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WIMAX

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WIMAX

WIMAX (Worldwide Interoperability for Microwave Access ):

Protocol of communication network without wire, basedon the standard IEEE 802.16

Allows communications over long distances than WiFi,

and a greater bandwidth. Cover approximately 40km.

Field of application:

Better price points for both home and business

customers.

WIMAX allow competitors joint access to any subscriberin areas without preexisting physical cable or telephone

networks

would allow gamers access to ad hoc local networks of

other players with the same gear- without any internetaccess.

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Introduction

Goal of WIMAX:Provide high-speed Internet access to home andbusiness subscribers, without wires.

Frequency range:

10-66 GHz and sub 11 GHz

Supports: Legacy voice systems

Voice over IP TCP/IP Applications with different QoS requirements.

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Introduction

802.16 consists of the access point, BS(Base Station) and

SSs (Subscriber Stations).

All data traffic goes through the BS, and the BS control

the allocation of bandwidth on the radio channel.

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Introduction

During a communication, all the informationcoming from a SS go to the BS and are

retransmit to the right SS.

Base stations (BS) can handle thousands ofsubscriber stations (SS).

Two type of link are defined: The downlink: From the BS to the SS.

The uplink: From the SS to the BS.

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Introduction

Infrastructure of WIMAX

A WIMAX tower: similar in concept to a cell-phone

tower. A single WIMAX tower can provide coverage

to a very large area (~8,000 km).

A WIMAX receiver : The receiver and antenna could

be a small box or PCMCIA card, or could be built into

a laptop.

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Introduction

A WIMAX tower An example of WIMAX receiver

: PCMCIA card

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PLAN

I Transmission of the data

II ARQ

III Scheduling

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PLAN


II ARQ

III Scheduling

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1 Mac and physical layers

2 Structure of a SDU

3 structure of a PDU

4 fragmentation and packing

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The physical level:

Specify the frequencies

diagram of modulation

synchronizations

speeds,

techniques of cutting in the time (of type

TDMA: Time Division Multiple Access)

techniques of detection and correction oferror.

1Mac and physical layers

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The MAC level :

Located at the top of the physical level, itmanages the allowance of the slots and usesthe method Rammed-tdma.

The interface of communication with theapplications :

this layer concentrates on the management oflevel IP and the encapsulation of packages IP inthe screen adapted to the section of time.

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The MAC is comprised of three sublayers.

The Service Specific Convergence Sublayer (CS)provides any transformation or mapping of external

network data, received through the CS service

access point (SAP), into MAC SDUs received by the

MAC Common Part Sublayer (MAC CPS) through theMAC SAP.

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2 structure of a SDU

Higher-layer PDUs shall be encapsulated in the MAC SDU format.

For some payload protocols, each payload consists of an 8-bit payloadheader suppression index (PHSI) field followed by actual payload.

Other protocols map the higher layer PDU directly to the MAC SDU. A

value of zero in the PHSI indicates no payload header suppression hasbeen applied to the PDU.

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3 Structure of a MPDU

MSB LSB

Generic MAC header Payload (optional) CRC (optional)

The maximum length of the MAC PDU is 2048 bytes,including header, payload, and Cyclic Redundancy Check

(CRC).

6 bytes Variable 4 bytes

The size of the payload is variable, the payload can contain

either data or management message.

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Two MAC header formats are defined:

Generic MAC Header that begins each MACPDU containing either MAC managementmessages or CS data.

Bandwidth Request Header used by the SSto request additional bandwidth.

The single-bit Header Type (HT) field

distinguishes this two header formats: HT=0 for Generic Header.

HT=1 Bandwidth Request Header.

Two MAC header formats are defined:

Generic MAC Header that begins eachMAC PDU containing either MACmanagement messages or CS data.

Bandwidth Request Header used by the SSto request additional bandwidth.

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The single-bit Header Type (HT) field

distinguishes this two header formats:

HT=0 for Generic Header.

HT=1 Bandwidth Request Header.

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4 Fragmentation and packing

To allow efficient use of the airlink resource, two

functions are included in the MAC layer:

Packing:

Several small MSDUs addressed to the same CID may

be concatenated by the transmitter to form a singleMPDU. At the reception, the SDU is reassembled by the

MAC layer.

