Mac layer

Chapter 4

The Medium Access
Control Sublayer

1

The Channel Allocation Problem
• The MAC (Medium Access Control) sublayer is
between the physical layer and the data link layer.
• The MAC sublayer is especially important in LANs,
nearly all of which use a multiaccess (or
broadcast) channel as the basis of their networks.
• This chapter mainly deals with LANs, other
broadcast networks and their protocols. The key
issue: how to determine who gets to use the
channel when there is competition for it ?
1. Static Channel Allocation in LANs and MANs
2. Dynamic Channel Allocation in LANs and MANs

2

Static Channel Allocation

• FDM (Frequency-Division Multiplexing):
Divide the bandwidth into equal sized portions
so that each user can be assigned one portion.

• Under what circumstances FDM is efficient ?

When there is only a small and fixed number of
users, and each of which has a heavy (buffered)
load of traffic (e.g., carriers' switching offices).

3

What's the problem with FDM ?
1. If fewer than N users are currently interested in
communication, some portions of spectrum will
be wasted.
2. If more than N users want to communicate,
some of them will be denied permission even if
some users with allocated frequency hardly ever
transmit anything.
3. Even the number of users is N and constant,
when some users are quiescent, no one else can
use their bandwidth so it is simply wasted.
4. For bursty data traffic (peak traffic to mean
traffic ratio of 1000:1), the allocated small
subchannel will be idle most of the time but
unable to handle the peak traffic. 4

Dynamic Channel Allocation in LANs and MANs
1. Station Model.
• Independent stations for generating frames.
• Once a frame has been generated, the station is blocked until
the frame has been transmitted.
1. Single Channel Assumption. A single channel for all
communication (send and receive), and all stations are equivalent.
2. Collision Assumption. If the transmission of two frames overlap in
time, a collision occurs. All stations can detect collisions. A
collided frame must be retransmitted.
3. Time assumption.
(a) Continuous Time.
(b) Slotted Time.
4. Sense assumption.
(a) Carrier sense. Stations can tell if the channel is in use
before trying to use it.
(b) No carrier sense. Stations cannot sense the channel before
trying to use it.
5

Multiple Access Protocols

• ALOHA
• Carrier Sense Multiple Access Protocols
• Collision-Free Protocols
• Limited-Contention Protocols
• Wavelength Division Multiple Access Protocols
• Wireless LAN Protocols

6

PURE ALOHA
The ALOHA system was used for ground-based radio
broadcasting, but the basic idea is applicable to any system
in which uncoordinated users are competing for the use of a
single shared channel.
Basic idea:
Let users transmit whenever they have data to be sent.
A sender can always find out whether or not its frame was
destroyed by listening to the channel output (due to the
feedback property of broadcasting).
If the frame was destroyed, the sender just waits a random
amount of time and sends it again.
Systems in which multiple users share a common channel in
a way that can lead to conflicts are known as contention
systems.

7

Pure ALOHA

In pure ALOHA, frames are transmitted at completely arbitrary times.
8

Pure ALOHA (2)

Vulnerable period for the shaded frame.
9

Analysis of Pure ALOHA protocol
a)Assume that there are a large number (N) of
stations in the network
b)All stations transmit frames with a fixed
(average) length of T seconds
c)Each station transmits with a fixed probability
(p) in the time period (T)
d)Thus, the average number of frames transmitted
in the system in the time period T will be Np

10

a) “Danger” period for a station’s transmission starts T
seconds before it initiates its frame transmission and
ends T seconds after it completes its frame
b)During this time period of 2T, the average number of
frames transmitted will be E=2Np=2G
c) A Poisson probability distribution indicates the
probability of k events occurring in a “unit time”

E k e− E
p( k ) =
k! 11

a) For transmission to be successful, no other
station should transmit during the unit time of
interest (2T). Thus the probability of a
successful transmission will be
p(k=0)=e-E=e-2G
a) Therefore, the system throughput for the time
period T will be S=Number of transmission
attempts in time period T x probability of
successful transmission, or
S=Ge-2G

12

Pure Aloha Throughput
0.2

0.18

0.16

0.14
T rou pu (S)

0.12
h gh t

0.1

0.08

0.06

0.04

0.02

0
0 1 2 3 4 5
A erage N m of fram per u it tim (G
v u ber es n e )

13

Operation of Protocol
a) Optimum Throughput occurs at G=0.5 or when
1
N=
2p
b)Average number of attempts to ensure
successful transmission is
∞
N av = ∑ n(1 − e − 2 G ) n −1e − 2 G = e 2 G
i =1
Optimum
N av = 2.72 attempts

