Transcript The Medium Access Sublayer

Chapter 4
The Medium Access Control
Sublayer
1
The Medium Access Control
Sublayer
• Network Classification
1. Use point-to-point connections - most WANs,
except satellite.
2. Use broadcast channels - most LANs.
• This chapter deals with broadcast networks and
their protocol.
• The objective is to allocate the channel to:
–
–
maximize channel utilization, and
minimize channel access delay.
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Channel Allocation
• Static Channel Allocation in LANs and MANs
• Dynamic Channel Allocation in LANs and
MANs
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Static Channel Allocation
• In static channel allocation, a subchannel is
statically assigned to each station (computer or
terminal).
• For example, in FDM, a frequency band is
assigned to each station.
• This is inherently inefficient (w.r.t. channel
utilization) for bursty traffic.
• Note, however, the channel access delay is
minimal.
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Static Channel Allocation
• We use the queueing system to analyze the above
scheme.
• Single station case:
Let C = channel capacity (in bps)
T = mean time delay to send one frame (in sec.)
λ = arrival rate (in frames/sec.)
1/μ = mean frame size (in bits/frame)
From queueing theory, we obtain: T = 1 / (μ C - λ)
For example, if 1/ μ = 10,000 bits/frame, C = 100
Mbps, and λ = 5000 frames/sec, then T = 1 /
((1/104)(108) - 5000) = 1 / 5000 = 200 μsec. per frame.
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Static Channel Allocation
• N station case:
Divide the channel up into N subchannels, each with
capacity C/N.
Let: λ /N = arrival rate at each station (divide the load).
Then, TFDM = 1 / (μ(C/N) - λ/N) = N / (μC - λ) = N T.
So, the N station case is N times worse than the 1
station case.
For example, as above, N = 10 TFDM = 2 msec. per
frame.
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Dynamic Channel Allocation
• Assumptions:
1. Station Model – N independent stations generate
frames.
2. Single Channel – A single channel is available for
all communication.
3. Collision – Frames that overlap in time destroy
each other; this is called a COLLISION. All
stations can detect collisions. The only errors are
those caused by collisions.
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Dynamic Channel Allocation
• Assumptions:
4. Continuous Time means that transmission of
frames can begin at any time.
Slotted Time means that time is divided into
discrete intervals, and frame transmission always
begins at the start of a slot.
5. Carrier Sense means that stations can tell if the
channel is in use by listening to the channel.
No Carrier Sense means that stations cannot tell
if the channel is in use by listening to the channel.
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Pure ALOHA
• In the Pure Aloha Protocol (by Abramson in
1970s), a station transmits the data whenever
there is data to be sent. Then, the station listens
to the channel to see if a collision occurred. If
the frame was destroyed, the station waits for a
random length of time and tries again.
• Systems in which multiple users share a
common channel in a way that can lead to
conflicts are widely known as contention
systems.
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Pure ALOHA
In pure ALOHA, frames are transmitted at completely arbitrary times.
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Pure ALOHA - Analysis
• Let FRAME TIME be the time required to transmit a
standard, fixed-length frame; that is, (1/μC).
• Assume there is an infinite population of stations that
transmit frames according to a Poisson process with a
mean of N frames transmitted per frame time.
• Note, if N>1, the channel will not be able to handle the
load. So, we expect 0<N<1. The offered load, G, is the
total number of frames sent on the channel per frame
time; that is, G = N + (number of retransmissions per
frame time).
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Pure ALOHA - Analysis
• Let P0 = probability that no collisions occur during one
frame time; that is, a transmission is successful. Then:
the throughput, S = GP0
• Since the offered load is a Poisson distribution, with
mean G, the probability that k frames are generated
during a frame time is: Pr[k] = (Gk * e-G) / k!
• So, Pr[0] = e-G. The probability of no other traffic
during the vulnerable period is e-G e-G = e-2G. Thus, P0
= e-2G, and it follows that S = Ge-2G
• dS/dG = e-2G (-2G + 1) = 0  G = 0.5
• Note, the maximum occurs when G = 0.5. S = 0.184.
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FYI: Poisson Distribution
• The Poisson Distribution is used to model the number of
events occurring within a given time interval.
– It is often used to model events such as the number of
telephone calls at a business or the number of accidents at an
intersection in a specific time period.