Fragmentation:MSDU might be fragmented by the transmitter to form

several MPDUs. At the reception the SDUs are

separated by the MAC layer.

4 F t ti d ki

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Reasons:

Lack of the frame time when allocatingthe air time to the given MSDU

High BER that requires employing

integrity check for smaller data blocks

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The number of fragments can not be more than 16.

When created, the MAC payload (MPDU) areassigned by:

- Fragment Serial Number (FSN) with possible

value 0 to 15.- Fragment Control code (FC) with the followingmeaning:

o 00 = non-fragmented MPDU.

o 01 = last fragment.

o 10 = first fragment.o 11 = continuing (middle) fragment.

The FSN is always transmitted within the same MAC

message as the fragment data.

4

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The sequence number allows the SS to recreate the

original payload and to detect the loss of any intermediate

packets.

Upon loss, the SS shall discard all MAC PDUs on theconnection until a new first fragment is detected or a non

fragmented MAC PDU is detected.

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If packing is turned on for a connection, the MAC

may pack multiple MAC SDUs into a single MAC

PDU. Packing makes use of the connection attribute

indicating whether the connection caries fixed-

length or variable-length packets.

The transmitting side has full discretion whether or

not to pack a group of MAC SDUs in a single MAC

PDU.

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Decrease the MAC overhead

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Packing and fragmentation can occur

in the same PDU.

Summarize

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Summarize

SDU 1 SDU 2 SDU 3 SDU 4

PDU 1 PDU 2 PDU 3

NO fragmentation

& NO packing Packing Fragmentation

MAC SDUs

MAC

PDUs

P

FEC 1 FEC 2 FEC 3Preamble

TC

PDUs

BURST

FEC encoding

PDU which has started in the

previous TC packet

First PDU which starts in this

TC packet

Second PDU which starts in

this TC packet

II ARQ

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II ARQA ARQ

1. Stop and Wait

2. Sliding window technique

3. Feedback ( go back-N)

4. Selective repeat

B FEC

C H-ARQ

D Methods used by WIMAX

II ARQ

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II ARQ

Three methods are employed to makes the data

transmission reliable in a unreliable connection( airlink):

ARQ ( automatic repeat request)

FEC (Forward Error Correcting)

H-ARQ (hybrid ARQ= ARQ+FEC)

II ARQ

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II ARQA ARQ

1. Stop and Wait



4. Selective repeat

B FEC

C H-ARQ


II ARQ

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II ARQ

Three methods are employed for the ARQ

wireless transmissions:

Stop and Wait

Feedback ( go back-N)

Selective repeat

Both feedback and selective algorithm arebased on sliding window technique

II ARQ

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II ARQ

A ARQ1. Stop and Wait



4. Selective repeat

B FEC

C H-ARQ


Stop and Wait

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Stop and Wait

to each reception of a package, the receiversends a particular message (ACK) to showreception.

the transmitter preserves a copy of the emittedpackage and await the reception of the

acknowledgement.

after a certain time (time out), the package isretransmitted (and the transmitter waits again).

Stop and Wait

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Stop and Wait

Transmitting

Data

1 32 3Time

Received Data 1 2 3

Time

ACK

ACK

NAK

Output Data 1 2 3

Time

Error

ACK: Acknowledge

NAK: Negative ACK

retransmission

Stop and Wait

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Stop and Wait

Disadvantages of this method:

Problem of acknowledgement of deliverytransmission not very effective

time between the emission of two packages

Transmissions on the network in only onedirection at the same time

=> use of the sliding window technique

II ARQ

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II ARQ




4. Selective repeat

B FEC

C H-ARQ


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One emits several packages before awaiting an

acknowledgement.

The number of packages is defined by the size ofthe window.

In each acknowledgement, the window shifts

(slips).

The sliding window technique:

II ARQ

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II ARQ

The sliding window technique

Without window With a sliding windowsize=0 (stop and wait) size=3

Tran

sm

itte

r

Rec

ep

tor

Trans

mitte

r

Re

cep

tor

II ARQ

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II ARQ




4. Selective repeat

B FEC

C H-ARQ


Feedback ( go back N)

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Based on the sliding window technique

When an MPDU is lost, the transmitter is requiredto retransmit all the PDU starting from first MPDUwas lost

Disadvantage of this method:

Very bandwidth inefficient: some frames may berepeated several times while there are wellreceived.