14

Slotted ALOHA protocol
a) Enhancement of pure ALOHA in that stations
can only start to transmit frame so that it
arrives at the destination at the beginning of
defined time slots of duration T
b)“Danger” period for this system is only the T
seconds prior to the start of station’s frame and
thus E=Np and S=Ge-G
c) For this system, optimum throughput occurs if
G=1

15

Slotted Aloha Throughput
0.4

0.35

0.3
h gh t )

0.25
T rou pu (S

0.2

0.15

0.1

0.05

0
0 1 2 3 4 5
Average N m of fram per u it tim (G
u ber es n e )

16

Efficiency of Slotted ALOHA
a)Successful throughput S read from
graph (e.g. Soptimum=0.368 or 36.8% of
timeslot contain successful
transmissions)
b)Number of frames with no
transmissions can be found from
Poisson distribution p(k=0)=0.368 or
36.8% slots
c)Remaining time slots must contain
collisions
17

Slotted ALOHA
Slotted ALOHA system:
• Time is divided up into discrete intervals, each interval corresponding to one
frame.
• A terminal is not permitted to send until the beginning of the next slot.

Slotted ALOHA peaks at G = 1, with S = 1/e ≈ 0.368, twice that of pure
ALOHA. The main reason for poor channel utilization of ALOHA (pure
or slotted) is that all stations can transmit at will, without paying
attention to what the other stations are doing.
18


• ALOHA

19

Carrier Sense Multiple Access protocols
Protocols in which stations listen for a carrier (i.e., a transmission)
and act accordingly are called carrier sense protocols.

1-persistent CSMA (Carrier Sense Multiple Access):
1. To send data, a station first listens to the channel to see if anyone else is
transmitting.
2. If so, the station waits (keeps sensing it) until the channel becomes idle.
Otherwise, it transmits a frame.
3. If a collision occurs, the station waits a random amount of time and
starts all over again.

It is called 1-persistent because the station transmits with a
probability of 1 whenever it starts sensing the channel and finds
the channel idle.

20

Collisions in CSMA
How could collisions happen in CSMA ?
Whenever more than one station detect an idle channel and their
transmission times overlap.
Discussions:
1. What's the effect of signal propagation delay ?
The longer the delay, the more the collisions, and the worse
the performance of the protocol.

2. How about zero propagation delay ?
There still exist chances of collisions.

3. Is this protocol any better than ALOHA (both pure and
slotted) ?
Yes, because both stations have the decency to desist from
interfering with the third station's frame.
21

Non-persistent and P-persistent CSMA
Non-persistent CSMA
1. To send data, a station first listens to the channel to see if anyone else is transmitting.
2. If so, the station waits a random period of time (instead of keeping sensing until the end
of the transmission) and repeats the algorithm. Otherwise, it transmits a frame.
3. If a collision occurs, the station waits a random amount of time and starts all over again.
This protocol has better channel utilization and longer delays than 1-persistent
CSMA.

P-persistent CSMA(applied to slotted channels):
1. To send data, a station first listens to the channel to see if anyone else is transmitting.
2. If the channel is idle, it transmits with a probability p. With a probability q = p – 1, it
defers until the next slot. If the next slot is also idle, it either transmits or defers again,
with probabilities p and q. This process is repeated until either the frame has been
transmitted or another station has begun transmitting. In the latter case, it waits a
random time and starts again.
3. If a collision occurs, the station waits a random amount of time and starts all over again.
4. If the channel is (initially) busy, it waits until the next slot and apply the above
algorithm.

22

Persistent and Non-persistent CSMA

Comparison of the channel utilization versus load for various
random access protocols. 23

CSMA with Collision Detection
Persistent and nonpersistent CSMA protocols improve ALOHA by ensuring
that no station begins to transmit when it senses the channel busy.

CSMA/CD (Carrier Sense Multiple Access with Collision Detection)
protocol further improves ALOHA by aborting transmissions as soon
as a collision is detected.

The conceptual model:
• To send data, a station first listens to the channel to see if anyone else
is transmitting.
• If so, the station waits until the end of the transmission (1-persistent)
or wait a random period of time and repeats the algorithm (non-
persistent). Otherwise, it transmits a frame.
• If a collision occurs, the station will detect the collision, abort its
transmission, waits a random amount of time, and starts all over again.

24

CSMA/CD can be in one of three states:
contention, transmission, or idle

• When two stations both begin transmitting at exactly the same time, how long
will it take them to realize that there has been a collision ?
The minimum time to detect the collision is the time it takes the signal to
propagate from one station to the other.
• How long could the transmitting station be sure it has seized the network ? ( τ or
2τ ? where τ is time equal to the full cable propagation)
• It is worth noting that no MAC-sublayer protocol guarantees reliable delivery.
Even in the absence of collisions, the receiver may not have copied the frame
correctly due to various reasons (e.g., lack of buffer space or a missed interrupt).
25


• ALOHA

26

Collision-Free Protocols
• Can we avoid collision all together, even during the
contention period ?
Yes. But how ?
• One assumption: there are N stations, each with a unique
address from 0 to N –1 ``wired'' into it.
Which station gets the channel after a successful transmission ?

