then number
• of bits wasted in access coordination is
  2tpropR.

And suppose average length of packets is L. Then
efficiency in use of the medium is
1
L
1
Efficiency


tpropR
L 2tpropR
12a
1 2
L
atpropR / L i.e., the ratio of (one-way)
delay-bandwidth product
to the average packet length. Suppose a
0.01, then efficiency 1/1.02 0.98 a
0.1, then efficiency 1 / 2 0.50
14
Examples of efficiency

• Ethernet (CSMA-CD)
• Efficiency 1/(16.44a) where a tpropR/L.
• Token-ring networks
• Efficiency 1/(1a ) where a ring-latency in
  bits/L where ring-latency contains
• The sum of bit delays introduced at each ring
  adapter.
• Delay-bandwidth product where delay is the time
  required for a bit to circulate around the ring.

15
Typical LAN structure and network interface card
(a)
A LAN connects servers, workstations, Printers,
etc., together to achieve sharing

• NIC is parallel with memory
• but serial with network
• 2. ROM stores the implementation of MAC
• 3. Unique physical address burn into ROM
• 4. A hardware in NIC recognizes physical,
• broadcast multicast addresses.
• 5. NIC can be Set to promiscuous mode
• to catch all transmissions.

(b)
Ethernet Processor
ROM
Figure 6.10
16
IEEE 802 LAN standards
Network Layer
Network Layer
LLC
802.2 Logical Link Control
Data Link Layer
802.11 Wireless LAN
Other LANs
802.3 CSMA-CD
802.5 Token Ring
MAC
Physical Layer
Various Physical Layers
Physical Layer
OSI
IEEE 802
One LLC and several MACs, each MAC has an
associated set of physical layers. MAC provides
connectionless transfer. Generally no error
control because of relatively error free. MAC
protocol is to direct when they should transmit
frames into shared medium.
Figure 6.11
17
The MAC sublayer provides unreliable datagram
service
Unreliable Datagram Service
Important all three MAC entities must cooperate
to provide datagram service, I.e., the
interaction between MAC entities is not between
pairs of peers, but rather all entities must
monitor all frames.
Figure 6.12
18
LLC provides three HDLC services 1.
Unacknowledged connectionless service, recall
HDLC has unnumbered frames 2. Reliable
connection-oriented service in the form of HDLC
ABM mode 3. Acknowledged connectionless service,
need to add two unnumbered frames to HDLC frame
set.
C
A
A
C
Reliable Packet Service
LLC
LLC
LLC
LLC can provide reliable packet transfer service
Figure 6.13
19

• LLC provides additional addressing, i.e., SAP
  (Service Access Point). Like PPP, LLC can
• support several different network connections
  with different protocols at the same time.
• Typical SAPs IP 06, IPX E0, OSI packets FE
  etc.
• In practice, LLC SAP specifies in which buffer
  the NIC places the frame, thus allowing the
• appropriate network protocol to retrieve the
  data.

1 byte
1
1 or 2
Destination SAP Address
Source SAP Address
Information
Control
Source SAP Address
Destination SAP Address
C/R
I/G
7 bits
1
7 bits
1
I/G Individual or group address
C/R Command or response frame
LLC PDU structure and its support for several SAPs
Figure 6.14
20
Header overhead TCP IP gt20
LLC 3 or 4
MAC 26
IP
IP Packet
LLC PDU
LLC Header
Data
MAC Header
FCS
MAC frame
LLC PDU and MAC frame
Figure 6.15
21
IEEE 802.3 MAC frame
802.3 MAC Frame
6
1
6
2
4
7
Destination Address
Source Address
Information
FCS
Pad
Preamble
Length
SD
Synch
Start frame
64 to 1518 bytes

• Destination address is either single address
• or group address (broadcast 111...111)
• Addresses are defined on local or universal
  basis
• 246 possible global addresses

0
Single address
Group address
1
0
Local address
1
Global address
Unicast address a single host address, multicast
address a group of hosts, Broadcast address
all hosts.
Figure 6.52
22
Random Access

• Why random access?
• Reaction time (i.e., 2 times of propagation
  delay) is very important for performance, e.g.,
  in Stop-and-Wait, when reaction time is small
  (i.e., the ACK will arrive soon) the performance
  is very good, however, if reaction time is large,
  then performance is very bad.
• Therefore, proceed the transmission without
  waiting for ACK and deal with collision/error
  after the fact, i.e., random access.
• Three types of random accesses
• ALOHA, slotted ALOHA, and CSMA-CD

23
ALOHA

• Basic idea
• let users transmit whenever they have data to be
  sent.
• When collision occurs, wait a random time ( why?
  ) and retransmit again.
• Differences between regular errors collision
• Regular errors only affect a single station
• Collision affects more than one
• The retransmission may collide again
• Even the first bit of a frame overlaps with the
  last bit of a frame almost finished, then two
  frames are totally destroyed.