Feedback ( go back N)

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1Time

NAK

Time

Error

Go-back 3

2 3 4 5 3 44 5 6 7 5

1 2 3 44 5

Error

NAK

Go-back 5

1 2 3 44 5

II ARQ

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II ARQ




4. Selective repeat

B FEC

C H-ARQ


Selective repeat

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Selective repeat

Based on the sliding window technique

Only the lost MPDU is retransmitted

Selective repeat

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Selective repeat

1 Time

NAK

Error

Retransmission

2 3 4 5 3 6 7 8 9 7

1Time

2 4 3 6 8 7

Error

NAK

Retransmission

5 9

1Time

2 4 3 6 8 75 9

1Time

2 43 6 875 9

II ARQ

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Q




4. Selective repeat

B FEC

C H-ARQ

DMethods used by WIMAX

FEC

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FEC

Error Detection Process:

Transmitter For a given frame, an error-detecting code (check bits)

is calculated from data bits

Check bits are appended to data bits

Receiver

Separates incoming frame into data bits and check bits

Calculates check bits from received data bits

Compares calculated check bits against received checkbits

Detected error occurs if mismatch

FEC

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Transmitter

Forward error correction (FEC) encoder maps

each k-bit block into an n-bit block codeword.

Codeword is transmitted.

Receiver

Incoming signal is demodulated

Block passed through an FEC decoder

FEC

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FEC

FEC

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No errors present

Codeword produced by decoder matches originalcodeword.

Decoder detects and corrects bit errors.

Decoder detects but cannot correct bit errors;reports uncorrectable error.

Decoder detects no bit errors, though errors arepresent

II ARQ

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2. Sliding window technique3. Feedback ( go back-N)

4. Selective repeat

B FEC

C H-ARQ

DMethods used by WIMAX

H-ARQ

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H-ARQ= FEC+ARQ

FEC: turbo codes/ convolutional codes/

block codes/

ARQ: selective repeat (SR) / stop and wait(SAW)/ go-back-N (GBN)

II ARQ

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2. Sliding window technique3. Feedback ( go back-N)

4. Selective repeat

B FEC

C H-ARQ


Methods used by WIMAX

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y

ARQ mechanism is an optional part of theMAC layer in WIMAX.

WIMAX can use ARQ ,FEC or H-ARQ.


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1 ARQ :

Uses the sliding window technique:

Selective repeat is selected by default

Feedback algorithm in specified case

More details:

ARQ parameters shall be specified and negotiated duringconnection creation or change

A connection can not have a mixture of ARQ and non-ARQ traffic

The ARQ feedback information can be sent as a standalone MAC

management message on the appropriate basic managementconnection or piggybacked on an existing connection

ARQ feedback cannot be fragmented.

y

1 ARQ :

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Transmitter state

:

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Receiver state


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2 FEC:

y

802.16 specifies the concatenation of a Reed-Solomon

(RS) outer code and a rate-compatible convolutional innercode, on both uplink and downlink.

The encoding is performed by first passing the data in

block format through the RS encoder and then passing it

through a zero-terminating convolutional encoder.

Turbo convolutional codes (TC) and Turbo Block (TB)

codes are specified as optional FEC schemes in the

standard.

Low density parity check (LDPC) codes are a new type of

FEC codes that are gaining in popularity and might be

specified as optional FEC scheme in 802.16e version


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3 H-ARQ :

H-ARQ schema is basically a stop and wait

protocol: Each H-ARQ packet is encoded and 4 subpackets are

generated from the encoded result

The transmitter shall send the packet labeled 00 at he fisttransmission

Then the receiver attempts to decode the original encoder

packet

If it succeeds the receiver sends an ACK to the transmitter so

that the transmitter stops sending additional subpackets.

Otherwise the transmitter sends a NACK and the transmitter

may send one among the fourth subpackets.

y


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3 H-ARQ :

These procedure go on until the SS successfullydecodes the encode packet.

The transmitter may send one among subpacketslabeled 00,01,10,11 in any order.

The transmitter can send more than a copy of anysub packet and can omit any subpacket exceptthe subpacket labeled 00.