– It is also useful in ecological studies, e.g., to model the
number of prairie dogs found in a square mile of prairie.
• The formula for the Poisson probability mass function
is:
p(x, λ) = e-λ λx / x! for x = 0, 1, 2, 3, ….
λ is a is the shape parameter which indicates the
average number of events in the given time interval.13
FYI: Poisson Distribution
• The following is the plot of the Poisson probability density
function for four values of λ.
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Pure ALOHA
Vulnerable period for the shaded frame.
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Slotted ALOHA
• In slotted Aloha (by Roberts in 1972) A computer is
not permitted to send whenever a carriage return is
typed but wait for a time slot.
• Time is divided into fixed slots of one frame time each.
A station waits until the start of the next slot before
transmitting a frame. Thus, P0 = e-G (the vulnerable
period is only one time slot). S = G e-G Note that as G
increases, the number of collisions increases
exponentially.
• Note, the maximum occurs when G = 1. S = 0.368.
• Slotted Aloha can be used to allocate a shared cable
channel.
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Pure ALOHA
Throughput versus offered traffic for ALOHA systems.
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Carrier Sense Multiple Access (CSMA)
• Protocols in which stations listen for a carrier (i.e.
transmission) and act accordingly are called carrier
sense protocols.
1. 1-persistent CSMA
Channel Busy  Continue sensing until free and then grab.
Channel Idle  Transmit with probability 1.
Collision  Wait for a random length of time and try again.
2. nonpersistent CSMA:
Channel Busy  Does not continually sense the channel. Wait
for a random length of time and try again.
Channel Idle  Transmit.
Collision  Wait for a random length of time and try again.
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Carrier Sense Multiple Access (CSMA)
3. p-persistent CSMA:
Channel Busy  Continue sensing until free (same as idle).
Channel Idle  Transmit with probability p, and defer
transmitting until the next slot with probability q = 1-p.
Collision  Wait for a random length of time and try again.
• The nonpersistent CSMA has better channel utilization but
longer delays than 1-persistent CSMA.
• CSMA are an improvement over ALOHA because they ensure
that no station begins to transmit when it senses the channel
busy.
• Another improvement is for stations to abort their
transmissions as soon as they detect a collision. Quickly
terminating damaged frames saves time and bandwidth. This
protocol is called CSMA/CD (CSMA with Collision
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Detection).
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various
random access protocols.
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CSMA with Collision Detection
CSMA/CD can be in one of three states: contention,
transmission, or idle.
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CSMA with Collision Detection
• Stations detect collisions using analog hardware
and abort transmissions immediately.
• Let τ be the propagation delay (the time for a
signal to propagate between the two farthest
stations be τ). The contention interval is
modeled as a slotted Aloha system with slot
width 2τ.
• Then, 2τ is the time required for a station to
detect collision with certainty.
For example, on a 1-km long coaxial cable, τ = 5
μsec. 2τ = 10 μsec to detect a collision.
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Collision-Free Protocols
• Assumptions:
– There are N stations, uniquely numbered 0, 1, 2, …, N – 1.
– Each contention period consists of N slots.
– Data frames consist of d time units.
1. Basic Bit-Map Protocol
– Low load  bit map is simply repeated over and over.
• Low # stations: wait ~ 1.5N slots
• High # stations: wait ~ 0.5N slots
– The overhead to transmit a data frame is N bits. The
channel efficiency at low load is d/(N + d)
– The channel efficiency at high load dN /(N + dN) = d/(1 +
d)
2. Binary Countdown – stations overwrite the low
numbered stations, and low numbered stations give up.
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Collision-Free Protocols
The basic bit-map protocol.
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Collision-Free Protocols
The binary countdown protocol. A dash indicates silence.
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Limited-Contention Protocols
• Contention  Limited  Collision free
(CSMA)
contention (Binary Countdown)
small delay for
Good channel efficiency
low load stations
at high load
• Analysis: Contention protocols (symmetric case)
Let k = # of stations
P = prob. that one station successfully requires the
channel during a given slot.
ρ = prob. that a station transmits a frame during a
given slot.