A basic bit-map protocol:
• Each contention period consists of exactly N slots, with one slot time
being at least 2τ.
• If station i (0 ≤ i ≤ N -1) has a frame to send, it transmits 1 bit during
the ith slot; otherwise, it transmits 0 bit during the ith slot.
• After all slots have passed by, stations begin transmitting in numerical
order.
• After the last ready station has transmitted its frame, another N-bit
contention period is begun.

27

The basic bit-map protocol

• Protocols like this in which the desire to transmit is broadcast
before the actual transmission are called reservation protocols.
• Low load situation: the bit map repeats over and over.

28

The binary countdown protocol
A problem with the basic bit-map protocol
is that overhead is 1 contention bit slot per
station. We can do better than that by using
binary station addresses.
1. Each station has a binary address. All
addresses are the same length.
2. To transmit, a station broadcasts its address
as a binary bit string, starting with high-
order bit.
3. The bits in each address position from
different stations are BOOLEAN ORed
together (so called Binary countdown).
4. As soon as a station sees that a high-order
bit position that is 0 in its address has been
overwritten with a 1, it gives up.
5. After the winning station has transmitted its
frame, there is no information available
telling how many other stations to send, so
the algorithm begins all over with the next
frame.

29


• ALOHA

30

Limited-Contention Protocols
Two basic channel acquisition strategies studied:
a) Contention methods. E.g., ALOHA, CSMA.
b) Collision-free methods. E.g., basic bit map.

Two performance measures:
a) Delay at low load. Contention methods are preferable.
b) Channel efficiency at high load. Collision-free methods are
preferable.

Could we combine the best properties of both the contention and
collision-free protocols ?
Yes. Such new protocols, called limited contention protocols,
use a contention method at low loads, but use a collision-
free technique at high loads.
31

Basic idea of Limited-Contention Protocols
• Stations are divided into groups.
• Each group is allocated a slot for transmission.
• Members of one group compete for one slot only.

How to assign stations to slots (groups) ?
Special cases:
1. Each group with only one member (e.g., basic bit-map). Collision-
free.
2. Each group with two stations. The probability that both will try to
transmit during a slot is p**2, which for small p is negligible.
3. A single group containing all stations (i.e., slotted ALOHA). Zero
delay at low load but very high collisions at high load.

What we need is a way to assign stations to slots dynamically,
with many stations per slot when the load is low and few
station per slot when the load is high.
33

Adaptive Tree Walk Protocol
The algorithm devised by US Army for testing soldiers for
syphilis during WW II:
1. The Army took a blood sample from N soldiers.
2. A portion of each sample was poured into a single test
tube. The mixed sample was then tested.
3. If no antibodies were found, all the solders in the
group were declared healthy.
4. Otherwise, two new mixed samples were prepared,
one for soldiers 1 through N/2 and one from the rest.
5. The process was repeated recursively until the
infected soldiers were determined.
The computer version of this algorithm organizes the
stations in a binary tree, as illustrated below.
34


35

Each bit slot (group) is associated with a particular
node in the tree, with the root node corresponding to
slot 0 for all stations in one group.
• Starting from the root node: all stations are
permitted to try to acquire the channel.
• If a collision occurs, the search continues
recursively with the left and right children of that
node.
• If a bit slot goes idle or if there is exactly one
station that transmits into it, the searching of its
node can stop.

36

• ALOHA
• Wavelength Division Multiple Access
Protocols

37

Wavelength Division Multiple Access Protocols
A different approach to channel allocation is to divide the channel into
subchannels using FDM, TDM, or both, and dynamically allocate them as
needed. This is commonly used on fiber optic LANs.

Wavelength division multiple access.
38


• ALOHA

39

Wireless LAN Protocols
A system of computers (e.g., notebook PCs) that communicate by radio can
be regarded as a wireless LAN.

Wireless LANs have different properties than conventional wired LANs and
require special MAC sublayer protocols.
All radio transmitters have some fixed range, so not all stations are necessarily
within range of one another.
a) A transmitting. C is unable to detect A’s transmission, so it may falsely
conclude that it may transmit to B, resulting in collision. This is called the
hidden station problem)
b) B transmitting. C is able to detect A’s transmission, so it may falsely
conclude that it may not send to D, which is called the exposed station
problem. 40

Wireless LAN Protocols (2)

The MACA (Multiple Access with Collision Avoidance) protocol.
(a) A sending an RTS to B. (b) B responding with a CTS to A.

