Lecture set 4

Communication Networks
Sanjay K. Bose
Lecture Set IV
LANs and Media Access Control (MAC)

Multiple Access Communications
• Shared media basis for broadcast networks
– Inexpensive: radio over air; copper or coaxial cable
– M users communicate by broadcasting into medium
• Key issue: How to share the medium?

1
2
3
4
5M
Shared multiple
access medium

Static
channelization
Dynamic medium
access control
Scheduling Random Access
Approaches to Media Sharing
 Partition medium
 Dedicated
allocation to users
Used in Satellite
Transmissions and
Cellular Telephony
 Polling: take turns
 Request for slot in
transmission
schedule
Used in Token Ring
Wireless LANs
 Loose coordination
 Send, wait, retry if
necessary
Used in Aloha and
Ethernet

Random Access Protocols
 When node has packet to send
 transmit at full channel data rate R.
 no a priori coordination among nodes
 two or more transmitting nodes ➜ “collision”,
 random access MAC protocol specifies:
 how to detect collisions
 how to recover from collisions (e.g., via delayed
retransmissions)
 Examples of random access MAC protocols:
 slotted ALOHA
 ALOHA
 CSMA, CSMA/CD, CSMA/CA

Pure ALOHA
 When frame first arrives, transmit immediately and wait for ACK
 If ACK is received, frame transmission was successful
 If ACK is not received, wait for random time and try transmission again;
repeat this until frame is successfully transmitted
In example shown, all
frames are of unit
length.
Node i’s frame is
successful if no other
transmission starts
between (t0-1, t0) or
between (t0, t0+1)
ACK and frames can share the same channel or the frames can be sent
on a “forward” channel while the ACKs are sent on a “reverse” channel

Poisson Process of rate : Probability of k arrivals in any interval of
length T is given by

 
!
k
TT
e
k
 
Performance Analysis of Pure ALOHA
(Infinite User Population Model)
Assume that –
(a) Packets of same length
(b) Time measured in units of packet length (i.e. packet length=1)
(c) ACKs are always successful
(d) New packets come at rate of from a Poisson process
(e) Unsuccessful packets retransmitted after random rescheduling delay
(f) New and Retransmitted packets come at rate from a Poisson process
O
R
Channel+
O R O R SP 
(1 )R SP 
Pure ALOHA channel in Steady State

(Infinite User Population Model)
Channel+
O R O R SP 
(1 )R SP 
2 R
SP e 

2 R
O Re 
  

Throughput ThroughputLoad
Capacity = Maximum Throughput = 1/(2e)≈18% at = 1/2R
The capacity offered by Pure ALOHA is quite low, i.e. only 18%. As
a matter of fact, stability considerations will actually force the
system to be operated at substantially below this level.

(Finite User Population Model)
t0 t0 +
1
t0 - 1
1
n
 
For n , P{no arrival in slot Δ}=
PS =
 1 R
n
 
 
 
Arrival of interest
2
2
lim 1 R
n
R
n
e
n
 

 
  
 
Therefore,
as obtained earlier
2 R
O Re 
  

Throughput Load
S = packets/unit time G = packets/unit timeO R

Throughput of ALOHA
G
success GeGPS 2

0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
00.00781250.015625
0.03125
0.0625
0.125
0.25
0.5
1
2
4
G
S
 Collisions are means
for coordinating
access
 Max throughput is
Smax= 1/2e (18.4%)
 Bimodal behavior:
Small G, S≈G
Large G, S↓0
 Collisions can snowball
and drop throughput
to zero
e-2 = 0.184

Slotted ALOHA
 Time is slotted in slots equal to the packet length (say unity)
 Stations synchronized to frame times
 Stations transmit frames in first slot after frame arrival
 Backoff intervals in multiples of slots
t
(k+1)k
t0 +1+2tprop+ B
Vulnerable
period
Time-out
Backoff period B
t0 +1+2tprop
Only frames that arrive during the previous slot can collide
t0

Throughput of Slotted ALOHA
Gnn
success
Ge
n
G
GpG
GP
GPGPS




)1()1(
intervals]ninarrivalsno[
seconds]Xinarrivalsno[
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.01…
0.03…
0.0625
0.125
0.25
0.5
1
2
4
8
Ge-G
Ge-2G
G
S
0.184
0.368
Capacity of Slotted ALOHA
is 1/e ≈ 36.8% at G=1

Discrete-Time Markov Chain Analysis for
Slotted ALOHA
• Time axis divided into slots with system examined just after each
slot boundary
• Let m be the total number of users
• Let backlogged users be the users who tried unsuccessfully to
transmit in an earlier slot. Each such user will attempt to transmit
with probability qr in every subsequent slot after its unsuccessful
attempt and will continue doing so until it finally succeeds – once
that happens it will no longer be considered a backlogged users
• Users who are not backlogged can generate new packets for
transmission in a given slot with probability qa
• Let n be the number of backlogged users at the beginning of a
given slot (each of them can try to transmit with probability qr ).
• Each of the remaining m-n unbacklogged users can choose to
transmit a new packet in a given slot with probability qa . Note that
if any such attempt is unsuccessful then the corresponding user
joins the backlogged user set.

Slotted ALOHA
p14
Qa (i, n) = P{i unbacklogged users transmit in given slot}
Qr (i, n) = P{i backlogged users transmit in given slot}
i
a
inm
a qq
i
nm 





 
 )1(
i
r
in
r qq
i
n 






 )1(
The state transition probabilities, pij ‘s, of the Markov Chain can then be
obtained using Qa (i, n) and Qr (i, n) (see next slide)
m1 2
p22
p00 p11
p01
p10
p02
p12
p03
p13
p21
p23
p32
p24
p25
0

Slotted ALOHA
1),1(),0(
0)],1(1)[,0(),0(),1(
1)],0(1)[,1(
2),(,




inQnQ
inQnQnQnQ
inQnQ
nminiQp
ra
rara
ra
ainn
The state transition probabilities of this Markov Chain will then be -
It can be shown that the steady state probabilities Pn of the Markov
Chain (i.e. the probability of the system being in state n) will satisfy -
 



m
n
n
n
i
inin PpPP
0
1
0
1&
Given the transmission probabilities, qa and qr and the user
population m, the system can then be solved to obtain the state
probabilities, Pi for i=0,1,…..,m