24
ALOHA random access scheme
Suppose L the average frame length, R rate,
XL/R frame time 1. Transmit a frame at tt0
(and finish transmission of the frame at t0X
) 2. If ACK does not come after t0X2tprop or
hear collision, wait for random time B 3.
Retransmit the frame at t0X2tpropB Two modes
collide only from time to time and snowball
effect collision
First transmission
Retransmission
t
t0
t0X
t0-X
t0X2tprop??
t0X2tprop
Vulnerable period
Backoff period B
Time-out
Retransmission if necessary
When collision occurs?
Vulnerable period t0-X to t0X, (2X seconds) if
any other frames are transmitted during
the period, the collision
will occur.
Therefore the probability of a successful
transmission is the probability that there is no
additional transmissions in the vulnerable
period.
Figure 6.16
25
The performance of ALOHA

• Let S be the arrival rate of new frames in units
  of frames/X seconds,
• S is also the throughput of the system.
• Let G be the total arrival rate in units of
  frames/X seconds, G
• contains the new and retransmissions and is
  the total load.
• Assume that aggregate arrival process resulting
  from new and
• retransmitted frames has a Poisson distribution
  with an average
• number of arrivals of 2G frames/2X seconds,
  i.e.,

(2G)k
Pk transmissions in 2X seconds
e-2G
, k0,1,2,
k!
Therefore, the throughput of the system is
SGPno collision GP0 transmission in 2X
seconds
(2G)0
e-2G G e-2G
G
0!
26
What results can be obtained from the graph?
1.peak value at G0.5 with S0.184
2.for any given S, there are two values of G,
corresponding to the two modes occasional
collision mode with S ? G and frequent
collision mode with G gtgt S
0.368
Ge-G
S
0.184
Ge-2G
G
Throughput S versus load G for ALOHA and slotted
ALOHA
Figure 6.17
27
Slotted ALOHA

• Synchronize the transmissions of stations
• All stations keep track of transmission time
  slots and are allowed to initiate transmissions
  only at the beginning of a time slot.

• Suppose a packet occupies one time slot
• Vulnerable period is from t0-X to t0, i.e., X
  seconds long.

Therefore, the throughput of the system is
SGPno collision GP0 transmission in X
seconds
(G)0
G
e-G G e-G
0!
28
Slotted ALOHA random access scheme
t0(k1)X
First transmission
Retransmission
t
(k1)X
nX
kX
t0X2tprop??
t0X2tprop
Backoff period B
Time-out
Retransmission if necessary
Vulnerable period t0-X to t0 , i.e., X seconds
long
Figure 6.16
29
Peak value at G1 with S0.368 for slotted
ALOHA, double compared with ALOHA. In LAN,
propagation delay may be negligible and
uncoordinated access of shared medium is
possible but at the expense of significant
wastage due to collisions and at very low
throughput. Throughput of ALOHAs is not
sensitive to the reaction time because stations
act independently.
0.368
Ge-G
S
0.184
Ge-2G
G
Throughput S versus load G for ALOHA and slotted
ALOHA
Figure 6.17
30
CSMA (Carrier sensing multiple access)

• Problem with ALOHAs low throughput because the
  collision wastes transmission bandwidth.
• Solution avoid transmission that are certain to
  cause collision, that is CSMA. Any station
  listens to the medium, if there is some
  transmission going on the medium, it will
  postpone its transmission.