41


• ALOHA

42

IEEE 802 Standards

The 802 working groups. The important ones are marked with *. The
ones marked with  are hibernating. The one marked with † gave up.
43

Ethernet
IEEE standard 802.3 and Ethernet
This standard is for a 1-persistent CSMA/CD LAN:
• When a station wants to transmit, it listens to the cable.
• If the cable is busy, the station waits until it goes idle,
otherwise it transmits immediately.
• If a collision occurs, all colliding stations terminate
their transmission, wait a random time, and repeat the
whole process all over again.

44

Ethernet Cabling

The most common kinds of Ethernet cabling.

Three kinds of Ethernet cabling.
(a) 10Base5, (b) 10Base2, (c) 10Base-T. 45

Cable topologies

(a) Linear, (b) Spine, (c) Tree, (d) Segmented.

Repeaters are used to connect multiple cables together. It receives, amplifies,
and retransmits signals in both directions. Repeaters are physical layer
devices.
No two transceivers may be more than 2.5 km apart, and no path between any
two transceivers may traverse more than four repeaters.
46

Manchester Encoding
None of the Ethernet versions use straight binary encoding since it leads to ambiguities:
it cannot tell the difference between an idle sender (0 volts) and a 0 bit (0 volts).

What is needed is a way for receivers to unambiguously determine the start, end, or
middle of each bit without reference to an external clock.

(a) Binary encoding, (b) Manchester encoding, (c) Differential Manchester encoding.
All Ethernet systems use Manchester encoding due to its simplicity. The high signal is volts
and the low signal is volts, giving a DC value of 0 volts.
47

Ethernet MAC Sublayer Protocol

Frame formats. (a) DIX Ethernet, (b) IEEE 802.3.
Preamble of 8 bytes, each with the bit pattern 10101010. For synchronization between the
sender and the receiver. Start of frame (SOF) byte
contains 10101011 to denote the start of the frame.
Destination and source addresses, each with 2-6 bytes.
• Ordinary address:
the high-order bit
value is 0.
• Multicast address:
the high-order bit
value is 1.
• Broadcast address:
all bits have a value
48 of 1.
• Local address:

Collision detection can take as long as 2τ
Length: 0 (46) - 1500 bytes. To make it easier to distinguish valid frames from
garbage, 802.3 states that valid frames must be at least 64 bytes.

Another more important reason for having a minimum length frame is to prevent a
station from completing the transmission of a short frame before the first bit has
even reached the far end of the cable, which it may collide with another frame, as
illustrated below.

49

The binary exponential backoff algorithm
After a collision, the station waits for a random time and try again. How the
randomization is done ?
Time is divided into discrete slots whose length is equal to the worst case round
trip propagation time (2τ).
1. After the first collision, each station waits either 0 or 1 slot time at random.
2. After the second collision, each station waits either 0, 1, 2, or 3 slot times at
random.
3. After i collisions, a random number between 0 and 2**i – 1 is chosen, and that
number of slots is skipped.
4. After 10 collisions have been reached, the randomization interval is frozen at
1023 slots.
5. After 16 collisions, the controller gives up and reports failure.

What happens if the randomization interval for all collisions is always 1023?
Think about the average delay when only a few stations collide.
What happens if the randomization interval for all collisions is always 2?
Think about the time needed to resolve the collision when 100 stations try to
send at once. This might take years to resolve.
The mean number of times per transmission is e ≈ 2.72.
50

Ethernet Performance

Efficiency of Ethernet at 10 Mbps with 512-bit slot times.
51

Switched Ethernet

The heart of the system is a switch containing a high-speed (typically over 1
Gbps) backplane and room for multiple plug-in cards (typically 4 to 32),
with one to eight connectors in each card.
• To transmit, a station outputs a standard 802.3 frame to a plug-in card in
the switch.
• The plug-in card checks to see if the frame is destined for one of the
other stations connected to the same card.
If so, the frame is copied there. If not, the frame is sent over the high-
speed backplane to the destination station's card.
52

Switched Ethernet (2)
What happens if two machines attached to the same plug-in card transmit frames at
the same time ?

In case that all the ports on the card are wired together to form a local on-card LAN:
• Collisions on this on-card LAN will be detected and handled the same as any other
collisions on a CSMA/CD network.
• Only one transmission per card is possible at any instant, but all the cards can be
transmitting in parallel.

In case that each input port is buffered, so incoming frames are stored in the card's
on-board RAM as they arrive:
• Once a frame has been completely received, the card checks to see if the frame is
destined for another port on the same card, or for a distant port. In the former case
it can be transmitted directly to the destination.
• In the latter case, it must be transmitted over the backplane to the proper card.
• All input ports can receive (and transmit) frames at the same time, for parallel,
full-duplex operation.