Slotted ALOHA
Define the “drift in state n”, Dn , of the system as the average
change in the number of backlogged users over one slot time, starting
in state n.
succan PqnmD  )(
Psucc = P{successful transmission in one slot}
= Average number of successful transmissions in one slot
),1(),0(),0(),1( nQnQnQnQP rarasucc 
Note that, when the system is in state n, the expected number G(n)
of transmission attempts made in a slot will be -
ra nqqnmnG  )()(
For qa <<1, qr <<1, we can show that Psucc ≈G(n)e-G(n) , as obtained before

Carrier Sensing Multiple Access (CSMA)
A
Station A begins
transmission at
t = 0
A
Station A captures
channel at t = tprop
 A station senses the channel before it starts transmission
 If busy, either wait or schedule backoff (different options)
 If idle, start transmission
 Vulnerable period is reduced to tprop (due to channel capture effect)
 When collisions occur they involve entire frame transmission times
 If tprop >packet length, no gain compared to ALOHA or slotted ALOHA

CSMA collisions
Note that collisions can
still occur:
propagation delay means two
nodes may not hear each other’s
transmission
In case of collision:
entire packet transmission
time wasted
spatial layout of nodes

 1-persistent CSMA (most greedy)
• Start transmission as soon as the channel becomes idle
• Low delay and low efficiency
 Non-persistent CSMA (least greedy)
• Wait a backoff period, then sense carrier again
• High delay and high efficiency
 p-persistent CSMA (adjustable greedy)
• Wait till channel becomes idle, transmit with prob. p; or wait one
mini-slot time & re-sense with probability 1-p
• Delay and efficiency can be balanced
CSMA Variations
(Based on transmitter behavior when busy channel is sensed)
p-persistent sensing

Analysis of Non-Persistent CSMA
S = Throughput = Carried Traffic
G = Load = Offered Traffic
a = Propagation Delay
• Assume time measured in units
of packet length (i.e. average
transmission time of packet =1)
• Arrival process for G assumed to
follow a Poisson model
Busy BusyIdle IdleIdle
Transmission
starting the
Busy period
Transmission
ending for the
Busy period
End of the Busy
period
a
Gt
k
e
k
Gt
tkP 

!
)(
),(
P{k arrivals from
offered traffic in
time interval t }

B = Length of Busy Period (random variable with mean )
I = Length of Idle Period (random variable with mean )
B
I




0
1
)(
G
GdtetI Gt
Mean length of an Idle Period
Note that the time interval is divided into Cycles, where each
cycle is an Idle Period followed by a corresponding Busy Period.
Let U denote the time during a cycle that the channel is used
without conflicts (i.e. no collisions).
Then as a busy period has a successful transmission
with no collisions only if there are no arrivals in the initial time
interval of length a of a busy period
aG
eU 


Throughput, S=
IB
U

Since we already know and , we need
to get to get the throughputB
IU
Successful
Transmission
Period
Idle Idle
a
Unsuccessful
Transmission
Period
a
1
a
Y
Let t = start of the busy period
t +Y = time of last arrival in interval (t, t +a)
Then
1
aYB 1 We therefore need to get the
throughput
Y

)1(
1
)(
}{)(
)(
0
)(
)(
aGyaG
a
yaG
Y
yaG
Y
e
G
adyGeyY
ayGeyf
ayeyYPyF







Therefore aG
aG
eaG
Ge
IB
U
S 





)21(
will be the throughput of a non-persistent CSMA system for
given load G and given values of the average packet length
and the propagation delay (assumed to be fixed)

Throughput of 1-Persistent CSMA
Similar derivation for 1-persistent CSMA gives the following
result -
)1(
)21(
)1()1()21(
2
11
aGaG
aG
eaGeaG
e
aG
GaGGG
S 
















Similar approach may also be followed to analyze a p-persistent
CSMA but the approach and the resultant expression are much
more complicated

0
0.1
0.2
0.3
0.4
0.5
0.6
0.015625
0.03125
0.0625
0.125
0.25
0.5
1
2
4
8
16
32
64
0.53
0.45
0.16
S
G
a  0.01
a =0.1
a = 1
Throughput of 1-Persistent CSMA
 Better than Aloha &
slotted Aloha for
small a
 Worse than Aloha for
a > 1
propt
a
packet duration


0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.01…
0.04…
0.125
0.35…
1
2.82…
8
22.6…
64
0.81
0.51
0.14
S
G
a = 0.01
Throughput of Non-Persistent CSMA
a = 0.1
a = 1
 Higher maximum
throughput than 1-
persistent for small a
 Worse than Aloha for
a > 1
propt
a
packet duration


CSMA with Collision Detection (CSMA/CD)
 Monitor for collisions & abort transmission
 Stations with frames to send, first do carrier sensing
 After beginning transmissions, stations continue listening to the
medium to detect collisions
 If collisions detected, all stations involved stop transmission,
reschedule random backoff times, and try again at scheduled
times
 In CSMA collisions result in wastage of the entire time
spent transmitting a complete frame
 CSMA-CD reduces this wastage to the time taken to detect
a collision and abort the corresponding transmission

Binary Exponential Back off
(used in IEEE 802.3 and Ethernet)
• On detecting a collision, the transmitter aborts and sends
a 48-bit jamming signal. It then enters the exponential
back off phase where it waits for a random delay before
attempting to retransmit.
• The random delay is chosen as K*512 bit times with K
random
After the nth collision in a row for the same frame
(1n16), choose K randomly from the set {0, 1, .......,
2m-1}, where m=min(n, 10)

Binary Exponential Back off
(used in IEEE 802.3 and Ethernet)
In effect, this means the following
• For the first ten (10) retransmission attempts, the mean
value of the random delay is doubled
• Thereafter, the mean value of the random delay remains
the same for six (6) additional attempts
What happens after that, i.e. after 16 retransmission
attempts?
• After sixteen (16) retransmission attempts the station
gives up the attempt to transmit the packet and reports an
error