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Limited-Contention Protocols
• Analysis: Contention protocols (Symmetric case)
P = kρ(1 – ρ)k-1
↓
prob. That the other stations do not transmit
dP/dρ = –kρ(k – 1)(1 – ρ)k-2 + k(1 - ρ)k-1
= k(1 – ρ)k-2 [(ρ(k – 1) + (1 – ρ)]
= k(1 – ρ)k-2 (-ρk + ρ + 1 – ρ) = 0
 ρk = 1  ρ = 1/k
P = k (1/k)(1 – 1/k)k-l = [(k – 1)/k]k-1
(4.4)
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Limited-Contention Protocols
Acquisition probability for a symmetric contention channel.
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Limited-Contention Protocols
• Idea: Limit the number of stations contending
for a slot.
• Question: How to assign stations to a slot?
• Static assignment:
One station/slot (group) Binary countdown
Two stations/slot
P = 1 – ρ2 ≈ 1
(ρ  0 collision is small)
All stations/slot
Slotted ALOHA
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Adaptive Tree Walk Protocol
• Use the algorithm devised by U.S. Army test for
syphilis in 1943)
• Example: There are 8 stations. Suppose that stations 0,
2, 4, and 5 want to transmit
Slot 0 – All try Collision
Slot 1 B Subtree try  Collision
Slot 2 D Subtree try No collision – 0
Slot 3 E Subtree try No collision – 2
Slot 4 C Subtree try Collision
Slot 5 F Subtree try Collision
Slot 6 4
Slot 7 5
Slot 8  G Subtree try  No Collision
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Adaptive Tree Walk Protocol
The tree for eight stations.
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Wavelength Division Multiple Access Protocols
• Each station is assigned two channels: a narrow
band control channel and a wide band data
channel.
• Each channel is divided into groups of time
slots. All channels are synchronized by a single
global clock.
• The protocol support three traffic classes:
1. Constant data rate connection-oriented traffic such as
uncompressed video.
2. Variable data rate connection-oriented traffic such as file
transfer.
3. Datagram traffic such as UDP packet-oriented traffic. 32
Wavelength Division Multiple Access Protocols
Wavelength division multiple access.
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Wireless LAN Protocols
• Because signal strength is not uniform throughout the space in
which wireless LANs operate, carrier detection and collision
may fail in the following ways:
- Hidden nodes:
• Hidden stations: Carrier sensing may fail to detect another station.
For example, A and D.
• Fading: The strength of radio signals diminished rapidly with the
distance from the transmitter. For example, A and C.
- Exposed nodes:
• Exposed stations: B is sending to A. C can detect it. C might want to
send to E but conclude it cannot transmit because C hears B.
• Collision masking: The local signal might drown out the remote
transmission.
• The result scheme is carrier sensing multiple access with
collision avoidance (CSMA/CA).
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Wireless LAN configuration
A
B
C
Laptops
radi o obs truc tion
Palmtop
Server
D
E
Wireless
LAN
Bas e station/
acc es s point
LAN
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Wireless LAN Protocols
• Hidden station problem: A is transmitting to B. C
cannot hear A. If C starts transmitting, it will interfere
at B.
• Exposed station problem: B is transmitting to A. C
concludes that it may not send to D but the interference
exists only between B and C.
A wireless LAN. (a) A transmitting. (b) B transmitting.
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MACA and MACAW
• The sender transmits a RTS (Request To Send) frame.
• The receiver replies with a CTS (Clear To Send)
frame.
• Neighbors
– see CTS, then keep quiet.
– see RTS but not CTS, then keep quiet until the CTS is back
to the sender.
• The receiver sends an ACK when receiving an frame.
– Neighbors keep silent until see ACK.
• Collisions
– There is no collision detection.
– The senders know collision when they don’t receive CTS.
– They each wait for the exponential backoff time.
• MACAW (MACA for Wireless) is a revision of
MACA.
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Wireless LAN Protocols
The MACA protocol. (a) A sending an RTS to B.
(b) B responding with a CTS to A.
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Ethernet
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Ethernet Cabling
Manchester Encoding
The Ethernet MAC Sublayer Protocol
The Binary Exponential Backoff Algorithm
Ethernet Performance
Switched Ethernet
Fast Ethernet
Gigabit Ethernet
IEEE 802.2: Logical Link Control
Retrospective on Ethernet
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Ethernet Cabling
• 10Base2 means that is operates at 10 Mbps, uses
baseband signaling, and support segments up to 200
meters.
• 10Base-T became dominant due to its use of existing
wiring and the ease of maintenance .
The most common kinds of Ethernet cabling.