53

Fast Ethernet
The Fast Ethernet (IEEE 802.3u) is backward compatible with but faster than the existing
802.3 standard.
All the 802.3 packet formats, interfaces, and procedural rules are kept, but the bit time is
reduced from 100 (10 Mbps) nsec to 10 nsec (100 Mbps).
Fast Ethernet is based entirely on twisted pair or fiber wiring, as shown below.

1. 100Base-T4 uses four twisted pairs (three for traffic from the station to the hub, one
for the reverse traffic). Each pair runs at a signaling speed of 25 MHz. 4 bits (by the
8B6T scheme) are transmitted per clock cycle via three twisted pairs (to give
100Mbps), and another twisted pair provides a reverse channel at 33.3 Mbps.
2. 100Base-TX uses two twisted pairs (one for each direction), each running at a
signaling speed of 125 MHz. The 4B5B scheme is used to transmit 4 bits per 5 clock
cycles via each twisted pair (to give a full-duplex 100 Mbps).
3. 100Base-F uses two strands of multimode fiber, one for each direction, to give full
duplex 100 Mbps. The distance between the station and the hub can be up to 2 km.
54

Gigabit Ethernet
The Gigabit Ethernet (IEEE 802.3z) is backward compatible with but faster than the
existing 802.3 and 802.3u standards, but goes 10 times faster than 802.3u.

All configurations of gigabit Ethernet are point-to-point.

(a) A two-station Ethernet. (b) A multistation Ethernet.
55

Gigabit Ethernet cabling
Gigabit Ethernet supports both copper and fiber cabling.

Gigabit Ethernet uses new encoding schemes, rather than Manchester encoding.

Gigabit Ethernet supports flow control: one end can send a special control frame
to the other end telling it to pause for some period of time. The time unit is
512 nsec, allowing for pause as long as 33.6 msec.

IEEE 802 committee has produced the 10-gigabit Ethernet standard 802.3ae in
2002.
56

FEATURES COMMON TO ALL ETHERNETS

•IEEE 802.3

•BROADCASTING

•CSMA/CD

•BASEBAND TRANSMISSION

•FRAME SIZE 64-1518 BYTES

•FRAME FORMAT

•BACKWARD COMPATIBILITY57

FEATURES VARIATIONS IN ETHERNETS

•TOPOLOGY

•SPEED

•CABLE TYPE

58

Token Ring
Operation

59

Token Ring (802.5)
a) 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

802.5 Physical Layer
a)Data Rate 4 16 100
b)Medium UTP,STP,Fiber
c) Signaling Differential Manchester
d)Max Frame 4550 18200 18200

Note: 1Gbit in development

61

IEEE 802.2: Logical Link Control
LLC hides the differences between the various kinds of 802 networks by
providing a single format and interface to the network layer.
LLC provides three service options: unreliable datagram service, acknowledged
datagram service, and reliable connection-oriented service.

(a) Position of LLC. (b) Protocol formats.
62

Wireless LANs
• Wireless LANs are increasingly popular and deployed in office buildings,
airports and other public places.
• Wireless LANs can operate with or without a base station.
• IEEE 802.11 LAN standard makes provision for both arrangements.

Part of the 802.11 protocol stack. 63

The 802.11 physical layer
• In 1997, three transmission techniques were allowed in the
physical layer:
1. The infrared method uses the same technology as television remote controls do.
2. The other two techniques, called FHSS (Frequency Hopping Spread Spectrum)
and DSSS (Direct Sequence Spread Spectrum, similar to CDMA), use short-
range radio spectrum (a part of the 2.4 GHz ISM band).
All techniques operate at 1 or 2 Mbps(a major disadvantage)

• In 1999, two new techniques, called OFDM (Orthogonal
Frequency Division Multiplexing, used by 802.11a) and HR-
DSSS (High Rate Direct Sequence Spread Spectrum, used by
802.11b), were introduced to achieve higher bandwidth, 54
Mbps (narrow range) and 11 Mbps (wider range), respectively.

• In 2001, a second OFDM modulation (like 802.11a) but at the
narrow 2.4 GHz ISM band (like 802.11b) was introduced and
can operate at up to 54 Mbps (802.11g).
64

The 802.11 MAC Sublayer Protocol

(a) The hidden station problem.
(b) The exposed station problem. 65

DCF (Distributed Coordination Function) mode
The DCF mode uses the CSMA/CA (Collision Avoidance)
protocol, in which, there is no central control, and
stations compete for air time, just as they do with
Ethernet:
• When a station wants to transmit, it senses the channel. If it is idle, it
just starts transmitting. It does not sense the channel while
transmitting but emits its entire frame, which may well be destroyed
at the receiver due to interference there.
• If the channel is busy, the sender defers until it goes idle and then
starts transmitting.
• If a collision occurs, the colliding stations wait a random time, using
Ethernet binary exponential backoff algorithm, and then try again
later.