Collision Detection
 Collision is detected by changes in the amplitude and pulse width
of the signal (from expected values)
 Potential problem as signal will attenuate on the line as it travels.
If the colliding stations are too far apart, then the collision may not
even be detectable
 IEEE 802.3 standards avoid this by restricting the maximum
length of the coaxial cable and the minimum frame size; minimum
frame must be larger than twice the total propagation delay
(including repeater delays) between the two farthest nodes of
the network (see subsequent slides)
 With a star-topology using a Hub, collision detection may be
simplified by using logic at the hub. Activity at more than one input
at the hub is declared to be a collision and a special collision
presence signal is generated and sent out to the stations connected
to the hubs

CSMA/CD Slot Time
Tp = propagation delay between S1 and S2
including the processing time at
repeaters, switches etc. in between
S1 and S2
= tp+tpr
S1 S2
Tp
Tp

CSMA/CD Slot Time
 Slot time is the time it will take for S1 to find out about the
collision between its packet and that of S2
Note that this depends only on the propagation (and
processing delay) between S1 and S2
 For the network, this will be the worst case time delay that
any node on the network must wait before it can find out that
its transmission has collided with that of another node.
S1 S2
Tp
Tp
CSMA/CD
Slot Time
= 2Tp

CSMA/CD Slot Time
 In CSMA/CD, the amount of wasted capacity is reduced
to the time it takes to detect a collision
 This makes the slot time, mentioned earlier, important
from the viewpoint of system performance
 In the worst case, it will be no greater than the twice
the end-to-end propagation delay (including the applicable
processing delays) in the network
 An important rule in IEEE 802.3 also is that “frames
should be long enough to allow collision detection before
the end of transmission” …. otherwise, collisions may go
undetected

CSMA-CD Model
 Assumptions
 Collisions can be detected and resolved in 2tprop
 Time slotted in 2tprop slots during contention periods
 Assume n busy stations, and each may transmit with
probability p in each contention time slot
 Once the contention period is over (a station successfully
occupies the channel), it takes X seconds for a frame to
be transmitted (Packet Duration=X)
 It takes tprop before the next contention period starts.
Busy Contention Busy
Time
Idle Contention Busy

Contention Resolution
 How long does it take to resolve contention?
 Contention is resolved (“success’) if exactly 1 station transmits in
a slot:
1
)1( 
 n
success pnpP
 Psuccess maximized for p=1/n
ennn
nP nn
success
1
)
1
1()
1
1(
1 11max
 
 On average, 1/Pmax = e = 2.718 time slots to resolve contention
secondsPeriodContentionAverage 2 etprop
n stations, each
transmitting with
probability p

Non-Perisitent CSMA/CD Throughput
 At maximum throughput, systems alternates between contention
periods and frame transmission times
    LRdeaeettX
X
propprop /121
1
121
1
2
max








Time
Busy Contention Busy Contention Busy Contention Busy
 where:
R bits/sec, L bits/frame, X=L/R seconds/frame
a = tprop/X normalized propagation time
meters/sec. speed of light in medium
d meters is “diameter” of system
2e +1 ≈ 6.44

max
1
1 6.44
Rd
vL
 


Throughput for Random Access MACs
0
0.2
0.4
0.6
0.8
1
0.01 0.1 1
ALOHA
Slotted ALOHA
1-P CSMA
Non-P CSMA
CSMA/CD
a
max
 For small a: CSMA-CD has best throughput
 For larger a: Aloha & slotted Aloha better throughput

ETHERNET (CSMA-CD Application)
 First Ethernet LAN standard used CSMA-CD
 1-persistent Carrier Sensing
 R = 10 Mbps
 tprop = 51.2 microseconds
• 512 bits = 64 byte slot
• accommodates 2.5 km + 4 repeaters
 Truncated Binary Exponential Backoff
• After nth collision, select backoff from {0, 1,…, 2k – 1},
where k=min(n, 10)

Ethernet Topology
 bus topology popular through mid 90s
 all nodes in same collision domain (can collide with each
other)
 today: star topology prevails
 active switch in center
 each “spoke” runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
Bus
(e.g. on coaxial cable)
Star
(e.g. using point-to-point
twisted wire pairs)

LAN Technologies
LAN Technologies
 Ethernet
 hubs, bridges, switches

MAC Addresses on LANs
LAN (or MAC or physical or Ethernet) address:
 used to get datagram from one interface to another physically-
connected interface (same network)
 48 bit MAC address (for most LANs)
burned in the adapter ROM
This is different from the 32-bit IP address which is a
 network-layer address
 used to get datagram to destination IP network (recall IP
network definition)
In effect, the IP Address ensures that the packet is delivered up to the
network (LAN) which has the destination station. The MAC address is
then used to deliver the packet to the destination station

MAC Addresses on LANs
Each adapter on LAN has unique LAN address

MAC Address Features
 MAC address allocation administered by IEEE
 Manufacturer buys portion of MAC address space
(to assure uniqueness)
 Analogy:
(a) MAC address: like IC Numbers
(b) IP address: like postal address
 MAC flat address => portability
 can move LAN card from one LAN to another
 IP hierarchical address NOT portable
 depends on IP network to which node is attached

ARP: Address Resolution Protocol
Question: How do you determine MAC address of B if you know only B’s IP
address?
Answer: You either know it (it is in your cache) or you ask B ....exactly
what ARP does
Broadcast the query on
the LAN, B hears it and
replies to A telling A the
MAC address of B
Needed so that incoming frame can be delivered to B

How does one send packets to a node in
another network?
Datagram from A to B via R, assuming A only knows B’s IP address
A
R
B
Router R would know B’s MAC Address (or can use ARP to find this)
and will use this to deliver datagram to B

Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble:
 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011
 used to synchronize receiver, sender clock rates

Ethernet Frame Structure (more)
 Addresses: 6 bytes
 if adapter receives frame with matching destination
address, or with broadcast address (eg ARP packet), it
passes data in frame to net-layer protocol
 otherwise, adapter discards frame
 Type: indicates the higher layer protocol, mostly
IP but others may be supported such as Novell
IPX and AppleTalk)
 CRC: checked at receiver, if error is detected, the
frame is simply dropped

