Cellular Network Planning and Optimization Part11

8/12/2019 Cellular Network Planning and Optimization Part11

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The Medium Access ControlSublayer

Chapter 4

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The Medium Access ControlSublayer

• Network Classification1. Use point-to-point connections - most WANs,

except satellite.

2. Use broadcast channels - most LANs.

• This chapter deals with broadcast networks andtheir protocol.

• The objective is to allocate the channel to: – maximize channel utilization, and – minimize channel access delay.





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Static Channel Allocation• Using an M/M/1 queueing system, we will 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 = 100Mbps, and λ = 5000 frames/sec, then T = 1 /((1/10 4)(10 8) - 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 withcapacity C/N.Let: λ /N = arrival rate at each station (divide the load).Then, T FDM = 1 / (µ(C/N) - λ /N) = N / (µC - λ ) = N T.So, the N station case is N times worse than the 1station case.For example, as above, N = 10 TFDM = 2 msec. perframe.



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Dynamic Channel Allocation• Assumptions:

1. Station Model – N independent stations generateframes.

2. Single Channel – A single channel is available forall communication.

3. Collision – Frames that overlap in time destroyeach other; this is called a COLLISION. Allstations can detect collisions. The only errors arethose 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 intodiscrete 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 tellif 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 itis available. Then, the station listens to thechannel to see if a collision occurred. If theframe was destroyed, the station waits for arandom length of time and tries again.

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Pure ALOHA - Analysis

• Let FRAME TIME be the time required to transmit astandard, fixed-length frame; that is, (1/µC).

• Assume there is an infinite population of stations thattransmit frames according to a Poisson process with amean of N frames transmitted per frame time.

• Note, if N>1, the channel will not be able to handle theload. So, we expect 0<N<1. The offered load, G, is thetotal number of frames sent on the channel per frametime; that is, G = N + (number of retransmissions perframe time).



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Pure ALOHA - Analysis• Let P 0 = probability that no collisions occur during one

frame time; that is, a transmission is successful. Then:the throughput, S = GP 0

• Since the offered load is a Poisson distribution, withmean G, the probability that k frames are generatedduring a frame time is: Pr[k] = (G k * e -G) / k!

• So, Pr[0] = e -G. Thus, P 0 = e -2G , and it follows that S =Ge -2G

• Note, the maximum occurs when G = 0.5. S = 0.184.

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Pure ALOHA

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



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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 isnot permitted to send whenever a carriage return istyped 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 beforetransmitting a frame. Thus, P 0 = e -G (the vulnerable period is only one time slot). S = G e -G Note that as Gincreases, the number of collisions increasesexponentially.

• 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 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. nonpersisten CSMA:Channel Busy Wait for a random length of time and tryagain.Channel Idle Transmit.Collision Wait for a random length of time and try again.



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Carrier Sense Multiple Access (CSMA3. p-persistent CSMA:

Channel Busy Continue sensing until free (same as idle).Channel Idle Transmit with probability p, and defertransmitting until the next slot with probability q = 1-p.Collision Wait for a random length of time and try again.

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Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for variousrandom access protocols.



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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 contention

interval is modeled as a slotted Aloha systemwith slot with 2 τ.

• Then, 2 τ 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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CSMA with Collision Detection

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



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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.

• 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 channel efficiency d/(N + d) – The channel efficiency at high load dN /(N + dN) = d/(1 + d)

• Binary Countdown – stations overwrite the lownumbered 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 stationsP = 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 = k ρ(1 – ρ)k-1

↓

prob. That the other stations do not transmitdP/d ρ = –k ρ(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) = 0ρk = 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 countdownTwo 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 A,

C, E, and F want to transmitSlot 0 – All try CollisionSlot 1 2 Subtree try CollisionSlot 2 4 Subtree try No collision – ASlot 3 5 Subtree try No collision – CSlot 4 3 Subtree try CollisionSlot 5 6 Subtree try CollisionSlot 6 ESlot 7 FSlot 8 7 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 datachannel.

• Each channel is divided into groups of timeslots. All channels are synchronized by a singleglobal clock.

• The protocol support three traffic classes:1. Constant data rate connection-oriented traffic such as

uncompressed video2. Variable data rate connection-oriented traffic such as file

transfer 3. Datagram traffic such as UDP packet-oriented traffice



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Wavelength Division Multiple Access Protocols

Wavelength division multiple access.

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Wireless LAN Protocols

A wireless LAN. (a) A transmitting. (b) B transmitting.



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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• 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

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

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

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Ethernet Cabling

(a) Binary encoding, (b) Manchester encoding,(c) Differential Manchester encoding.



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Ethernet MAC Sublayer Protocol

Frame formats. (a) DIX Ethernet, (b) IEEE 802.3.

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Ethernet MAC Sublayer Protocol

Collision detection can take as long as 2 . τ



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Ethernet Performance

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

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Switched Ethernet

A simple example of switched Ethernet.