66

The use of virtual channel sensing using CSMA/CA

A wants to send to B.
C is within range of A.
D is within range of B but not within range of A.
RTS: Request To Send
CTS: Clear To Send.
NAV: Network Allocation Vector
67

A fragment burst

To deal with the problem of noisy channels, 802.11 allows frames
to be fragmented into smaller pieces, each with its own
checksum.
Fragments are individually numbered and acknowledged using a
stop-and-wait protocol.
68

PCF (Point Coordination Function) mode
In the PCF mode, the base station pools the other stations, asking them
if they have any frames to send. No collisions ever occur.
PCF and DCF can coexist within one cell, which works by carefully
defining the interframe time interval. Four different intervals are
defined, as depicted below.

Interframe spacing in 802.11.
69

The 802.11 Frame Structure

The 802.11 data frame.
Version: two versions of the protocol can operate at the same time in the same cell.
Type: data, control, or management.
Subtype: RTS or CTS.
To/from DS: the frame is going to or coming from intercell distribution system.
MF: more fragments will follow.
Retry marks a retransmission of a frame sent earlier.
Power is used by the base station to put/take the receiver into/outof sleep state.
More indicates the sender has additional frames for the receiver.
W specifies that the frame has been encrypted by Wired Equivalent Privacy algorithm.
O tells that a sequence of frames with this bit on must be processed strictly in order.
70

The 802.11 Frame Structure (2)

Duration: tells how long the frame and its ack will occupy the channel, which is
used to manage NAV mechanism.
Addresses: Two addresses are used for source and destination, and other two
used for the source and destination base stations for intercell traffic.
Sequence: 12 bits for identifying frame and 4 bits for fragment.
Data: payload up to 2312 bytes.

Management frames are similar to data frames, without one of the base station
addresses because management frames are restricted to a single cell.
Control frames are even shorter, having only one or two addresses, no Data
field, and no Sequence field. Key information is in the Subtype field.
71

802.11 Services
Distribution Services:
• Association: used by mobile stations to connect themselves
to base stations.
• Disassociation: used by both mobile stations and the base
station to break up their association relationship.
• Re-association: used by a mobile station to change its
preferred base station.
• Distribution: determine how to route frames sent to the
base station.
• Integration: handle the translation from the 802.11 format
to the format required by the non-802.11 destination
network.

72

802.11 Services
Intracell Services (used after association has taken place):
• Authentication: used by a mobile station to authenticate
itself (proving it possesses a secret key).
• De-authentication: used by a mobile station when leaving
the network
• Privacy: manage the encryption and decryption.
• Data Delivery: data transmission over 802.11 is not 100%
reliable (the same as Ethernet). Higher layers must deal
with detecting and correcting errors.

73

Broadband Wireless
This section is about wireless local loop or wireless MAN.
Comparison of 802.11 and 802.16:
• They were all designed in providing high-bandwidth wireless communication.
• 802.16 provides service to buildings. Buildings are not mobile (a
simplification). Building can have more than one computer in them (a
complication). Building owners are willing to spend more money for
communication gear than individual notebook owners, so 802.16 can use full-
duplex communication.
• 802.16 runs over part of a city, covering distance of several kilometers, which
means the perceived power at the base station can vary widely, which affects
signal-to-noise ratio, which, dictates multiple modulation schemes. Open
communication over a city means that security and privacy are essential and
mandatory.
• Each 802.16 cell has many more users than an 802.11 cell, forcing 802.16 to
operate in 10-to-66 GHz frequency range, which has different physical
properties and thus requires a different physical layer. Also, error handling is
more important.
• 802.16 is expected to support telephony and multimedia usage for both
residential and business use.
• In short, 802.11 was designed to be mobile Ethernet, whereas 802.16 was
designed to be wireless, but stationary, cable television.
74

The 802.16 Protocol Stack

The 802.16 MAC sublayer uses the base station to manage the channel
allocation. It is connection-oriented in order to provide quality-of-service
guarantees for telephony and multimedia communication.

75

The 802.16 Physical Layer

The 802.16 transmission environment.
76

The 802.16 Physical Layer (2)

Frames and time slots for time division duplexing.
77

The 802.16 MAC Sublayer Protocol

Service Classes
• Constant bit rate service
• Real-time variable bit rate service
• Non-real-time variable bit rate service
• Best efforts service