Ethernet: Unreliable, Connectionless operation
 Connectionless: No handshaking between sending
and receiving adapter
 Unreliable: receiving adapter doesn’t send ACKs or
NACKs to sending adapter

Ethernet uses CSMA/CD
 adapter doesn’t transmit
if it senses that some
other adapter is
transmitting, that is,
carrier sense
 transmitting adapter
aborts when it senses
that another adapter is
transmitting, that is,
collision detection
 Before attempting a
retransmission,
adapter waits a
random time following
Binary Exponential
Backoff (mentioned
earlier)

Ethernet CSMA/CD algorithm
1. Adaptor gets datagram from
and creates frame
2. If adapter senses channel idle,
it starts to transmit frame. If
it senses channel busy, waits
until channel idle and then
transmits
3. If adapter transmits entire
frame without detecting
another transmission, the
adapter is done with frame !
4. If adapter detects another
transmission while transmitting,
aborts and sends jam signal
Jam Signal: To make sure all
other transmitters are aware
of collision; length=48 bits
5. After aborting, adapter enters
exponential backoff: after the
mth collision, adapter chooses a
K at random from {0,1,2,…,2m-1}.
Adapter waits K*512 bit times
and returns to Step 2

Ethernet Technologies: 10 Mbps
Bus topology with multiple segments connected by
repeaters.
Check out the web for the 5-4-3 rule for setting up a “thin”
Ethernet network!

Manchester Encoding
 Used in 10BaseT, 10Base2
 Each bit has a transition
 Allows clocks in sending and receiving nodes to synchronize
to each other
 no need for a centralized, global clock among nodes!

Table 16.2, “Data and Computer Communications” William Stalling

Fast Ethernet
 100 Mbps rate is called “fast ethernet”
 Nodes connect to a hub: “star topology”; 100 m max distance
between nodes and hub
 Hubs are essentially physical-layer repeaters:
 bits coming in one link go out all other links
 no frame buffering
 no CSMA/CD at hub: adapters detect collisions
 provides some net management functionality
hub
nodes Hubs may be interconnected through
other hierarchically higher hubs

Table 16.3, “Data and Computer Communications” William Stalling

Evolution of the Ethernet Standards
10 Mbps
10Base5
10Base2
10Base-T
10Base-F
1 Gbps
802.3z 1000Base-X
802.3ab 1000Base-T
100Mbps
100Base-TX
100Base-FX
100Base-T4
Fast Ethernet
10 Gbps
802.3ae
Gigabit Ethernet

The Gigabit Ethernet Approach
 As in the case of Fast Ethernet, Gigabit Ethernet retains
the CSMA/CD Protocol and Ethernet Frame Format of
10/100 Mbps Ethernet while defining new medium and
transmission specifications.
 Fully compatible formats with 10/100 Mbps Ethernets
allow easy internetworking and smooth migration
 Initial deployment foreseen as backbones in switching
hubs but gradually expected to move towards end-systems as
bandwidth requirements of end applications grow

A Gigabit Ethernet Scenario
Gigabit Ethernet Links

Scaling up the speed of CSMA/CD
What is involved if one wants to scale
up the speed of a network using
Ethernet’s CSMA/CD as the MAC
protocol?

 Ethernet has a minimum frame size of 64 bytes
 The reason for enforcing a minimum frame size is to
ensure that collision detection can be done even between
nodes that are the farthest from each other.
This must be such that it is longer than twice the
maximum propagation delay between the two most distant
nodes in the network (adding in all the hub and repeater
delays in between). This time is also referred to as the
slot time of the Ethernet system
Since the minimum frame size is fixed, this effectively limits the
maximum propagation (and repeater/hub) delay that may be allowed
in an Ethernet network.........and hence limits the cable lengths

In the original 10 Mbps Ethernet standards, this was
ensured by having -
 Minimum Frame Size = 64 bytes
 Maximum Cable Length of 2.5 Km with a maximum of
four repeaters on any path
 Minimum frame size duration is comfortably larger than
the slot time for the above!
(Incidentally, it may also be noted that Ethernet also prescribes a
maximum frame size as one with 1500 bytes of data.)

Going from 10 Mbps to 100 Mbps
 Minimum Frame Size still 64 bytes for compatibility
 Maximum Cable Length has to be reduced
Maximum cable length from 100 Mbps hub to a
node is restricted to 100 m
Note that the designers of 100 Mbps Fast Ethernet could
have made the minimum frame size ten times longer but they
chose not do that in order to keep the system completely
compatible with 10 Mbps Ethernet!

Going from 100 Mbps to 1 Gbps
 Not practical to just reduce the distance to 10 m and
keep frame size the same .............. and imagine then what
will happen for a speed of 10 Gbps!
 In the interests of compatibility, one would not want to
change the frame format either, i.e. use longer frames
which are incompatible with earlier Ethernet
specifications!
..........so what can be done to solve this problem?

Solution is to use Carrier Extension which keeps the same
frame size (ensuring compatibility) but extend the carrier
event for small frames so that collisions can be detected even
with long cable lengths (see next slide)
Extension uses special extension symbols
that cannot occur in the payload

Maximum Hub to Node Distance for 1 Gbps
Ethernet
LAN
MAN/WAN

 Carrier Extension is needed to keep the maximum cable
lengths reasonably large enough so that practical systems
may be implemented
 Unfortunately, padding a frame with extension symbols
means lower operating efficiency, especially when a
number of short frames have to be sent
Packet Bursting is proposed as an extension of Carrier
Extension to mitigate this problem to some extent. It may
be viewed as effectively being “carrier extension plus a
burst of packets ”

Extension Symbols
Extension symbols on subsequent packets added
as required to fill up the inter-packet gaps.
Packet Bursting: When a station has a number of packets to
send, the first packet is padded to the slot time if necessary
using carrier extension. Subsequent packets are transmitted
back to back with until a Burst Timer (of 1500 bytes) expires

10-Gbps Ethernet (IEEE 802.3ae)
WAN/MAN

10-Gbps Ethernet (IEEE 802.3ae)
 Full Duplex only technology
 Fiber-only technology
 Does not need the CSMA/CD protocol of slower speed
Ethernet at all
 No distance limitation since there is no collision
 Retains the frame format of original Ethernet for
complete inter-operability