III Downlink/Uplink Scheduling

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Radio resources have to be scheduled according tothe QoS (Quality of Service) parameters

WIMAX Downlink scheduling:

the flows are simply multiplexed

the standard scheduling algorithms can be used :

WRR (Weighted Round Robin) VT (Virtual Time)

WFQ (Weighted Fair Queueing)

WFFQ (Worst-case Fair weighted Fair Queueing)

DRR (Deficit Round Robin) DDRR (Distributed Deficit Round Robin)


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p g

Plan

A Downlink Scheduling

B Uplink Scheduling


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p g

Plan


B Uplink Scheduling


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g

Plan

1. RR

2. WRR

3. VT

4. WFQ5. WFFQ

6. DRR

7. DDRR


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Plan

1. RR

2. WRR

3. VT

4. WFQ5. WFFQ

6. DRR

7. DDRR

g

RR

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111VCC 1 (Source 1)

22VCC 2 (Source 2)

333VCC 3 (Source 3)

3 3

12 3112

WRR

scheduler

Counter

Reset

Cycle

1 32 233 .

Round-Robin algorithm equitably distributes

the load between each waiter whatever the

current number of connections or the response

times

RR

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This algorithm is adapted if the waiters of thecluster have the same processing capacities

if not, certain waiters are likely to receivemore requests than they can treat. Somewhile others will use only part of theirresources.

The WRR algorithm solves this problem.


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Plan

1. RR

2. WRR

3. VT

4. WFQ5. WFFQ

6. DRR

7. DDRR

o Sc edu g

WRR

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The WRR algorithm is based on the Round Robin algorithm but it takes

into account the processing capacity of each waiter.

The administrators manually assign a coefficient of performance to eachwaiter. ( 1, 2 and 3 in the example).

CounterReset

Cycle111

VCC 1 (Source 1)

22VCC 2 (Source 2)

333VCC 3 (Source 3)

3 3

2

1

3

123111 2 33333

WRR

scheduler

Coefficients of performance

.

WRR

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A sequence of scheduling is generated automatically

according to this value.

The requests are then assigned to the variouswaiters according to a sequence of alternate

repetition

VCC 2 (Source 2)

VCC 3 (Source 3)

111VCC 1 (Source 1)

22

3333 3

2

1

3

123111 2 33333

WRR

scheduler

Counter

ResetCycle

Sequence of scheduling

.


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Plan

1. RR

2. WRR

3. VT

4. WFQ5. WFFQ

6. DRR

7. DDRR

g

VT

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VT

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VT : aims to emulate the TDM (Time Division Multiplexing)

connection 1 : reserves 50% of the link bandwidth

connection 2, 3 : reserves 20% of the link bandwidth

Connection 1Average inter-arrival : 2 units



First-Come-First-Served

service order

Virtual times

Virtual Clock service

order


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g

Plan

1. RR

2. WRR

3. VT

4. WFQ5. WFFQ

6. DRR

7. DDRR

WFQ

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It is not practical to have one queue for each

conversation so the WFQ employs a hashingalgorithm which divides the traffic over a limited

number of queues to be selected by the user or fixed

by default.

WFQ is like having several doors. When a packet

arrives it is classified by the classifier and assigned

to one of the doors. The door is the entry to a queue

that is served together with some other in a

weighted round-robin order. This way the service is'fair' for every queue.

WFQ

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The packet arrives, then the classifier reads itsheader.

Calculates a number between "1" and "number

of queues by using information contained onthe header (source address ,destinationaddress, ip precedence, protocol, ...)

Then, it locates the packet in the queueidentify by this number.

WFQ

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WFQ

Scheduler

flow 1

flow 2

flow n

Classifier

Buffer

management

WFQ

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WFQ

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Each flow i given a weight (importance) wi

WFQ guarantees a minimum service rate to flow i ri = R * wi/ (w1 + w2 + ... + wn)

Implies isolation among flows (one cannot mess up another)

w1

w2

wn

R

Packet queues

WRR algorithm

WFQ

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w1

water pipesw2

w3

t1

t2

w2 w3

water buckets

w1

WFQ

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If flows can be served one bit at a time

WFQ can be implemented using bit-by-bit weighted

round robin

During each round from each flow that hasdata to send, send a number of bits equal to

the flows weight

WFQ

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FFQ (Fluid Fair Queue) : head-of-the line processorsharing service discipline

