Allan C. Cleary

Voice-Data Channel Access Integration in Third Generation Microcellular Wireless Network : Design and Performance Evaluation

Allan C. Cleary

Chapter 2 : Separating Contending Voice and Data Packet Transmissions when every slot is an Information Slot

T ECHNICAL U NIVERSITY OF C RETE

Problem

Integrated Voice-Data packet access of a slotted wireless communication channel


Goals

• Design and evaluation multiple access schemes for efficiently integration voice-data traffic.

• Maximize system capacity.

• Satisfy Quality of Service Requirements (QoS) :

Voice packet dropping probability

Access delay

• These goals are complicated by :

The radio channel bandwidth.

The contradictory nature of voice and data traffic.

System Model

• Channel time is organized into periodic time frames of fixed duration

• The slots within a frame are classified as being reserved (R) or available (A)


• During a slot three event will occur : empty, success, collision.

Voice-Data Integration Scheme

• Every time slot is an information slot and the contending terminals follow a random

access algorithm to compete for available slots within a frame.

• A voice packet involved in a collision is retransmitted until it is dropped or successfully

received, whichever occur first.

• A data packet involved in a collision is retransmitted until it is received successfully.

• On successfully transmission on an available slot :

a voice terminal receive a reservation for the corresponding slot in successive frames

a data terminal does not.

• A voice terminal with a reservation transmits exclusively during its reserved slot and retain

its reservation for as long as it continues to transmit packet in successive frames.


• To eliminate the competition for available slots between voice and data terminals :

at the beginning of each frame only the contending voice terminals are

permitted to transmit into the available slots (giving rise to the voice contention

period (VCP)).

the VCP ends when the voice access algorithm marks it as ended or when the

frame end.

from the end of the VCP until the end of frame, only data terminals may transmit

into the available slots.


Parameter Value

Speech rate (bps) 8000

Channel rate (bps) 270000

Packet size (bits) 552

Speech or data (bits) 472Header (bits) 80Frame duration (ms) 59

Slots per frame 28Voice delay limit, Dmax (slots) 28Mean talkspurt duration (s) 1.0

Mean silence duration (s) 1.35

Experimental system parameters


Quality of Service (QoS) Requirements

• Voice packet delay requirements are more stringent than those for data packets.

• Each voice packet must be delivered within a specified maximum delay to be usefull. If the delay of a voice packet is greater than the maximum threshold, then the voice packet is dropped.

• Speech can withstand a small (1-2%) amount of dropped packets without suffering a quality degradation which can be perceived by humans.

• Data traffic is often extremely bursty and more tolerant of delays (e.g., delays up to 200 ms are often acceptable); but 100% delivery of correct packet is often required.


Reservation Random Access Protocols

Voice Terminals

• Priority Queue (RRA-Q)

• Ideal Controlled ALOHA (PRMA)

• Controlled Aloha (RRA-CE)

• Two-Cell Stack (RRA-2S)

• Three-Cell Stack (PPA-3S)

Data Terminals

• First Come First Served (FCFS)

• Two-Cell Stack (RRA-2S)


Priority Queue (RRA-Q)

• Contending voice packets are maintained in a priority queue sorted in non increasing

order from oldest to youngest.

• When the queue is not empty, voice packets are transmitted one at a time into

available voice slots.

• Because channel access for the contending terminals is perfectly scheduled the random access

component of RRA-Q is ideal and this algorithm can not be implemented in practice.

• Provides an upper bound for the voice capacity (the maximum number of active voice

terminals with voice packet dropping less than approximately 1%) and a lower bound

for voice access delay over all implementable voice transmission protocols.


Ideal Controlled ALOHA (RRA-CI)

• A contending terminal may transmit its packet only if the slot is available and the

terminal has permission to transmit.

• The permission probability is :

where , c represents the actual number of contending voice terminals.

• The value of c can not be known to every participation station (RRA-CI is not implementable).

Aloha (PRMA)



• The permission probability is determined by a pseudo number generator with

probability, p, in each time slot.

• The performance of PRMA is sensitive to the choice of the design parameter , p.

• For our system p = 0.35.

cp 1


Controlled Aloha (RRA-CE)



• The permission probability is

where is an estimate of the number of contending voice terminals at the beginning of the

current available slot.

• The estimation procedure is as follow:

(i) at the beginning of each frame

ecp

1

activity speech theis

frame upcoming inslot reserved ofnumber theis

yprobabilit n transitio talkspurt tosilence theis

,1max,1max11

,1max

T

ST

SSTST

STT

STe

p

R

p

pNppp

Rp

pRpc


ec

otherwisec

occurscollisionawhencc

e

ee ,)1(,1max

,1

(ii) after the first available slot

• The permission probability decreases on collision and increase on non-collision • Any terminal can conclude that the VCP is finished when :

the base station feedback for the current available slot is NC

• We observe that RRA-CE does not permit the terminals to immediately and uniquely recognize the end of the VCP because the estimate of may not be accurate.

1ec


ec

Two-Cell Stack

1. At the beginning of each frame, every contending voice terminal initializes

its counter r to 0 or 1 with equal probability 0.5.

2. Contenders with r = 0 transmit into the first available voice slot.

Let x be the feedback for that transmission. Then

a. if x = no collision:

if r = 0, the request packet was transmitted successfully.

if r = 1, then r = 0.

b. if x = collision:

if r = 0, then reinitialise r to 0 or 1 with probability 0.5.

if r = 1, then r = 1.

3. Repeat steps 1-2, until two consecutive feedbacks indicating non-collision occur (end of VCR).


• Three contending terminals try to transmit their packets at the beginning of the frame.