10-Gbps Ethernet Usage
 High-speed Metropolitan Area Network (MAN)
interconnecting high-speed LANs for faster access
 High-speed LANs as backbones for campus networks
 Storage area networks interconnecting high-capacity
storage devices, servers and access points
 Applications in Wide Area Networks (WAN) for use by
ISPs and service providers or as the network connecting
geographically distributed, high-speed LANs belonging to
the same organization

Wireless Networks
IEEE 802.11
WiFi
IEEE 802.16
WiMax
Ad-hoc
(Mesh)
Networks
Infrastructure
Networks

B D
CA
Ad Hoc Communications
Temporary association of group of stations
 Within range of each other
 Need to exchange information
 Examples: Presentation in meeting, distributed computer game, war
or disaster scenarios
Nodes not only act as source and destination nodes but also
function as intermediate nodes (routers) for others

A2 B2
B1
A1
AP1
AP2
Distribution System
Server
Gateway to
the InternetPortal
Portal
BSS A
BSS B
Infrastructure Network

A transmits data frame
(a)
Data Frame Data Frame
A
B C
C transmits data frame
& collides with A at B
(b)
C senses medium,
station A is hidden from C
Data Frame
B
CA
Hidden Terminal Problem in WiFi
WiFi Networks need new MAC: CSMA-CA (CSMA with Collision Avoidance)
Collision Detection will not work in a wireless network

RTS
A requests to send
B
C
(a)
CTS CTS
A
B
C
B announces A ok to send
(b)
Data Frame
A sends
B
C remains quiet
(c)
CSMA-CA, CSMA with Collision Avoidance

Exposed Terminal Problem in WiFi
(unnecessarily reduces throughput)
A wants to send to B, and C to D
A is outside the radio range of D and C is outside the
radio range of B, but D is within the radio range of B
A
B
C
D
D hears B’s CTS and will not send back a CTS to C. This is even
though A can transmit to B and C can transmit to D simultaneously
without causing any collision Loss of Throughput Capability

IEEE 802.11 (WiFi) Definitions
 Basic Service Set (BSS)
 Group of stations that coordinate their access using a given
instance of MAC
 Located in a Basic Service Area (BSA)
 Stations in BSS can communicate with each other
 Distinct collocated BSS’s can coexist
 Extended Service Set (ESS)
 Multiple BSSs interconnected by Distribution System (DS)
 Each BSS is like a cell and stations in BSS communicate with
an Access Point (AP)
 Portals attached to DS provide access to Internet

A2 B2
B1
A1
AP1
AP2
Distribution System
Server Gateway to
the InternetPortal
Portal
BSS A
BSS B
Infrastructure Network

Distribution Services
 Stations within BSS can communicate directly with each other
 DS provides distribution services:
 Transfer MAC SDUs between APs in ESS
 Transfer MSDUs between portals & BSSs in ESS
 Transfer MSDUs between stations in same BSS
Multicast, broadcast, or stations’s preference
 ESS looks like single BSS to LLC layer

Infrastructure Services
 Select AP and establish association with AP
Can then send/receive frames via AP & DS
 Reassociation service to move from one AP to another AP
 Dissociation service to terminate association
 Authentication service to establish identity of other stations
 Privacy service to keep contents secret

IEEE 802.11 MAC
 MAC sublayer responsibilities
 Channel access
 PDU addressing, formatting, error checking
 Fragmentation & reassembly of MAC SDUs
 MAC security service options
 Authentication & privacy
 MAC management services
 Roaming within ESS
 Power management

MAC Services
 Contention Service: Best effort
 Contention-Free Service: time-bounded transfer
 MAC can alternate between Contention Periods (CPs) & Contention-Free
Periods (CFPs)
Physical
Distribution coordination function
(CSMA-CA)
Point coordination
function
Contention-
free service
Contention
service
MAC
MSDUs MSDUs

Distributed Coordination Function (DCF)
 DCF provides basic access service
 Asynchronous best-effort data transfer
 All stations contend for access to medium
 CSMA-CA
 Ready stations wait for completion of transmission
 All stations must wait Interframe Space (IFS)
DIFS
DIFS
PIFS
SIFS
Contention
window
Next frame
Defer access
Wait for
reattempt time
Time
Busy medium

Priorities through Interframe Spacing
 High-Priority frames wait Short IFS (SIFS)
 Typically to complete exchange in progress
 ACKs, CTS, data frames of segmented MSDU, etc.
 PCF IFS (PIFS) to initiate Contention-Free Periods
 DCF IFS (DIFS) to transmit data & MPDUs
DIFS
DIFS
PIFS
SIFS
Contention
window
Next frame
Defer access
Wait for
reattempt time
Time
Busy medium

Contention & Backoff Behavior
 If channel is still idle after DIFS period, ready station can
transmit an initial MPDU
 If channel becomes busy before DIFS, then station must
schedule backoff time for reattempt
 Backoff period is integer # of idle contention time slots
 Waiting station monitors medium & decrements backoff
timer each time an idle contention slot transpires
 Station can contend when backoff timer expires
 A station that completes a frame transmission is not allowed to
transmit immediately
 Must first perform a backoff procedure

RTS
CTS CTS
Data Frame
A requests to send
B
C
A
A sends
B
B
C
C remains quiet
B announces A ok to send
(a)
(b)
(c)
ACK B
(d)
ACK
B sends ACK

Carrier Sensing in 802.11
 Physical Carrier Sensing
 Analyze all detected frames
 Monitor relative signal strength from other sources
 Virtual Carrier Sensing at MAC sublayer
 Source stations informs other stations of transmission time
(in msec) for an MPDU
 Carried in Duration field of RTS & CTS
 Stations adjust Network Allocation Vector to indicate when
channel will become idle
 Channel busy if either sensing is busy

Data
DIFS
SIFS
Defer Access Wait for
Reattempt Time
ACK
DIFS
NAV
Source
Destination
Other
Stations
Transmission of MPDU without RTS/CTS