78

The 802.16 Frame Structure

(a) A generic frame. (b) A bandwidth request frame.
79

Bluetooth
• In 1994, Ericsson, IBM, Intel, Nokia, and Toshiba formed SIG
to develop a wireless standard for interconnecting computing
and communication devices and accessories using short-range,
low-power, inexpensive wireless radios. This project was
named Bluetooth, after Harald Blaatand (Bluetooth) II (940-
981), a Viking king who unified Denmark and Norway, also
without cables.
• The original idea was just to get rid of the cables between
devices, but it soon began to expand in scope and encroach on
the area of wireless LANs. It creates competition with 802.11
and even interferes with 802.11 electronically (using the same
2.4-GHz ISM band).
• In 1999, Bluetooth SIG issued a 1500-page specification V1.0
(from physical to application layers), part of which (physical
and data-link layers) was adopted by IEEE as the basis for a
wireless PAN (Personal Area Network) standard, 802.15.1
(2002). 80

Bluetooth Architecture

a) The basic unit of a Bluetooth system is a piconet, which consists of a master
node and up to seven active slave nodes within a distance of 10 meters.
b) There can be up to 255 parked nodes in a piconet, which are devices that the
master has switched to a low-power state to reduce the drain on their batteries.
c) All communication is between the master and a slave, using TDM; no direct
slave-slave communication is possible.
d) Multiple piconets can exist in the same room and be connected via a bridge
node. An interconnected collection of piconet is called a scatternet.
81

Bluetooth Applications

The Bluetooth profiles.
Conway’s law: if you assign n people to write a compiler, you will get an n-pass
compiler, or more generally, the software structure mirrors the structure of the
group that produced it. 82

The Bluetooth Protocol Stack

The 802.15 version of the Bluetooth protocol architecture.
83

The Bluetooth Frame Structure

A typical Bluetooth data frame.
84

Data Link Layer Switching

• Bridges from 802.x to 802.y
• Local Internetworking
• Spanning Tree Bridges
• Remote Bridges
• Repeaters, Hubs, Bridges, Switches, Routers, Gateways
• Virtual LANs

85

LANs can be connected by devices called bridges, which
operate in the data link layer. This means that bridges do not
examine the network layer header and can thus copy IPv4,
IPv6, AppleTalk, ATM, IPX, and OSI packets equally well.

Most bridges connect 802 LANs, so we will concentrate
primarily on 802 bridges.

Reasons why a single organization may end up with multiple
LANs and bridges are needed:
1. Different goals of different departments.
2. Geographical distance (cheaper and shorter delay).
3. Accommodation of the heavy load.
4. The matter of reliability and security.

86


Multiple LANs connected by a backbone to handle a total load higher
than the capacity of a single LAN.

87

Hubs
and
Switches
88

Bridges
a) Ability to expand beyond single LAN
b) Provide interconnection to other LANs/WANs
c) Use Bridge or router
d) Bridge is simpler
– Connects similar LANs
– Identical protocols for physical and link layers
– Minimal processing
a) Router more general purpose
– Interconnect various LANs and WANs
– see later
89

Why Bridge?
a)Reliability
b)Performance
c)Security
d)Geography
e)Org Structure
f)To accommodate load

90

Functions of a Bridge
a) Read all frames transmitted on one LAN and accept those
addressed to any station on the other LAN
b) Using MAC protocol for second LAN, retransmit each frame
c) Do the same the other way round

91

Bridge Operation

92

Bridge Design Aspects
a) No modification to content or format of frame
b) No encapsulation
c) Exact bitwise copy of frame
d) Minimal buffering to meet peak demand
e) Contains routing and address intelligence
– Must be able to tell which frames to pass
– May be more than one bridge to cross
a) May connect more than two LANs
b) Bridging is transparent to stations
– Appears to all stations on multiple LANs as if they are
on one single LAN
93

Bridge Protocol Architecture
a) IEEE 802.1D
b) MAC level
– Station address is at this level
a) Bridge does not need LLC layer
– It is relaying MAC frames
a) Can pass frame over external comms system
– e.g. WAN link
– Capture frame
– Encapsulate it
– Forward it across link
– Remove encapsulation and forward over LAN link

94

Connection of Two LANs

95

Fixed Routing
a) Complex large LANs need alternative routes
– Load balancing
– Fault tolerance
a) Bridge must decide whether to forward frame
b) Bridge must decide which LAN to forward frame on
c) Routing selected for each source-destination pair of LANs
– Done in configuration
– Usually least hop route
– Only changed when topology changes

96

Spanning Tree

a)Bridge automatically develops routing
table
b)Automatically update in response to
changes
c)Frame forwarding
d)Address learning
e)Loop resolution 98

Frame forwarding
a) Maintain forwarding database for each port
– List station addresses reached through each port
a) For a frame arriving on port X:
– Search forwarding database to see if MAC address is
listed for any port except X
– If address not found, forward to all ports except X
– If address listed for port Y, check port Y for blocking or
forwarding state
• Blocking prevents port from receiving or transmitting
– If not blocked, transmit frame through port Y
99

Address Learning
a) Can preload forwarding database
b) Can be learned
c) When frame arrives at port X, it has come form the LAN attached to
port X
d) Use the source address to update forwarding database for port X to
include that address
e) Timer on each entry in database
f) Each time frame arrives, source address checked against forwarding
database

100

Spanning Tree Algorithm
a) Address learning works for tree layout
– i.e. no closed loops
a) For any connected graph there is a spanning tree that maintains
connectivity but contains no closed loops
b) Each bridge assigned unique identifier
c) Exchange between bridges to establish spanning tree

101

Loop of Bridges

102

Bridges from 802.x to 802.y

Operation of a LAN bridge from 802.11 to 802.3.
103

Bridges from 802.x to 802.y (2)

The IEEE 802 frame formats. The drawing is not to scale.

