A Multiplex-Multicast Scheme that Improves System Capacity of Voice- over-IP on Wireless LAN by 100% * B 葉仰廷 B 陳柏煒 B 林易增 B 謝秉諺

Outline Introduction VoIP Multiplex-Multicast Scheme Capacity Analysis Delay Performance Conclusions

Introduction This paper considers the support of VoIP over b WLAN. WLAN capacity can potentially support more than 500 VoIP sessions when using GSM 6.10 codec. But various overheads bring WLAN capacity only 12 VoIP sessions when using GSM 6.10 codec.

Introduction b, which can support data rates up to 11Mbps. A VoIP stream typically requires less than 10Kbps. 11M/10K = 1100, which corresponds to about 550 VoIP sessions, each with two VoIP streams.

Introduction The efficiency at the IP layer for VoIP: A typical VoIP packet at the IP layer consists of 40-byte IP/UDP/RTP headers. A payload ranging from 10 to 30 bytes, depending on the codec used. less than 50%!!

Introduction At the MAC/PHY layers: Attributed to the physical preamble, MAC header, MAC backoff time, MAC acknowledgement, and inter-transmission times of packets and acknowledgements …. The overall efficiency drops to less than 3%!!

Outline Introduction VoIP Multiplex-Multicast Scheme Capacity Analysis Delay Performance Conclusions

每日一詞 Unicast Broadcast Multicast

Multiplex-Multicast Scheme An WLAN is referred to as the basic service set (BSS) in the standard specification. There are two types of BSSs: Independent BSS and Infrastructure BSS.

Multiplex-Multicast Scheme Independent(ad hoc) BSS

Multiplex-Multicast Scheme Infrastructure BSS

Multiplex-Multicast Scheme This paper focuses on infrastructure BSSs. We assume that all voice streams are between stations in different BSSs. Each AP has two interfaces, an interface which is used to communicate with wireless stations, and an Ethernet interface which is connected to the voice gateway.

Multiplex-Multicast Scheme

Within a BSS, there are two streams for each VoIP session. M-M Scheme idea : Combine the data from several downlink streams into a single packet for multicast over the WLAN to their destinations.

Multiplex-Multicast Scheme

multiplexer(MUX), demultiplexer(DEMUX) Add miniheader In miniheader, there is an ID used to identify the session of the VoIP packet.

Multiplex-Multicast Scheme Header data1 Header data3 Header data2 Header Minih.+Data1+Minih.+data2+Minih.+data3 MUX DEMUX

Multiplex-Multicast Scheme Reduce the number of VoIP streams in one BSS from 2n to 1 + n, where n is the number of VoIP sessions. The MUX sends out a multiplexed packet every T ms, which is equal to or shorter than the VoIP inter-packet interval. For GSM 6.10, the inter-packet interval is 20 ms.

Multiplex-Multicast Scheme MUX DEMUX

Multiplex-Multicast Scheme Problem: Security!?

Outline Introduction VoIP Multiplex-Multicast Scheme Capacity Analysis Delay Performance Conclusions

Capacity Analysis consider the continuous-bit-rate(CBR) voice sources voice packets are generated at the voice codec rate focus on the GSM 6.10 codec the payload is 33 bytes the time between two adjacent frames is 20 ms

Capacity Analysis n : maximum number of sessions that can be supported T down & T up : transmission times for downlink and uplink packets T avg : average time between the transmissions of two consecutive packets in a WLAN N P : number of packets sent by one stream in one second 1/T avg = number of streams * N P

Capacity of Ordinary VoIP over WLAN OH hdr = H RTP + H UDP + H IP + H MAC OH sender if unicast packet: OH receiver T down = T up = (Payload + OH hdr ) * 8 / dataRate + OH sender + OH receiver

Capacity of Ordinary VoIP over WLAN n downlink and n uplink unicast streams T avg = (T down + T up ) / 2 1/T avg = 2n *N p n = 11

Capacity of Multiplex- Multicast Scheme over WLAN the RTP, UDP and IP header of each packet is compressed to 2 bytes T down = [(Payload + 2) *n + H UDP + H IP + H MAC ] * 8 / dataRate + OH sender T avg = (T down + n *T up ) / (n + 1) 1/T avg = (n + 1) *N p n = 21.2

VoIP Capacities assuming Different Codecs CodecsOrdinary VoIP Multiplex-Multicast Scheme GSM G G G G

Simulations increase the number of VoIP sessions until the per stream packet loss rate exceeds 1% system capacity = max number of sessions assume that the retry limit for each packet is 3

Simulations for ordinary VoIP over WLAN, the system capacity is 12 exceeding the system capacity leads to a large surge in packet losses for the downlink streams

Analysis vs. Simulation Capacity of Ordinary VoIP and Multiplex- Multicast Schemes assuming GSM 6.10 codec Different SchemesAnalysisSimulation Original VoIP Multiplex-Multicast Scheme

Outline Introduction VoIP Multiplex-Multicast Scheme Capacity Analysis Delay Performance Conclusions

Delay Performance voice quality: packet-loss rates & delay performance with ordinary VoIP: local delay: only the access delay within the WLAN at the AP: time between the packet ’ s arrival until it ’ s successfully transmitted or dropped at the client: time from when the packet is generated until it leaves the interface card

Delay Performance with the M-M scheme: local delay: access delay & the MUX delay incurred at the VoIP multiplexer (only downlink) MUX delay: time from the packet ’ s arrival until the next one is generated we set a requirement: no more than 1% of packets should suffer a local delay of more than 30 ms

Access Delay ordinary VoIP scheme (12 sessions): in the AP: average delay and delay jitter are 2.5 ms and 1.4 ms in the wireless station: average delay & delay jitter are 1.2 ms and 1.0 ms if normally distributed: less than 0.27% of the packets would suffer local delays larger than 30 ms

Access Delay Access Delays in AP and a Station in Original VoIP over WLAN when there are 12 Sessions

Access Delay M-M scheme (22 sessions): in the AP: average delay and delay jitter are 0.9 ms and 0.2 ms in the wireless station: average delay & delay jitter are 2.0 ms and 1.5 ms no link layer retransmissions for the packets when collisions occur

Access Delay Access Delay in AP and a Station in M-M Scheme when there are 22 Sessions

Extra Delay Incurred by the Multiplex-Multicast Scheme when a VoIP packet waits for the MUX to generate the next multiplexed packet we set the multiplexing period to be at most one audio-frame period 20 ms if GSM 6.10 codec is used random variable M : the MUX delay assume M to be uniformly distributed between 0 and 20 ms

Delay Distribution for Ordinary VoIP When System Capacity of 12 is Fully Used Access delay for the AP Access delay for the station Pr[A ≤ 0.01s] Pr[A ≤ 0.03s]11 Pr[A ≤ 0.05s]11

Delay Distributions for Multiplex-Multicast Scheme When System Capacity of 22 is Fully Used Access delay for the AP plus MUX delay in the MUX Access delay for the station Pr[M + A ≤ 0.01s]0.455Pr[A ≤ 0.01s]0.996 Pr[M + A ≤ 0.02s]0.955Pr[A ≤ 0.02s]1 Pr[M + A ≤ 0.03s]1Pr[A ≤ 0.03s]1

Outline Introduction VoIP Multiplex-Multicast Scheme Capacity Analysis Delay Performance Conclusions

M-M scheme can reduce the large overhead when VoIP traffic is delivered over WLAN it requires no changes to the MAC protocol at the wireless end stations more readily deployable over the existing network infrastructure. it makes the voice capacity nearly 100% higher than ordinary VoIP