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Ethernet Cabling
Three kinds of Ethernet cabling.
(a) 10Base5, (b) 10Base2, (c) 10Base-T.
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Ethernet Cabling
• Different ways of wiring a building are shown as
follows:
Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.
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Ethernet Cabling
• Devices used for connecting Ethernet cable:
– A repeater is a physical layer device that receives,
amplifies and retransmits the signal.
– A hub will take a network packet and transmit it to
all other ports on the hub.
– A switch will take a network packet and transmit it
only to a specific port on the switch that the packet
is addressed to. So, a switch greatly reduces
network transmissions thus providing better
network throughput.
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Ethernet Cabling
• All Ethernet systems use Manchester encoding due to
its simplicity (high at 0.85 V and low at -0.85 V).
(a) Binary encoding, (b) Manchester encoding,
(c) Differential Manchester encoding.
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Ethernet MAC Sublayer Protocol
• Preamble – used for sender and receiver to synchronize their
clock.
• Addresses
–
–
–
–
unique, 48-bit unicast address assigned to each adapter
example: 8:0:e4:b1:2
broadcast: all 1s, the set of all recipient nodes
Multicast: first bit is 1,a group of recipient nodes
Frame formats. (a) DIX Ethernet, (b) IEEE 802.3.
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Ethernet Transmit Algorithm
• If line is idle:
– send immediately
– upper bound message size of 1500 bytes
– must wait some time (9.6µs) between back-to-back
frames
• If line is busy:
– wait until idle and transmit immediately
– called 1-persistent (a frame to send transmits with
probability 1)
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The Binary Exponential Backoff Algorithm
• If collision
– Time is divided into discrete slots whose length is equal to
the worst-case round-trip propagation time on the ether (2τ).
• minimum frame is 64 bytes (header + 46 bytes of data) =
512 bits
• Channel capacity 10 Mbps, 512/10 M = 51.2µ
– delay and try again (binary exponential backoff)
• 1st time: waits 0 or 1 slotted time (51.2µs)
• 2nd time: waits 0, 1, 2, or 3 slotted time
• nth time: k slotted time, for randomly selected k=0..2n - 1
• Frozen at 1023 slots and give up after several tries
(usually 16)
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Ethernet MAC Sublayer Protocol
Collision detection can take as long as 2 .
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Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times.
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Switched Ethernet
• To deal with increased load, the switched Ethernet is devised
with a switch to route the frame to the destination.
A simple example of switched Ethernet.
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Fast Ethernet
• The 802.3 committee decided to keep 802.3 for the
fast Ethernet (802.3u).
– Backward compatible
– A new protocol might have problems.
– Get job done before the technology changed.
• 100Base-T4 – 4 twisted pairs achieve 100 Mbps.
The original fast Ethernet cabling.
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Gigabit Ethernet
• 802.3z is the standard for gigabit Ethernet.
• All configurations of gigabit Ethernet are point-topoint rather than multidrop with two possible
configurations:
– Two computers are directly connected to each other.
– Multiple computers are connected through a switch
and a hub.
• Two operation mode:
– Full-duplex – connected by a switch. CSMA/CD is not used.
– Half-duplex – connected by a hub.
• 802.3z increase the radius by carrier extension
(padding to extend the frame to 512 bytes).
• Gigabit Ethernet supports flow control.
• 802.3ae is 10-gigabit Ethernet standard.
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Gigabit Ethernet
(a) A two-station Ethernet. (b) A multistation Ethernet.
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Gigabit Ethernet
Gigabit Ethernet cabling.
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IEEE 802.2: Logical Link Control
• All Ethernet protocols offer a best-efforts datagram
service.
• LLC (Logical Link Control) forms the upper half of
the data link layer. MAC forms the lower sublayer.
– error-controlled, flow-controlled
– Adds an LCC header, containing sequence and
acknowledgement numbers.
• LLC provides three service options:
– Unreliable datagram service
– Acknowledged datagram service
– Reliable connection-oriented service
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IEEE 802.2: Logical Link Control
(a) Position of LLC. (b) Protocol formats.
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Retrospective on Ethernet
• Ethernet has been around for over 20 years.
• The main reason for its success is simple and flexible.
– Simple translates into reliable, cheap, and easy to
maintain.
• IP is a connectionless protocol, so it fits perfectly with
Ethernet. IP fits much less well with ATM.