Data
SIFS
Defer access
Ack
DIFS
NAV (RTS)
Source
Destination
Other Nodes
RTS
DIFS
SIFS
CTS
SIFS
NAV (CTS)
NAV (Data)
Transmission of MPDU with RTS/CTS

Collisions, Losses & Errors
 Collision Avoidance
 When station senses channel busy, it waits until channel becomes
idle for DIFS period & then begins random backoff time (in units of
idle slots)
 Station transmits frame when backoff timer expires
 If collision occurs, recompute backoff over interval that is twice as
long
 Receiving stations of error-free frames send ACK
 Sending station interprets non-arrival of ACK as loss
 Executes backoff and then retransmits
 Receiving stations use sequence numbers to identify duplicate
frames

Point Coordination Function
 PCF provides connection-oriented, contention-free service
through polling
 Point coordinator (PC) in AP performs PCF
 Polling table up to implementor
 CFP repetition interval
 Determines frequency with which CFP occurs
 Initiated by beacon frame transmitted by PC in AP
 Contains CFP and CP
 During CFP stations may only transmit to respond to a poll from PC
or to send ACK

CF
End
NAV
PIFS
B D1 +
Poll
SIFS
U 1 +
ACK
D2+Ack+
Poll
SIFS SIFS
U 2 +
ACK
SIFS SIFS
Contention-free repetition interval
Contention period
CF_Max_duration
Reset NAV
D1, D2 = frame sent by point coordinator
U1, U2 = frame sent by polled station
TBTT = target beacon transmission time
B = beacon frame
TBTT
PCF Frame Transfer

IEEE 802.11 MAC Layer
 MAC Layer Acknowledgement for the transmitted
fragment (Multicast packets are not acknowledged in this way)
 If the fragment being acknowledged did not suffer from a
collision, then its ACK will also not have to undergo a collision.
This is ensured by the NAV values of the RTS and
CTS frames and by the fact that the DIFS interval is
longer than the SIFS interval
 Each fragment of a multi-fragment packet PDU is
separately acknowledged

IEEE 802.11 Fragmentation & Reassembly
 LAN protocols, like Ethernet, use large packets
(LANPDUs), e.g. Ethernet packets may have upto 1518
bytes of data
 In a Wireless LAN, smaller packet sizes may be
preferred (higher probability of packet error with large
packets, smaller packets incur smaller overheads due to
packet loss)
 Fragmentation may therefore happen earlier in an
IEEE 802.11 LAN than in an IEEE 802.3 LAN

 802.11 MAC receives a MAC Service Data Unit
(MSDU) of length up to 2304 bytes from its higher
layer and can optionally divide each MSDU into
several smaller MAC Protocol Data Units (MPDU).
This is the Fragmentation process. (Each MPDU has
its own header and CRC.)
 Fragments are sent to the destination within the
same RTS/CTS exchange using a Stop-&-Wait
protocol. The destination node does Reassembly of
the fragments to get the original MSDU and pass it
on to its higher layer.

IC0101, LAN Infrastructure 97
MSDU
H CRC H CRC
Fragment 1 Fragment 2
ACK
S
I
F
S
S
I
F
S
Timing Between Fragments of a MSDU

IEEE 802.11: Procedure for a Station to
Join an Existing Cell (BSS)
To join a cell (after power-up, sleep or entering a new cell),
the station needs to get synchronization information from
the AP of that cell. This can be done in one of the two
following ways -
Passive Scanning: Station waits to receive a
Beacon Frame from the AP. This is sent out
periodically with the synchronization information
Active Scanning: Station tries to locate a
reachable AP by sending a Probe Request Frame
and waits for the Probe Response Frame from
the AP for synchronization.
Both methods are valid and any one of them may be chosen. The
choice is decided by considerations like power consumption
and/or performance.

IEEE 802.11: Authentication Process
 Once a station locates an AP and decides to join
its BSS, it has to go through an Authentication
Process to identify itself as a valid user of the
Wireless LAN facility and also verify the identity of
the AP.
 This is done by the station and AP exchanging
information with appropriate passwords.

IEEE 802.11: Association Process
 This is started after the Authentication Process has
been successfully completed
 This involves exchange of information about the
station and the BSS and their respective capabilities
 This allows the overall network (i.e. the set of APs)
to know about the current location of the station and
the AP it is currently associated with.
 A station can actually transmit/receive data frames
only after the Association Process is over

IEEE 802.11: Roaming
This is the process of a station moving from one BSS to
another without losing the connection.
 The transition from one BSS to another is performed
between packet transmissions
 Temporary disconnection during roaming will have a bad
effect on overall performance as it would require
retransmissions to be done by the higher layers

IEEE 802.11: Roaming
The 802.11 protocol does not define how
roaming is to be done but does define the basic
tools required such as active/passive scanning
and a re-association process to change a
station from the AP of one BSS to another.

IEEE 802.11: Security Issues
 Intruders should not be able to get unauthorized access to
the resources of the Wireless LAN
 The Authentication Process is expected to prevent this from
happening
 Potential eavesdroppers should not be able to capture and
interpret the traffic on the Wireless LAN
 Prevented by 802.11’s WEP algorithm (Wired Equivalent
Privacy) which encrypts the transmissions on the wireless
medium.
 WEP is a simple algorithm based on RSA which is reasonably
strong.

IC0101, LAN Infrastructure 104
IEEE 802.11: Power Saving Issues
 Power saving is important at the mobile stations as battery
power is a scarce resource
 802.11 standards directly address this issue and provide the
mechanism for stations to go into sleep mode without losing data
 The AP keeps a continually updated record of the stations
currently in Power Saving mode. Data intended for these
stations is buffered at the AP until either the station sends a
polling request for it or it changes its operational mode.
 Apart from synchronization information, Beacon Frames from
the AP also send information about which Power Saving Stations
have frames buffered at the AP. The indicated stations can
then download their data as per their convenience.

