Distributed delay estimation and call admission control in ieee 802 11 wlans
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Distributed Delay Estimation and Call Admission Control in IEEE 802.11 WLANs. Kenta Yasukawa, Andrea G. Forte and Henning Schulzrinne Ericsson Research Japan, Department of Computer Science Columbia University IEEE ICC 2009 proceedings. 報告者:李宗穎. Outline. Introduction

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Distributed delay estimation and call admission control in ieee 802 11 wlans

Distributed Delay Estimation and Call Admission Control in IEEE 802.11 WLANs

Kenta Yasukawa, Andrea G. Forte and Henning Schulzrinne

Ericsson Research Japan, Department of Computer Science Columbia University

IEEE ICC 2009 proceedings

報告者:李宗穎


Outline
Outline IEEE 802.11 WLANs

  • Introduction

  • Delay Estimation using Time between Idle Time

  • Call Admission Control using TBIT

  • Evaluation

  • Conclusion


Introduction
Introduction IEEE 802.11 WLANs

  • After the number of concurrent voice calls in the BSS surpasses the AP capacity

    • high delay, high collision rate

    • even worse in scenarios where both voice traffic and data traffic


Mobile station based cac
Mobile-Station-Based CAC IEEE 802.11 WLANs

  • NOT requiring any probing of the medium

    • any STA in the BSS can monitor the shared medium and calculate the TBIT

  • NOT requiring any changes in the infrastructure and the protocol (client-based)

    • deployment feasibility ( it is not realistic to assume a new CAC mechanism will be widely deployed on existing APs )


Time between idle time tbit
Time Between Idle Time (TBIT) IEEE 802.11 WLANs

  • “idle time” as a period during which the medium is idle and long enough to represent a transmission opportunity

    • if it takes “too long” for the AP/STA to empty its queue, it means that the packets stay in the queue for a longer time

    • the time between idle times gives us a direct estimate of the level of congestion


Delay estimation using tbit
Delay Estimation using TBIT IEEE 802.11 WLANs

  • the wireless medium is shared and every STA can hear packets that other STAs and the AP send and receive (STAs can still “hear” ACK in hidden nodes)

The queuing delay for the new packet is t2− t1 and it is maximized when t0 = t1

The TBIT is equal to t2− t0 and represents a direct measure of the maximum queuing delay at the AP at the time of the observation


Idle time threshold i th
Idle Time Threshold IEEE 802.11 WLANsIth

  • if the AP has more packets to send, it has to wait for DCF inter frame space (DIFS) plus the time needed to decrement a random backoff counter to zero

Expected Backoff Time


Expected backoff time 1 2
Expected Backoff Time (1/2) IEEE 802.11 WLANs

  • The Ith second term is the expected backoff time, paper defines it as :

  • We consider N equal to its upper bound, that is, equal to CW and we set CW to its lower bound, that is, to CWmin.


Expected backoff time 2 2
Expected Backoff Time (2/2) IEEE 802.11 WLANs

  • CW is equal to CWmin for more than 95% of the time, and paper selected (N = CW = CWmin)

    • Ithis big enough not to count as transmission opportunities those idle periods due to backoff, and small enough not to make our method too conservative


Cac using tbit
CAC using TBIT IEEE 802.11 WLANs

  • Let P and λ be its packet size and packet rate, respectively, and let Ttx(P) be the time needed to send a packet whose size is P

  • if we denote the frequency of idle times that are longer than Ttx(P) by μ, we can say that the new call will not cause congestion if μ is larger than λ


Obviously the inverse of tbit
Obviously the inverse of TBIT IEEE 802.11 WLANs

  • Paper can inverse of TBIT and is obtained in the same way as in the delay estimation. The only change we need to apply here is to set the idle time threshold as Ttx(P), that is:


How to get p and
How to get IEEE 802.11 WLANsP and λ

  • both caller and callee usually negotiate which codec to be used in the call setup phase

    • SIP is used codecs are negotiated in the initial INVITE−200 OK handshake

    • By substituting the codec information obtained through the negotiation, it is possible for a STA to check if λ is larger than μ


Common network setup
Common Network Setup IEEE 802.11 WLANs

Network Simulator 2 (ns-2)

IEEE 802.11B


Implementation and testbed
Implementation and Testbed IEEE 802.11 WLANs

  • The TBIT listener was based on Java and libpcap version 0.9.8

  • Paper used three T42 and one R51 IBM Thinkpad laptops to make the same topology as Fig. 3 (Linux OS)


Delay estimation experiments
Delay Estimation Experiments IEEE 802.11 WLANs

  • The PC connected to the AP by Ethernet sent packets with time-stamps to the TBIT listener and the TBIT listener plotted the actual one-way delay

  • The other laptop PC was used to generate background traffic so to increase the queuing delay at the AP

  • The clocks of all the PCs were synchronized by using the NTP every second (the clocks to much less than 1ms)



Limitation of tbit listener
Limitation of TBIT listener IEEE 802.11 WLANs

  • The limitation of our current implementation is that, TBIT is computed by capturing packets, and packets can be lost due to errors and collisions

  • This problem can be avoided by accessing the status of the medium directly from the wireless card (Such implementation will be reserved for future study)


Cac experiments
CAC Experiments IEEE 802.11 WLANs

  • Paper emulated G.711 CBR VoIP calls by creating a simple program which sends out packets with a given constant interval

  • The idle time threshold Ithwas set to 1650μs according to TBIT equation



Simulation study
Simulation Study IEEE 802.11 WLANs

  • Paper did simulations assuming both homogeneous and heterogeneous scenarios,

    • only one kind of VoIP codec is used

    • different kinds of VoIP codecs are mixed together



Contour of 90 th percentile delays in ms and cac decisions made using tbit g 711 vbr g 723 1 cbr
Contour of 90 IEEE 802.11 WLANsthpercentile delays in ms and CAC decisions made using TBIT (G.711 VBR + G.723.1 CBR)


Conclusions
Conclusions IEEE 802.11 WLANs

  • This paper introduce the concept of time between idle times (TBIT) and show how this can be used to estimate queuing delay and make CAC decisions in IEEE 802.11 networks


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