• Ethernet evolves with time. FDDI, Fibre Channel, and
ATM were faster when introduced, but they are
incompatible with Ethernet, complex, and hard to
manage. Only ATM is used within the telephone
system.
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Wireless LANs
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•
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•
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The 802.11 Protocol Stack
The 802.11 Physical Layer
The 802.11 MAC Sublayer Protocol
The 802.11 Frame Structure
Services
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802.11 Physical Layers
• IEEE 802.11 physical layers:
– Infrared – 1 Mbps and 2 Mbps
– FHSS (Frequency Hopping Spread Spectrum) uses
79 channels, each 1 MHz wide, starting in the 2.4
GHz band.
• A psudorandom number generator is used to produce
the sequence of frequencies hopped to.
• The amount of time spent at each frequency, dwell time,
is adjustable.
– DSSS (Direct Sequence Spread Spectrum) delivers
1 or 2 Mbps in the 2.4 GHz band.
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802.11 Physical Layers
• IEEE 802.11 physical layers:
– 802.11a uses OFDM (Orthogonal Frequency
Division Multiplexing) to deliver up to 54 Mbps in
the 5 GHz band.
– 802.11b uses HR-DSSS (High Rate Direct
Sequence Spread Spectrum) to achieve 11 Mbps in
the 2.4 GHz band.
– 802.11g uses OFDM to achieve 54 Mbps in the 2.4
GHz band.
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The 802.11 Protocol Stack
Part of the 802.11 protocol stack.
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Wireless LAN Standard
Standard Modulation
Spectrum
Max physical Working
Rate
distance
2 Mbps
≈100 m
802.11a
WDM, FHSS 2.4 GHz
DSSS
OFDM
5 GHz
54 Mbps
≈ 50 m
802.11b
HR-DSSS
2.4 GHz
11 Mbps
≈ 200 m
802.11g
OFDM
2.4 GHz
54 Mbps
≈ 200 m
802.11
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Wireless LANS Devices
a)wireless router
b) wireless network
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card
802.11 MAC Sublayer
• Problems: Hidden and exposed stations
• 802.11 supports two modes of operation:
– DCF (Distributed Coordination Function) does not use
any kind of central control.
– PCF (Point Coordination Function) uses the base station
to control all activity in its cell. Optional.
• CSMA/CA (CSMA with Collision Avoidance) is
based on MACAW.
– Use NAV (Network Allocation Vector) to indicate
the channel is busy.
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The 802.11 MAC Sublayer Protocol
(a) The hidden station problem.
(b) The exposed station problem.
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The 802.11 MAC Sublayer Protocol
The use of virtual channel sensing using CSMA/CA.
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The 802.11 MAC Sublayer Protocol
• The deal with the problem of noisy channels,
802.11 allows frames to be fragmented.
A sequence of fragments is called a fragment burst.
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The 802.11 MAC Sublayer Protocol
• In the PCF mode, the base station polls the other
stations, asking them if they have any frames to send.
– Completely controlled by the base station. No collisions
occur.
– A beacon frame which contains system parameters is
periodically (10 to 100 times per second) broadcasted to
invite new stations to sign up for polling service.
• PCF and DCF can coexist within one cell by
carefully defining the interframe time interval.
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The 802.11 MAC Sublayer Protocol
• The four intervals are depicted:
– SIFS (Short InterFrame Spacing) is used to allow the
parties in a single dialog the chance to go first including
letting the receiver send a CTS and an ACK and the sender
to transmit the next fragment.
– PIFS (PCF InterFrame Spacing) is used to allow the base
station to send a beacon frame or poll frame.
– DIFS (DCF InterFrame Spacing) is used to allow any
station to grab the channel and to send a new frame.
– EIFS (Extended InterFrame Spacing) is used only by a
station that has just received a bad or unknown frame to
report the bad frame.
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The 802.11 MAC Sublayer Protocol
Interframe spacing in 802.11.
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802.11 MAC Sublayer
• Three different classes of frames: data, control,
management.
