Transcript Colin Roby - present

Provision of Multimedia Services in 802.11-based
Networks
Colin Roby
CMSC 681 Fall 2007
Company
LOGO
Overview
1. Multimedia Streaming in 802.11 Networks
2. 802.11e Mac Layer Enhancement
3.Sustain Quality of Service in WLAN
4. Challenges for Quality of Service Provision
1.Multimedia Streaming in 802.11 Networks
Background Info on 802.11 Network
Contention-based network:
1) System can only transmit when the sharing media is free and clear of signals.
2) If multiple systems find the media is free and attempt to transmit, data collision will result.
3) Busy network will induce higher collision rate and reduce performance.
MAC – Media Access Control Protocol
Responsible for sharing the physical connection to the network among several computers.
1.Multimedia Streaming in 802.11 Networks
Common problems in sustaining quality of service in streaming real-time
multimedia in 802.11 networks
Technology limitations:
1) Random resources sharing nature of the networks affects QoS(quality of service)
2) Enforce common application level QoS control at MAC level - provide some priority mechanism
in the MAC protocol.
Flaws in the default ip-level differentiated service:
1) Use a single MAC queue to manage ip traffic, and service is at the same level.
2) High priority flow sometimes is treated same as non-priority (best effort) flow
3) The differentiated services is difficult to use variable MAC factor (parameters)
which determines the network condition.
Problems in the basic distributed coordination function (DCF) algorithm:
1) when network contention increases will result frequent collision, which introduces disparity
between services received by different hosts on the network
2) when different channel competing for the medium, the access order is not the same as the
backlogging order, creates the random nature of the 802.11 MAC protocol.
802.11e Mac Layer Enhancement
HCF (Hybrid coordination function)
EDCA
HCCA
Enhanced Distributed
Channel Access
HCF controlled
Channel Access
802.11e Mac Layer Enhancement
802.11e’s new MAC layer function HCF(hybrid coordination function)
EDCA – Enhanced Distributed Channel Access:
Goal: high priority traffic class receives more transmission time than low priority class.
It combines three different MAC parameters to achieve this goal:
1) Arbitration interface space (AIFS) parameter set
2) Minimum/maximum contention window parameter set
3) Transmission duration (optional)
* MAC parameter are customized towards each traffic class.
* Drawback: non-multimedia application will experience degraded performance,
can not be universally applied to all network configuration.
HCCA – HCF Controlled Channel Access:
Goal: reduce randomness and increase service guarantees in contention-based network access.
1) Channel pools each station from a predefined, continuously updated polling list
2) Each station provides a traffic specification that characterize the burstiness (TSPEC), each
station explicitly targets a particular scheduling method.
Most significant parameter:
Mean data rate, delay bound, maximum service interval, nominal MSDU size.
802.11e Mac Layer Enhancement
EDCA – Different class parameter value:
802.11e Mac Layer Enhancement
HCCA frame for traffic specification:
802.11e Mac Layer Enhancement
802.11e’s new MAC layer function HCF(hybrid coordination function)
(Continued)
ETSI HiperLan (European Technical Standard Institution)
European standard – an alternate method for media access –
Elimination Yield-Non Preemptive Multiple Access(EYNPMA)
Hiperlan breaks into three phases:
1) Prioritization phase – burst duration, jaming sequence
2) Elimination phase – same priority
3) Sensing phase – backoffed process detects idle network
Drawback: maintains the characteristic of high randomness, make it hard to guarantee fairness.
Service differentiation for distributed IEEE 802.11 networks
Service differentiation at EDCA is achieved through setting static parameters
for different traffic class. Each parameter influences a particular medium access priority.
1) AIFS – smaller value allows earlier start, limits flow collisions between different traffic class,
But increases contention between flow from same traffic class.
2) Frame Size/TXOP – changes the amount data to be send, results in higher or lower throughput.
3) Persistent Factor – scale up and down contention window
4) Backoff – various algorithms can be used to improve QoS performance – determines the spacing
between two successive transmission attempts.
802.11e Mac Layer Enhancement
802.11e’s new MAC layer function HCF(hybrid coordination function)
(Continued)
ECDA parameter effects:
1) AIF – minimize intra-class collision in heavily loaded networks
2) Frame Size – increase burst rate of data, but does not guarantee transmission time
3) Persistent Factor – large persistent factor will cause colliding traffic to rapidly (multiplicative)
Increase contention window.
4) Backoff Algorithm – adaptive in nature, monitoring network conditions to dynamically adjust
MAC parameters.
Sustain Quality of Service in WLAN
Bandwidth provision: Estimating
the achievable QoS performance
DBASE
Dynamic
Reservation
CARC
Resource Control
Method
DAC
Virtual
MAC
Sustain Quality of Service in WLAN
Bandwidth provision method:
1) DBASE – (Distributed Bandwidth Allocation/Sharing/Extension) protocol : Split contention
into two subperiods, one for real-time traffic, one for non-real-time traffic.
2) CARC – (Call Admission and Rate Control) : control each station’s arrival rate to achieve
desired minimum throughput, maximum delay, jitter or loss rate in the networks.
3) Combination of DAC (Distributed admission control) and two-level protection and guarantee
mechanisms – DAC uses statically allocated network bandwidth, each traffic class receives a fixed
share of bandwidth.
4) Virtual MAC and virtual source algorithms – a virtual MAC operates in parallel to the real MAC
Algorithm. VMAC estimates the collision probability. But not actually transmit any data. VMAC uses
Time stamped virtual packet, and calculate total delays to estimate the channel capacity to support
new demands.
5) Dynamic multiple-threshold reservation algorithm – use cellular infrastructure networks and
assign different priority to different traffic class. Cellular network assign each flow a fixed
transmission slots.
MAC Fairness
Inter-TC
Traffic flow
between same
traffic class
Intra-TC
Balance
Traffic flow
between
different
traffic class
MAC Fairness
DFS - Distributed fair scheduling
1) Determines which station should access the medium first
2) Initiate backoff process before transmitting each frame
3) Backoff process is proportional to packet size and inversely proportional to flow priority
Advantage: cause lower priority station to generate longer backoff intervals
Drawback: can not guarantee QoS to flows of the same priority, fairness is limited at throughput
Challenges for Qos Provision
A
Trade off between
achieved network
throughput
and delay guarantees
-Increasing flow’s
throughput beyond
certain threshold
results enqueuing
delays
B
Find optimal
network point –
maximizing the
number of Qosenabled service
regardless
network
configuration
C
Construct
admission
control
mechanism
enable all
possible perclass load
distribution
within the limits
of Qos metrics
D
Admission
decision should
be made at
different active
stations
rather than
admitting
station.
Reference
Reference:
1) Provisioning of Multimedia Services IN 802.11-Based Networks: Facts and Challenges
(IEEE Wireless Communications, October 2007)