31
Suppose tprop is propagation delay from one
extreme end to the other extreme end of the
medium.
When transmission is going on, a station can
listen to the medium and detect it.
After tprop, As transmission will arrive the
other end every station will hear it and
refrain from the transmission, so A captures the
medium and can finish its transmission.
But in ALOHAs, it is X or 2X
Vulnerable period tprop
In LAN,generally, tprop lt X
sense
sense
sense
sense
CSMA random access scheme
Figure 6.19
32
Three different CSMA schemes

• Based on how to do when medium is busy
• 1-persistent CSMA
• Non-persistent CSMA
• p-persistent CSMA

33
1-persistent CSMA
sense channel when want to transmit a packet, if
channel is busy, then sense continuously, until
the channel is idle, at this time, transmit the
frame immediately.
If more than one station are sensing, then they
will begin transmission the same time when
channel becomes idle, so collision. At this time,
each station executes a backoff algorithm to
wait for a random time, and then re-sense the
channel again.
Problem with 1-persistent CSMA is high collision
rate.
34
Non-persistent CSMA
sense channel when want to transmit a packet, if
channel is idle, then transmit the packet
immediately. If busy, run backoff algorithm
immediately to wait a random time and then
re-sense the channel again.
Problem with non-persistent CSMA is that when
the channel becomes idle from busy, there may be
no one of waiting stations beginning the
transmission, thus waste channel bandwidth,
35
p-persistent CSMA
sense channel when want to transmit a packet, if
channel is busy, then persist sensing the
channel until the channel becomes idle. If the
channel is idle, transmit the packet with
probability of p, and wait, with probability of
1-p, additional propagation delay tprop and
then re-sense again
36
Throughput versus load G for 1-persistent
(three different atprop/X )
S
0.53
1-Persistent CSMA
0.01
0.45
0.16
0.1
G
1
Figure 6.21 - Part 2
37
Throughput versus load G for non-persistent
(three different atprop/X )
S
0.81
Non-Persistent CSMA
0.01
0.51
0.14
0.1
G
1
1-persistent is sharper than non-persistent. atpr
op/X has import impact on the throughput. When a
approaches 1, both 1-persistent and
non-persistent is worse than ALOHAs.
Figure 6.21 - Part 1
38
CSMA-CD

• When the transmitting station detects a
  collision, it stops its transmission immediately,
  Not transmit the entire frame which is already
  in collision.
• The time for transmitting station to detect a
  collision is 2tprop.
• In detail when a station wants to transmit a
  packet, it senses
• channel, if it is busy, use one of above three
  algorithms (i.e., 1-persistent, non-persistent,
  and p-persistent schemes). The transmitter senses
  the channel during transmission. If a collision
  occurred and was sensed, transmitter stops its
  left transmission of the current frame moreover,
  a short jamming signal is transmitted to ensure
  other stations that a collision has occurred and
  backoff algorithm is used to schedule a future
  re-sensing time.
• The implication frame time X gt 2tprop, , since
  XL/R, which means that there is a minimum
  limitation for frame length.

39
The reaction time in CSMA-CD is 2tprop
It takes 2 tprop to find out if channel has been
captured
Figure 6.22
40

• When a is small, i.e, tprop ltlt X, the CSMA-CD is
  best and all CSMAs are
• better than ALOHAs.
• When a is approaching 1, CSMAs become worse than
  ALOHA.
• ALOHAs are not sensitive to a because they do not
  depend on reaction time.

CSMA/CD
1-P CSMA
Non-P CSMA
?max
Slotted Aloha
Aloha
a
tprop /X
Maximum achievable throughput of random access
schemes
Figure 6.24
41
Summary of random access schemes
(Continuous) ALOHA try to send a frame anytime,
if collision, wait random time, resend.
Vulnerable period 2X , maximum
throughput 0.184.
Slotted ALOHA send a frame at the beginning of a
time slot. If collision, wait a random
time to a new time slot, and resend
again.Vulnerable period X , maximum throughput
0.368.
1-persistent CSMA listen before transmission, if
busy, continuously listen until channel
become idle, then transmit immediately. If
collision, wait a random time, re-listen.
Vulnerable period tprop , throughput 0.53 for
atprop/X0.01
Non-persistent CSMA listen before transmission,
if busy, wait a random time, re-listen.
if idle, transmit. If collision, wait a random
time, re-listen. Vulnerable period
tprop , throughput 0.81 for atprop/X0.01
p-persistent CSMA persist listening to the
channel until idle. At this time, with
probability p, transmit the
packet, and with probability of 1-p, do not
transmit but wait additional
tprop and then re-listen. If collision, wait
random time, re-listen. Vulnerable period
tprop throughput dependent on p.
CSMA-CD using any one of above three. Listen
during transmission. if collision. If collision,
stop its transmission immediately. Vulnerable
period tprop. throughput gt0.90 for
atprop/X0.01
Important atprop/X affects performance.
Requirement frame length can not below
certain value for given tprop, (the distance of
LAN).