• The fields of data frame:
– Frame control – 2 bytes and 11 subfields
– Duration – 2 bytes indicate how long the frame and its
acknowledgement will occupy the channel
– Addresses – 24 bytes identify source and destination
addresses and source and destination addresses of base
stations
– Sequence – 12 bits identify the frame and 4 identify the
fragment
– Data – the payload up to 2312 bytes
– Checksum – 4 bytes
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802.11 MAC Sublayer
• The Frame control field:
– Version – allows two versions of the protocol operating at the
same time
– Type – data, control, management
– Subtype – RTS, CTS
– To/Frame DS indicate the frame is going to or coming from
the intercell distribution
– MF – more fragments will follow
– Retry – marks a retransmission frame
– Power management – puts the receiver into sleep stateor take
it out of sleep state
– The W bit specifies that the frame body has been encrypted
using the WEP (Wired Equivalent Privacy) algorithm.
– The O bit tells a sequence of frames with this bits must be
processed strictly in order.
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The 802.11 Frame Structure
The 802.11 data frame.
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802.11 Services
• Distribution Services
– Association – mobile stations connect themselves to
base stations
– Disassociation – the station or base station breaks
the association
– Reassociation – a station may change its preferred
base station
– Distribution – determines how to route frames sent
to the base station
– Integration – handles the translation from the
802.11 format to the format of the destination
network
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802.11 Services
• Intracell Services
– Authentication – a station must authenticate itself
before permitted to send data.
– Deauthentication – a authenticated station wanting
to leave the network is deauthenticated.
– Privacy – manages the encryption and decryption.
The algorithm specified is RC4 by Ronald Rivest of
MIT.
– Data Delivery – not reliable.
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Broadband Wireless
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•
•
•
•
Comparison of 802.11 and 802.16
The 802.16 Protocol Stack
The 802.16 Physical Layer
The 802.16 MAC Sublayer Protocol
The 802.16 Frame Structure
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Comparison of 802.11 and 802.16
• Similarity - Provide high-bandwidth wireless
communications.
• Differences
• 802.16 provides service to stationary buildings which can
have multiple computers. High quality full-duplex link is
used. Higher cost is affordable. 802.11 provides service to
individual mobile users.
• Longer transmission range security/privacy
• More user in each cell more spectrum is needed, operate in
10-66 GHz absorbed by water
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The 802.16 Protocol Stack
• Physical layer
– Physical medium dependent sublayer – narrow-band radio
(which means that it contains all of its power in a very
narrow portion of the radio frequency bandwidth, prone to
interference ) is used with conventional modulation schemes.
– Transmission convergence sublayer – hide the different
physical medium technologies from the data link layer.
• Data link layer
– Security layer – deal with privacy and security
– MAC sublayer common part – channel management
– Service specific convergence sublayer – integrate with both
datagram protocols (PPP, IP, and Ethernet) and ATM.
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The 802.16 Protocol Stack
The 802.16 Protocol Stack.
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The 802.16 Physical Layer
• Physical media:
– 10-to-66 GHz millimeter waves travel in the straight line.
– Transfer up to 155 Mbps
– 30 miles range
• The base station has multiple antennas, each pointing
at a different sector.
• The signal-to-noise ratio drops sharply with distance.
Three modulation schemes are used depending on
distance:
– QAM-64 (6 bits/baud)
– QAM-16 (4 bits/baud)
– QPSK (2 bits/baud)
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The 802.16 Physical Layer
The 802.16 transmission environment.
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Broadband Wireless
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The 802.16 Physical Layer
• Unlike equal bandwidth allocation in the cell phone
system, more bandwidth is allocated for downstream
than upstream traffic.
• Two schemes are used to allocate the bandwidth:
– FDD (frequency division duplex)
– TDD (time division duplex) – downstream, guard, and
upstream time slots
• The Hamming code is used for forward error
correction.
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The 802.16 Physical Layer
Frames and time slots for time division duplexing.
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The 802.16 MAC Sublayer Protocol
• Base station sends out frames
• Each frame includes a number of subframes, which
include a number of time slots
• The first two subframes are the downstream and
upstream maps, which tell what is in which time slot.
• Downstream subframes (channels) are straight
forward. The base station decides what to send
• Upstream channel is more complex, due to
competition.
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The 802.16 MAC Sublayer Protocol
• Four classes of service are defined and are connectionoriented:
– Constant bit rate service: transmit uncompressed voice
(similar to a T1 channel). Certain time slots are dedicated to
each user.
– Real-time variable bit rate service: compressed multimedia
and other soft real-time applications (bandwidth needed at
each instant may vary). Polling is used.