• Terminals in the bottom cell (r = 0) transmit in the first available slot and a collision (C) ensues.

• The subsequent transitions in time of the stack follow the rules 1-3.• Two consecutive non-collision (NCs) feedbacks indicate an empty stack, therefore

the end of the voice contention period (VCP).

C NC C NC NC

cell 1

cell 2

An example of Two-Cell Stack Algorithm


Three-Cell Stack

1. At the beginning of each frame, every contending voice terminal initializes

its counter r to 0, 1 or 2 with equal probability 1/3.

2. Contenders with r = 0 transmit into the first available voice slot.

Let x be the feedback for that transmission. Then

a. if x = no collision:

if r = 0, the request packet was transmitted successfully.

if r >1, then r = r-1.

b. if x = collision:

if r = 0, then reinitialise r to 0, 1 or 2 with probability 1/3.

if r >0, then no change.

3. Repeat steps 2, until three consecutive feedbacks indicating non-collision occur (end of VCR).


Reservation Random Access Protocols for Data Terminals

Two-Cell Stack

• A blocked access mechanism is established by the following first transmission rule for

newly generated data messages:

Terminals with new packets arrivals may not transmit during a collision resolution

period (CRP).

A CRP is defined as the interval of the time that begins with an initial collision

(if any) and ends with the successfully transmission of all data requests packets

involved in that collision (or, if no collision occurred ends with that slots)

In the first available data slot following the CRP, every terminal whose message

arrived within a prescribed allocation interval of maximum length transmit

with probability one.

• Terminals involved in a collision follow the Two-Cell Stack transmission rules.

• The maximum data throughput is 0.429 and is achieved by using

slots33.2


C CNC NC NC

t

t

t

t1

t1

t2

t2

C NCNC

Resolved Unexamined Interval

Examined = Δ Unexamined

Resolved Interval Examined

(CRP)

CRP

CRP:Collision Resolution Period

An example of Two-Cell Stack algorithm operation

Examined = min(t 1 + Δ, t)

Blocked Access Mechanism


First Come First Served (FCFS)

• Data terminals with newly arriving packets may not transmit during a CRP.

• When a CRP completed, every terminal with data packets that arrived within the allocation

interval, of maximum length Δ, transmit with probability one.

• In case of collision, the terminals involved are split into two subsets according to their

packets arrival times and the subset with the earlier packet arrival times is resolved first.

• The FCFS collision resolution algorithm includes two improvements :

First, when a collision is followed by an empty slot, the second subset splits before

transmitting to avoid a sure collision.

Second, whenever two consecutive collision occur, the second subset is removed from

the CRP and it is absorbed into the next allocation interval.

• The end of CRP is identified by the occurrence of two consecutive successful transmission

in available data slots (as a results of the second improvement).

• The maximum data throughput is 0.4871 and is achieved by using slots6.2


• The speech activity is modeled by a two state discrete time Markov chain

• The talkspurt and silence periods are geometrically distributed with means 1/pTS and 1/pST

frames respectively.

• The voice delay limit, Dmax, is equal to the duration of one frame.

• The channel is assumed error free and without capture.

• The reserved slots are deallocated immediately by the BS.

• The number of active voice terminals, N, in the system is assumed constant.

Voice Traffic Model

TS

TSST

STT

pp

pp

pp

1


System State Transitions for an Active Voice Terminal

• Silence : the terminal has no packets and does not require for channel resource

• Contender : terminal uses its packet to compete for available voice slots within a frame.

• Reserved : the terminal transmits one voice packet per frame into its assigned slot


Performance Metrics

Voice Traffic

• Voice capacity : the maximum number of voice terminals with dropping probability

smaller than 1%.

• Multiplexing gain : the ratio of the voice capacity to number of slots per frame.

Data Traffic

• Data throughput : the average number of successful data packet transmissions

per frame

• Data packet delay : the time between the packet arrival and the successful transmission.


Steady state voice packet dropping probability as a function of the number of active voice terminals


Steady state voice performance results


Steady state voice packet throughput as a function of the number of active voice terminals


VoiceAlgorithm

Pdrop

(percent)Voice Thr.

(packets/frame)Avail.Data Slots

(slots/frames)Data Delay

(frames)

RRA-CE 0.840.02 19.810.03 6.720.05 7.080.29

RRA-2S 1.060.03 19.810.04 6.220.03 12.160.03

RRA-3S 0.660.02 19.840.06 5.650.05 31.656.35

Voice-data traffic; N = 47, λ = 2.5 packets/frame


An Overview of Proposed MAC Algorithms for Wireless ATM

Daniel Sobirk, Johan M. Karlsson, Lars Falk, Christer Lind


Attributes

• Requires infrastructure (Yes/No)

• Intelligence (Central/Distributed/Both/None)

• Up/downlink channels (Different/Same)

• Time Division (Discrete/Continuous)

• Main access strategy ( Collision free/Contention based)

• Access request (With request packet/With data packet/Via polling)

• Reservation strategy

a) First packet in a burst (Contention based/Reservation/Fixed)

b) Remaining cells in a burst (None/Burst reservation)

c) Bandwidth allocation flexibility (Contention/Request/New Connection)

• Contention resolution strategy (None/Random binary backoff/Others)

• Multiplexing technique (CDMA/TDMA/FDMA/any)


• Carrier sense (Yes/No)

• Traffic integration ( None/Class Based/Seamless)

• Frame structure (Homogeneous/ Heterogeneous)


Allan C. Cleary

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