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Fast Ethernet

The original fast Ethernet cabling.

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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

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



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Wireless LANs• 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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The 802.11 Protocol Stack

Part of the 802.11 protocol stack.



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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 Protocol802.11 supports two modes of operation.

– DCF (Distributed Coordination Function), doesnot use any kind of central control (in that respect,similar to Ethernet).

– PCF (Point Coordination Function), uses the basestation to control all activity in its cell.

All implementations must support DCF but PCF is optional.



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The 802.11 MAC Sublayer ProtocolWhen DCF is employed, 802.11 uses a protocol called CSMAwith Collision Avoidance

– physical channel sensing• 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• if a collision occurs, the colliding stations wait a random

time using the Ethernet binary exponential backoffalgorithm.

– virtual channel sensing• other mode of CSMA/CA operation is based on MACAW

(Medium Access with Collion Avoidance for Wireless)

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The use of virtual channel sensing using CSMA/CA.



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A fragment burst.

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Interframe spacing in 802.11.



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The 802.11 Frame Structure

The 802.11 data frame.

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The 802.11 Frame StructureFrame Control field with 11 subfields

– Protocol version, which allows two versions of the protocol to operate atthe same time in the same cell.

– Type (data, control, or management) and Subtype fields (RTS or CTS). – To DS and From DS bits indicate the frame is going to or coming from

the intercell distribution system (e.g., Ethernet). – MF bit means that more fragments will follow. – Retry bit marks a retransmission of a frame sent earlier. – Power management bit is used by the base station to put the receiver into

sleep state or take it out of sleep state. – More bit indicates that the sender has additional frames for the receiver. – W bit specifies that the frame body has been encrypted using the WEP

(Wired Equivalent Privacy) algorithm. – O bit tells the receiver that a sequence of frames with this bit on must be

processed strictly in order.



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The 802.11 Frame Structure• Duration field, tells how long the frame and itsacknowledgement will occupy the channel

– for other stations manage the NAV mechanism• four addresses: source and destination hosts, and source

and destination base stations for intercell traffic.• Sequence allows fragments to be numbered: 12 bits

identify the frame and 4 bits identify the fragment.

• Data field contains the payload, up to 2312 bytes,followed by the usual Checksum.

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802.11 Services• Distribution Services

– Association• used by mobile stations to connect themselves to base stations

– Disassociation• station or base station may disassociate, breaking the relationship

– Reassociation• station changes its preferred base station

– Distribution• frames sent directly over the air or forwarded over the wired network to

the base station

– Integration• handles the translation from the 802.11 format to the format required by

the destination network



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802.11 Services• Intracell Services

– Authentication – Deauthentication – Privacy

• for encryption

– Data Delivery

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802.11 Services

• Intracell Services – Authentication – Deauthentication – Privacy – Data Delivery



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Broadband Wireless• 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 with 802.16

• 802.16• Less mobile• Much longer range• More users in each cell



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The 802.16 Protocol Stack

The 802.16 Protocol Stack.

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The 802.16 Physical Layer

The 802.16 transmission environment.



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The 802.16 Physical Layer

Frames and time slots for time division duplexing.

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• Service Classes – Constant bit rate service

• e.g. telephony

– Real-time variable bit rate service• e.g. compressed multi-media

– Non-real-time variable bit rate service• e.g. large file transfer

– Best efforts service• e.g. all the rest



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The 802.16 Frame Structure

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

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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



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Bluetooth Architecture

Piconets can be connected to form a scatternet.

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Bluetooth Applications

The Bluetooth profiles.



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The Bluetooth Protocol Stack

The 802.15 version of the Bluetooth protocol architecture.

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The Bluetooth Frame Structure

A typical Bluetooth data frame.F: Flow (receiver is full); A: Ack, S: Sequence





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Bridges from 802.x to 802.y

Operation of a LAN bridge from 802.11 to 802.3.

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Bridges from 802.x to 802.y

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



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Local Internetworking

A configuration with four LANs and two bridges.

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Spanning Tree Bridges

Two parallel transparent bridges.



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Spanning Tree Bridges

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

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Remote Bridges

Remote bridges can be used to interconnect distant LANs.



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Repeaters, Hubs, Bridges, Switches,

Routers and Gateways

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

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Repeaters, Hubs, Bridges, Switches,Routers and Gateways

(a) A hub. (b) A bridge. (c) a switch.



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Virtual LANs

A building with centralized wiring using hubs and a switch.

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Virtual LANs

(a) Four physical LANs organized into two VLANs, gray and white, by two bridges. (b) The same 15 machines organized into twoVLANs by switches.



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The IEEE 802.1Q Standard

Transition from legacy Ethernet to VLAN-aware Ethernet. The shadedsymbols are VLAN aware. The empty ones are not.

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The IEEE 802.1Q Standard

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



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Summary

Channel allocation methods and systems for a common channel.

Documents

Cellular Network Planning and Optimization Part11