– Non-real-time variable bit rate service: e.g., file transfer.
• User can request a poll to send constant rate
• If a station does not respond to a poll K times, the base station put it in
a multicast group. The user will content for service.
– Best efforts service
• No polling. User compete for channel in the time slots marked inthe
upstream map for contention.
• If a request is successful, it will be noticed in the next downstream
map.
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The 802.16 MAC Sublayer Protocol
• Generic frame:
– EC: tells whether the payroll is encrypted
– Type: frame type (fragmentation)
– CI: whether checksum presents (optional as FEC in physical
layer)
– EK: which encryption key is used
– Length: complete length including header
– Connection ID: which connection this frame belongs to
– CRC: checksum over header only
• Bandwidth request frame: indicate bytes needed
87
The 802.16 Frame Structure
(a) A generic frame.
(b) A bandwidth request frame.
88
Bluetooth
•
•
•
•
•
•
•
Bluetooth Architecture
Bluetooth Applications
The Bluetooth Protocol Stack
The Bluetooth Radio Layer
The Bluetooth Baseband Layer
The Bluetooth L2CAP Layer
The Bluetooth Frame Structure
89
Bluetooth
• In 1994, Ericsson was interested in connecting mobile
phone with other devices without cables
• Started by Ericsson, a SIG was formed with four other
companies (IBM, Intel, Nokia, and Toshiba) to develop
a wireless standard for short range, low power, and
inexpensive wireless radios.
• This project was named as Bluetooth, after Harald
Blaatand (Bluetooth), a Viking king who unified
Denmark and Norway, also without cable
• In 1999, 1500 page specification of V1.0
• IEEE 802.15 adopts the Bluetooth as a personal area
network standard.
90
Bluetooth Architecture
• The basic unit of Bluetooth is piconet, including a
master node, and up to seven active slave nodes within
10 m, and up to 255 parked nodes
• Multiple piconets can coexist. An interconnected
collection of piconets is called a scatternet.
91
Bluetooth Architecture
Two piconets can be connected to form a scatternet.
92
Bluetooth Applications
The Bluetooth profiles.
93
The Bluetooth Protocol Stack
• The Bluetooth Protocol Stack does not follow OSI
model and revised by IEEE to fix 802.11.
• The Physical Radio layer corresponds to the ISO
physical layer and deals with radio transmission and
modulation.
• The Baseband layer corresponds to MAC sublayer and
some physical layer and handles how the master
control time slots.
• The link manager handles establishment of logic
channel (power management, authentication, QoS).
• The logical link control adaptation protocol (L2CAP)
shields the upper layer from lower layer
94
The Bluetooth Protocol Stack
• Audio and Control protocols deal with audio and
control.
• LLC (Logical Link Control) is inserted by IEEE to
make it compatible with other 802.
• RFcomm (Radio Frequency communication) emulates
serial port for connecting mouse, keyboard, modem.
• Telephony protocol real-time speech, call setup and
termination
• The service discovery protocol is used to locate service
in a network.
• Each application uses a specific subset of protocols
95
The Bluetooth Protocol Stack
The 802.15 version of the Bluetooth protocol architecture.
96
The Bluetooth Frame Structure
• The address field identifies which of the eight active
devices the frame is intended for.
• The type field identifies the frame type.
• The flow bit is asserted by a slave when its buffer is
full.
• The acknowledge bit is used to piggyback an ACK.
• The sequence bit is used to number the frames to detect
retransmissions.
97
The Bluetooth Frame Structure
A typical Bluetooth data frame.
98
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
99
Bridges
• Connect LANs
• Do not exam payload of the frames (so it can support
any L3 protocols, IPv4, IPv6, etc)
• In contrast, routers exam addresses in packets
100
Data Link Layer Switching
Multiple LANs connected by a backbone to handle a total load higher
than the capacity of a single LAN.
101
Bridges from 802.x to 802.y
Operation of a LAN bridge from 802.11 to 802.3.
102
Bridges from 802.x to 802.y
• Difficulties
– Different frame format, copying data takes CPU
time
– Different data rate (gigabit Ethernet 11 Mbps
WLAN)
– Different maximum frame length
segmentation/reassembly
– Security: 802.11/802.16 support encryption in L2,
but Ethernet doesn’t.
– QoS: 802.11/802.16 support, but Ethernet doesn’t.