Frame Types
 Management frames
 Station association & disassociation with AP
 Timing & synchronization
 Authentication & deauthentication
 Control frames
 Handshaking
 ACKs during data transfer
 Data frames
 Data transfer

Address
2
Frame
Control
Duration/
ID
Address
1
Address
3
Sequence
control
Address
4
Frame
body
CRC
2 2 6 6 6 2 6 0-2312 4
MAC header (bytes)
Frame Structure
 MAC Header: 30 bytes
 Frame Body: 0-2312 bytes
 CRC: CCITT-32, 4 bytes over CRC over MAC header & frame
body

Address
2
Frame
Control
Duration/
ID
Address
1
Address
3
Sequence
control
Address
4
Frame
body
CRC
Protocol
version
Type Subtype
To
DS
From
DS
More
frag
Retry
Pwr
mgt
More
data
WEP Rsvd
2 2 6 6 6 2 6 0-2312 4
2 2
MAC header (bytes)
4 1 1 1 1 1 1 1 1
Frame Control (1)
 Protocol version = 0
 Type: Management (00), Control (01), Data (10)
 Subtype within frame type
 Type=00, subtype=association; Type=01, subtype=ACK
 MoreFrag=1 if another fragment of MSDU to follow

To
DS
From
DS
Address
1
Address
2
Address
3
Address
4
0 0
Destination
address
Source
address
BSSID N/A
0 1
Destination
address
BSSID
Source
address
N/A
1 0 BSSID
Source
address
Destination
address
N/A
1 1
Receiver
address
Transmitter
address
Destination
address
Source
address
Meaning
Data frame from station to
station within a BSS
Data frame exiting the DS
Data frame destined for the
DS
WDS frame being distributed
from AP to AP
Address
2
Frame
Control
Duration/
ID
Address
1
Address
3
Sequence
control
Address
4
Frame
body
CRC
Protocol
version
Type Subtype
To
DS
From
DS
More
frag
Retry
Pwr
mgt
More
data
WEP Rsvd
2 2 6 6 6 2 6 0-2312 4
2 2 4 1 1 1 1 1 1 1 1
To DS = 1 if frame goes to DS; From DS = 1 if frame exiting DS
Frame Control (2)

Address
2
Frame
Control
Duration/
ID
Address
1
Address
3
Sequence
control
Address
4
Frame
body
CRC
Protocol
version
Type Subtype
To
DS
From
DS
More
frag
Retry
Pwr
mgt
More
data
WEP Rsvd
2 2 6 6 6 2 6 0-2312 4
2 2
MAC header (bytes)
4 1 1 1 1 1 1 1 1
Frame Control (3)
 Retry=1 if mgmt/control frame is a retransmission
 Power Management used to put station in/out of sleep mode
 More Data =1 to tell station in power-save mode more data
buffered for it at AP
 WEP=1 if frame body encrypted

Physical
layer
LLC
Physical layer
convergence
procedure
Physical medium
dependent
MAC
layer
PLCP
preamble
LLC PDU
MAC SDU
MAC
header
CRC
PLCP
header
PLCP PDU
Physical Layers
 802.11 designed to
 Support LLC
 Operate over many physical layers

IEEE 802.11 Physical Layer Options
Frequency
Band
Bit Rate Modulation Scheme
802.11 2.4 GHz 1-2 Mbps Frequency-Hopping
Spread Spectrum, Direct
Sequence Spread
Spectrum
802.11b 2.4 GHz 11 Mbps Complementary Code
Keying & QPSK
802.11g 2.4 GHz 54 Mbps Orthogonal Frequency
Division Multiplexing
& CCK for backward
compatibility with 802.11b
802.11a 5-6 GHz 54 Mbps Orthogonal Frequency
Division Multiplexing

The LLC and MAC Sublayer Structure
Data link
layer
802.3
CSMA-CD
802.5
Token Ring
802.2 Logical link control
Physical
layer
MAC
LLC
802.11
Wireless
LAN
Network layer Network layer
Physical
layer
OSIIEEE 802
Various physical layers
Other
LANs

Logical Link Control Layer
PHY
MAC
PHY
MAC
PHY
MAC
Unreliable Datagram Service
PHY
MAC
PHY
MAC
PHY
MAC
Reliable frame service
LLCLLC LLC
A C
A C
IEEE 802.2: LLC enhances service provided by MAC

LLC PDU Structure
1
Source
SAP Address Information
1 byte
Control
1 or 2 bytes
Destination SAP Address Source SAP Address
I/G
7 bits1
C/R
7 bits1
I/G = Individual or group address
C/R = Command or response frame
Destination
SAP Address
1 byte
Examples of SAP Addresses:
06 IP packet
E0 Novell IPX
FE OSI packet
AA SubNetwork Access protocol (SNAP)

Encapsulation of MAC frames
IP
LLC
Header
Data
MAC
Header
FCS
LLC
PDU
IP
Packet

Interconnecting LAN Segments
 Hubs
 Bridges
 Switches
 Remark: switches are essentially multi-port
bridges.
 What we say about bridges also holds for
switches!

Interconnecting with Hubs
 Backbone hub interconnects LAN segments
 Extends max distance between nodes
 But individual segment collision domains become one
large collision domain
 if a node in CS and a node EE transmit at same time: collision
 Can’t interconnect 10BaseT & 100BaseT

Bridges
 Link layer device
 stores and forwards Ethernet frames
 examines frame header and selectively
forwards frame based on MAC destination
address
 when frame is to be forwarded on segment,
uses CSMA/CD to access segment
 transparent
 hosts are unaware of presence of bridges
 plug-and-play, self-learning
 bridges do not need to be configured
Transparent
SelfLearning

Bridges offer Traffic Isolation
 Bridge installation breaks LAN into LAN segments
 Bridges filter packets:
 same-LAN-segment frames not usually
forwarded onto other LAN segments
 segments become separate collision domains
bridge
collision
domain
collision
domain
= hub
= host
LAN (IP network)
LAN segment LAN segment

Forwarding Packets through Bridges
How to determine the LAN segment to which to forward frame?
Use Forwarding Data Base (FDB) with Self Learning
Bridge
Bridge
Bridge
Bridge
Segment/LAN
Segment/LAN
Segment/LAN
Segment/LAN
Segment/LAN

Self Learning
 A bridge has a bridge table
 entry in bridge table:
 (Node LAN Address, Bridge Interface, Time Stamp)
 stale entries in table dropped (TTL can be 60 min)
 bridges learn which hosts can be reached through
which interfaces
 when frame received, bridge “learns” location of
sender: incoming LAN segment
 records sender/location pair in bridge table