General problems:
1. Different frame formats. Solution: reformatting.
2. Different data rates (802.3z 1Gbps and 802.11b 11Mbps). Solution:
buffering.
3. Different maximum frame length (e.g., 802.3 1518 bytes, 802.11 2312 bytes,
802.16 1000 (?) bytes, and 802.15.1 2744 bytes). No solution.
4. Security: 802.11 and 802.16 support encryption but 802.3 does not.
5. Quality of service: 802.11 and 802.16 have but 802.3 does not.
104

Local Internetworking
Multiple LANs connected by transparent bridges do not need any change on their
hardware and software.
Transparent bridges operate in promiscuous mode, accepting every frame transmitted on
all the LANs to which it is attached.

A configuration with four LANs and two bridges.

When a frame arrives, a bridge must decide whether to discard or forward it, and
if the latter, on which LAN to put the frame. This decision is made by looking up
the destination address in a big (hash) table inside the bridge.
105

Local Internetworking (2)
A bridge maintains a table of destination addresses as follows:
1. Initially all the hash tables are empty.
2. Every incoming frame for an unknown destination is output on all the LANs to
which the bridge is connected except the one it arrived on (flooding).
3. By looking at the incoming frame's source address (backward learning), a
bridge is able to know which machine is accessible on which LAN, so it make
an entry in its hash table linking the source machine with the incoming LAN.
4. To handle dynamic topologies, whenever a hash table entry is made, the arrival
time of the frame is noted in the entry. The entry time is updated whenever a
frame from the address in that entry arrives.
5. Periodically, a process in the bridge scans the hash table and purges all entries
more than a few minutes old.

Routing procedure:
1. If destination and source LANs are the same, discard the frame.
2. If the destination and source LANs are different, forward the frame.
3. If the destination LAN is unknown, use flooding.
Special purpose VLSI chips exist to do the lookup and update the table entry, all
in a few microsecond. 106

Spanning Tree Bridges
To increase reliability, some sites use two or more bridges in parallel between
pairs of LANs. This arrangement creates loops in the topology.
What happens if a frame with an unknown destination arrives ?

The solution is for the bridges to communicate with each other and overlay the
actual topology with a spanning tree that reaches every LAN.
Large networks can be partitioned into multiple communicating spanning trees.
107

Spanning Tree Bridges (2)

(a) Interconnected LANs. (b) A spanning tree covering the
LANs. The dotted lines are not part of the spanning tree.
108

Remote Bridges

Remote bridges can be used to interconnect distant LANs.
109

Repeaters, Hubs, Bridges, Switches,
Routers and Gateways

(a) Which device is in which layer.
(b) Frames, packets, and headers.
110

Hubs, Bridges, and Switches

(a) A hub. Different from repeater, hubs do not amplify the incoming signals
and are designed to hold multiple line cards each with multiple inputs (one
collision domain). Like repeater, hubs do not examine the 802 addresses or
use them in any way.
(b) A bridge. A bridge has line cards (like a hub) for different network types and
speeds. However, each line is its own collision domain, in contrast to a hub.
(c) A switch. A switch is similar to bridge in its routing on frame addresses, but
is often used to connect individual computers (no collision).
111

Configure LANs logically rather than physically

A building with centralized wiring using hubs and a switch.
112

Virtual LANs

a) Four physical LANs organized into two VLANs, gray and white, by two
VLAN-aware bridges. A VLAN-aware bridge maintains configuration
tables which tell which VLANs are accessible via which lines.
b) The same 15 machines organized into two VLANs by VLAN-aware
switches.
113

The IEEE 802.1Q Standard

Transition from legacy Ethernet to VLAN-aware Ethernet. The shaded
symbols are VLAN aware. The empty ones are not.
114

The IEEE 802.1Q Standard (2)

The 802.3 (legacy) and 802.1Q Ethernet frame formats.
115

Summary

Channel allocation methods and systems for a common channel.
116

Mac layer

Recommandé

Recommandé

Contenu connexe

Tendances

Tendances (20)

Similaire à Mac layer

Similaire à Mac layer (20)

Dernier

Dernier (20)

Mac layer