103
Bridges from 802.x to 802.y
The IEEE 802 frame formats. The drawing is not to scale.
104
Local Internetworking
• Connecting multiple LANs (same network)
– Accept all frame
– Decide to discard it or forward it, by looking up the
destination address in a big table
• If destination is on the same LAN, discards it
• If destination is in on a different LAN, forward to that
LAN
• If destination is unknown, flooding
– At beginning, floods all frames
– Learn the destinations by looking at the source
address of received frames
– Each entry in the table has a life time
105
Local Internetworking
A configuration with four LANs and two bridges.
106
Spanning Tree Bridges
Two parallel transparent bridges.
107
Spanning Tree Bridges
(a) Interconnected LANs. (b) A spanning tree covering the
LANs. The dotted lines are not part of the spanning tree.
108
Remote Bridges
• Connect remote LANs by point-to-point line
• PPP can be used on the point-to-point lines
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
Repeaters and Hubs
• Repeater
– An analog device connecting two cable
– A signal appearing on one cable is amplified and put
out on the other
– E.g., classic Ethernet allow four repeaters
• Hub
–
–
–
–
Has a number of input lines that it joins electrically
Frames arriving on any of the lines are sent out on all others
Entire hub is a collision domain
All the lines coming into a hub must operate at the same
speed
– Don’t amplify signal
– Both hub and repeat don’t exam MAC addresses
111
Bridges and Switches
• Bridge
– Connects two or more LANs
– Maintains a table including hosts attached in the LANs
– When bridge receives frame, software in bridge extracts the
destination MAC address and forward frames
– May have different speed lines
• Switch
–
–
–
–
–
Similar to bridge, check MAC address
Switch is used to connect individual computers
No frame loss due to collision (if with buffer)
If frame come in faster than it can handle frame loss
Cut-through Switch: start forwarding frames as soon as the
destination address field comes in
112
Routers and Gateways
• Router
– Payload packet is extracted and passed to routing software
– A out-going line is chosen based on L3 address
• Gateway
– Transport gateway: connect two computers using different
connection-oriented transport protocols (TCP, ATM)
– Application gateway: translate format and contents of data.
E.g., email gateway: translate Internet message into SMS for
mobile phones.
113
Repeaters, Hubs, Bridges, Switches,
Routers and Gateways
(a) A hub. (b) A bridge. (c) a switch.
114
Virtual LANs
A building with centralized wiring using hubs and a switch.
115
Virtual LANs
• Network administrators like to group users on LANs to
reflect organizational structure. Issues are:
– Security: prevent one group from sniffing the data of another
group.
– Load: separate load, e.g., prevent the heavy traffic of
research group to affect the communication in accounting
group
– Broadcasting storm
• It can be done by plugging lines to hubs carefully. But
when one changes office frequently or changes group
frequently, the overhead comes.
• Solution: Virtual LAN (VLAN) is a way to rewire
building entirely in software.
116
VLAN
• VLAN is based on specially designed VLAN-aware
switches (or bridges). Administrators determine:
– How many VLANs needed
– Which computer on which VLAN
– What’s the names for VLANs (color)
• Configuration tables have to be set up in the switches (or
bridges)
– Tells which VLAN is accessible via which port
– E.g., a frame from gray VLAN must be forwarded on all the
ports marked G.
– A port may be labeled with multiple VLAN colors
117
Virtual LANs
(a) Four physical LANs organized into two VLANs, gray and white,
by two bridges. (b) The same 15 machines organized into two
VLANs by switches.
118
VLAN
• How to know the coming frame in which VLAN
– Ports - Only works if all machines on a port belong to the same
VLAN
– MAC address - Notebook can be docked in any place where
the dock station has aMAC address MAC address does not
always reflect the user
– IP address - Solve the problem. However, has to check the IP
address violate the fundamental rule in networking:
independence of layers
• Solution
– Put a label in MAC header, so that no need to check IP
– For 802.11 and 802.16, easy
– For 802.3, difficult because there is no free space in its header
119
802.1Q
The IEEE 802.1Q Standard
Transition from legacy Ethernet to VLAN-aware Ethernet. The shaded
symbols are VLAN aware. The empty ones are not. 120
The IEEE 802.1Q Standard
The 802.3 (legacy) and 802.1Q Ethernet frame formats.
121
Summary
122
Channel allocation methods and systems for a common channel.