Forwarding Data Base
(FDB)
7 8 9
Port 1 Port 2 Port N
Segment 1
Segment 2
Segment N
3
4 5 6
1 2
Forwarding Database
FDB
Station # Port #
1 1
2 1
3 1
4 2
5 2
6 2
7 N
8 N
9 N
Forwarding Database
(Time stamp not shown)

Bridge FDBs in Networks with Multiple
Bridges
Bridge
Port Port
Bridge
Port Port
1
1 1
2
2 2
1 2 3 4 5
Station # Port # Station # Port #
1
2
3
4
5
1
2
3
4
5
1
1
2
2
2
1
1
1
2
2
FDB FDB

Filtering/Forwarding
When a bridge receives a frame:
Examine FDB entry for the frame’s destination address
if entry found for destination
then{
if destination on segment from which frame arrived
then drop the frame
else forward the frame on interface indicated
}
else flood
forward on all interfaces other than
the one on which the frame arrived

Spanning Tree Algorithm (STA)
 A Spanning Tree is a path list of one and only one path
between all the nodes in an extended LAN (i.e. multiple LANs
connected by bridges)
 If the network is represented by a graph, then the spanning
tree maintains the connectivity of all the nodes in the graph but
removes all possible loops
 STA is needed because logical loops can lead to packets
circulating endlessly in the network
 Following the STA, the network automatically disables certain
bridges/ports. Note that these bridges are not removed since
they may be needed if the topology changes and the spanning
tree is reconfigured dynamically.

Spanning Tree Algorithm (STA)
To implement the STA in a network -
 each bridge must have a unique Bridge ID
 each port in the bridge must have a unique Port ID
 all the bridges on the LAN must recognize a unique MAC
group address
B1
P1 P2 P3
B1: Bridge ID
P1, P2, P3: Port IDs
Example

Sample Topology and Spanning Tree
B1
P1
P2
Segment 1
Segment
2
Segment
3
Segment 4
B2
P1
P2
B4
P1
P2
B5
P1
P2
B3
P1
P2
P3
A Sample Topology of a Bridged LAN
All LANs assumed
to have equal cost

B1
P1
P2
Segment 1
Segment
2
Segment
3
Segment 4
B2
P1
P2
B4
P1
P2
B5
P1
P2
B3
P1
P2
P3
B1 Root Bridge
R Root Port
D Designated Port/Bridge
R
R
R
RD
D
D
D

B1
P1
P2
Segment 1
Segment
2
Segment
3
Segment 4
B3
P1
P2
P3
R
D
D
D
D
Even though bridges B2,
B4 and B5 have not been
shown, they are still
present and may be used
if the topology changes
or if bridges or links
fail.

Spanning Tree Algorithm
Bridges participating in the Spanning Tree Algorithm do the
following together -
 Select a root bridge in the LAN - this is selected as the bridge
with the lowest bridge ID. Bridge priorities ignored for simplicity
 Determine the root port for each bridge except the root
bridge. The root port is the port with the least-cost path to the
root bridge. If there is a tie, the root port is chosen as the port
with the lowest port ID.
 Select a designated bridge for each LAN. This is the bridge
that offers the least-cost path from the LAN to the root bridge.
If there is a tie, the designated bridge is chosen as the one with
the lowest bridge ID. The port connecting the LAN to its
designated bridge is called a designated port. Note that all the
ports of the root bridge must be chosen as designated ports and
that the root port of a bridge cannot be chosen as a designated
port.
For the given root bridge, the resulting network
would be a minimum spanning tree.

Bridges vs. Routers
 both store-and-forward devices
 routers: network layer devices (examine network layer
headers)
 bridges are link layer devices
 routers maintain routing tables, implement routing
algorithms
 bridges maintain bridge tables, implement filtering,
learning and spanning tree algorithms

Routers vs. Bridges
Bridges + and -
+ Bridge operation is simpler requiring less packet
processing
+ Bridge tables are self learning
- All traffic confined to spanning tree, even when
alternative bandwidth is available
- Bridges do not offer protection from broadcast
storms

Routers vs. Bridges
Routers + and -
+ arbitrary topologies can be supported, cycling is
limited by TTL counters (and good routing protocols)
+ provide protection against broadcast storms
- require IP address configuration (not plug and play)
- require higher packet processing
 bridges do well in small (few hundred hosts) while
routers used in large networks (thousands of hosts)

Ethernet Switches
 Essentially a multi-
interface bridge
 layer 2 (frame) forwarding,
filtering using LAN
addresses
 Switching: A-to-A’ and B-
to-B’ simultaneously, no
collisions
 large number of interfaces
 often: individual hosts,
star-connected into switch
 Ethernet, but no
collisions!

Ethernet Switches
 cut-through switching: frame forwarded
from input to output port without awaiting
for assembly of entire frame
 slight reduction in latency
 combinations of shared/dedicated,
10/100/1000 Mbps interfaces

Summary Comparison
hubs bridges routers switches
traffic
isolation
no yes yes yes
plug & play yes yes no yes
optimal
routing
no no yes no
cut
through
yes no no yes

Physical
partition
Logical partition
Bridge
or
switch
VLAN 1 VLAN 2 VLAN 3
S17
2 3 4 5 61
8
9 Floor n – 1
Floor n
Floor n + 1
S2
S3
S4
S5
S6
S7
S8
S9
Virtual LAN (VLAN)

Logical partition
Bridge
or
switch
VLAN 1 VLAN 2 VLAN 3
S17
2 3 4 5 61
8
9 Floor n – 1
Floor n
Floor n + 1
S2
S3
S4
S5
S6
S7
S8
S9
Per-Port VLANs
Bridge only forwards frames to outgoing ports associated with same VLAN

Tagged VLANs
 More flexible than Port-based VLANs
 Insert VLAN tag after source MAC address in each frame
 VLAN protocol ID + tag
 VLAN-aware bridge forwards frames to outgoing ports
according to VLAN ID
 VLAN ID can be associated with a port statically through
configuration or dynamically through bridge learning
 IEEE 802.1q

Lecture set 4

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Lecture set 4