: guaranteed rate to connection i C : the link speed

: the set of non-empty queue

The service rate for a non-empty queue i

WFQ : picks the first packet that wouldcomplete service in the corresponding FFQ


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Plan

1. RR

2. WRR

3. VT

4. WFQ5. WFFQ

6. DRR

7. DDRR

WFFQ

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WFFQ is based on WFQ algorithm

WFFQ : picks the first packet thatwould complete service among the set

of packets that have started service in

the corresponding FFQ

WFFQ and WFQ

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EXAMPLE (1)

All packets have the same size 1 and link speed is 1

Guaranteed rate for connection 1 : 0.5

Guaranteed rate for connection 2-11 : 0.05

Connection 1 sends 11 back-to-back packets at time 0Connection 2-11 sends 1 packet at time 0

The completion time of connection 1 :

2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22

The completion time of connection 2 11 : 20

WFFQ and WFQ

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Connection 1

Connection 2

Connection 11

WFQ and WFFQ

WFQ Service Order

WFFQ Service Order

EXAMPLE (2)

VT and WFQ

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VT and WFQ

All packets are fixed size and require exactly onesecond to service

Starting at time zero, 1000 packets from connection1 arrive at a rate of 1 packet/second

Starting at time 900, 450 packets from connection 2arrive at a rate of 1 packet/second The completion times of the 901, 902, 903, packets of

connection 1 in FFQ system are 1802, 1904, 1806,

The completion times of the 1, 2, 3, packets ofconnection 2 in FFQ system are 901, 902, 903,


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Plan

1. RR

2. WRR

3. VT

4. WFQ5. WFFQ

6. DRR

7. DDRR

D R R

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Each connection is assigned a state variable called

the DC(Deficit Counter).

At the start of each round, DCi of queue i is

incremented by a specific service share (quantum)

If the length of the head of the line packet, Li, is less

than or equal to DCi,, the scheduler allows the ith

queue to send a

packet.

Once the transmission is completed DCi is

decremented by Li.

D R R

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Qi DCi

3500

35002800 7800 2000

1500

5000

700

1400

2800 7800 2000

2800 7800 2000

2800 7800 2000

2800 7800 2000

initializing

(1st round)

Serviced

Not serviced

Serviced

Serviced

(3rd round)

(4th round)

+3500

+3500

(2nd round)

+3500

-2000

-7800

-2800

Li


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Plan

1. RR

2. WRR

3. VT

4. WFQ5. WFFQ

6. DRR

7. DDRR

D D R R

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Each connection is assigned a state variable calledthe DC (Deficit Counter)

If the value of the DCiis positive then thescheduler allows the ith queue to send apacket

Once the transmission is completed DCi isdecremented by Li, the length of the transmittedpacket

At the start of the subsequent rounds, DCi

isincremented by a specific service share (quantum)

D D R R

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QiDCi

3500

35002800 7800 2000

1500

-6300

-2800

700

-2100

2800 7800 2000

2800 7800 2000

2800 7800 2000

2800 7800 2000

2800 7800 2000

initializing(1st round)

Serviced

Serviced

Not serviced

(2nd round)

Not serviced

(3rd

round)

Serviced

-2000

-7800

-2800

+3500

+3500

+3500

+3500


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Plan


B Uplink Scheduling

B Uplink Scheduling

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Uplink scheduling:

Responsible for the efficient and fair

allocation of the resources (time slots) in the

uplink direction

Uplink carrier :

Reserved slots

contention slots (random access slots)

The standard scheduling algorithms can be

used

Bibliography

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g p y

http://www.opalsoft.net/qos/WhyQos-2424.htm

http://www.math.tau.ac.il/~alx/courses/notes/Icc8_2.ppt

http://www.it.uu.se/edu/course/homepage/datakom/civinght04/schema/
http://www.opalsoft.net/qos/WhyQos-2424.htmhttp://www.math.tau.ac.il/~alx/courses/notes/Icc8_2.ppthttp://www.it.uu.se/edu/course/homepage/datakom/civinght04/schema/sliding_window.ppshttp://www.it.uu.se/edu/course/homepage/datakom/civinght04/schema/sliding_window.ppshttp://www.math.tau.ac.il/~alx/courses/notes/Icc8_2.ppthttp://www.opalsoft.net/qos/WhyQos-2424.htm

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