Delivering Voice over IEEE WLAN Networks

Transcription

1 Delivering Voice over IEEE WLAN Networks Al Petrick, Jim Zyren, Juan Figueroa Harris Semiconductor Palm Bay Florida Abstract The IEEE wireless LAN standard was developed primarily for packet data applications supporting data rates of 1 and 2Mbps operating in the 2.4GHz unlicensed ISM band. An extension to the physical layer of the standard is under development for specifying data rates up to 11Mbps. IEEE specifies a Carrier Sense Multiple Access / Collision Avoidance (CSMA/CA) protocol for the medium access control (MAC) layer. The Point Coordination Function (PCF) is a feature in the MAC that supports time bounded applications. This paper presents latency calculations for PCF when using the combination of voice, data and isochronous data packets at data rates of 1 and 11Mbps. Introduction This paper addresses the key issues for delivering time bounded services over an wireless LAN network uses a CSMA/CA protocol for data transmission over the medium. The MAC layer supports two (2) types of networks. They are; Ad-Hoc and Distributed Infrastructure. In the distributed infrastructure mode an access point (AP) is used to coordinate traffic between clients over a coverage area. This is analogous to base stations used in cellular networks. Traffic maybe transmitted over an medium using either the Distributed Coordinated Function (DCF) or PCF. DCF is used as the primary mode of operation for distributed infrastructure networks. What is PCF PCF is specified in the standard as an optional protocol framing method. PCF was designed to accommodate those services requiring both voice and data transactions, but does not guarantee to offer the same quality of service as DECT for example. The frame slotting structure of PCF is similar to the DECT -- TDMA scheme. In simplistic terms, it is viewed as a master - slave infrastructure network. The access point is the point coordinator (PC) for a given coverage area. The coverage area is commonly referred to as Basic Service Area (BSA). The access point is the master controller and the stations are the slave radios in a BSA. The PC maps a prioritizing scheme and determines which radio has the right to transmit. The access priority given by the PCF is used to create the contention free (CF) period. During this period all slave radios are polled in a slotted fashion. At the beginning of the CF period a beacon is broadcast to the slave radios, assigning priority. The radio with the highest priority is polled first. The slave radio responds with the data piggyback with a special acknowledgement (ACK) just after a short inter-frame space (SIFS) time out. Each slave radio is polled in priority until the NAV and CF period has expired. The timing of the slave radios is synchronized by resetting the network allocation vector (NAV) timer located in the MAC layer. This is set at the beginning of each CF interval. The CF interval polling process is repetitious from then on. An example of the PCF frame transfer is illustrated in Figure 1. This example illustrates four (4) slave radios for a given contention free period. Each device D1 through D4 including respective ACKs could be viewed as four (4) slot times within a frame.

2 Figure 1 PCF Framing Structure Integrated Voice - Data Service Office Network Latency analysis was modeled after the network environment illustrated in Figure 2. The network example assumes a traffic model that is common in offices today. The hardware and interfaces include the following: 1) a PSTN customer premise gateway 2) Six (6) full duplex users of toll grade voice 3) PC to PC data file and PC to printer transactions 4) Isochronous 16-bit 44Ksps digital audio configured as 2 channels, 5 channels, and 10 channels. PSTN PC Desktop/Server PC Desktop RF 2.4GHz Left Speaker Right Speaker Figure 2 Integrated Voice-Data and Isochronous Services

3 Voice Interface to the PHY and MAC Figure 3 illustrates the signaling flow of the PSTN interface to the physical and MAC layers of the radio. The PSTN interface is sampled at the Nyquist rate of 8Kbps with an equivalent CODEC rate of 32Kbps. The voice samples are stored in 1msec buffers. The buffered voice samples are transmitted once per client per PCF CW frame and interleaved with data transmitted over the medium. PSTN Codec 32Kbps Buffer Voice Samples MAC Protocol Frame Formator Mbps Based Radio 8Kbps Timing and Control Figure 3 MAC and PHY Signaling flow 1Mbps Data Pipe Overhead Assumptions The calculations included bit time delays contributed from the DSSS preamble and header structure. Included is 192 bits of preamble and header length and 28 bytes of MAC payload header information. As part of the frame timing considerations, a 20 msec slot time and a 10 usec SIF time were used together with a beacon period of 500 msec. All payloads assumed a packet length of 1000 bytes. Simulated Results Latency was calculated by determining the probability of success of transmitting voice packets in 1msec buffer increment and the mean time between drop outs. The calculations where modeled for several combinations of voice, data and isochronous traffic at 1 and 11Mbps data rates. Both voice and isochronous traffic were given the high priority in the frames. Table 1 illustrates results for latency buffer sizes of 1msec and 5msec. Comparisons were made at data rates of 1Mbps and 11Mbps. The rule-of-thumb for toll grade quality voice transmission is approximately 20 msec. A latency of msec was calculated for six (6) voice only users transmitting at 1Mbps. The latency in this case exceeded the 20 msec budget. This was due to the length of the preamble and the overhead associated with ACK responses within the PCF CW frame. Figure 4 illustrates further degradation of voice quality for increasing buffer sizes at 1Mbps. Latency calculations using combinations of voice and data was abandon because latencies greater than 30 msec are unacceptable for consumer quality voice communications. At 11Mbps there was sufficient bandwidth to accommodate up to 10 channels of isochronous audio together with six (6) users of toll grade voice and data file transfers. At 11Mbps, latency does not appear to be an issue when the protocol frame is loaded with 2, 5 and 10 channels of isochronous digital audio. Table 1 Calculated Latency Using PCF

4 Latency (max) Buffer Size (1 msec) Latency (max) Buffer Size (5 msec) Description Voice only at 1Mbps Voice only at 11Mbps Voice + Isoch 2 ch 44kHz Audio at 11Mbps Voice + Isoch 5 ch 44kHz Audio at 11Mbps Voice + Isoch 10 ch 44kHz Audio at 11Mbps Voice + Data file transfers (1000 byte packets) at 11Mbps Voice + Data file transfers +Isoch 2 ch 44kHz Audio at 11Mbps Voice + Data file transfers +Isoch 5 ch 44kHz Audio at 11Mbps Voice + Data file transfers +Isoch 10 ch 44kHz Audio at 11Mbps Figure 4 Six (6) Users Voice Only at 1Mbps Using IEEE Frames with PCF Latency (msec) Buffer sample (msec) Latency (ms) Figure 5 Six (6) Voice Users with Data and Isochronous Audio at 11Mbps Using IEEE Frames with PCF Latency (msec) Buffer samples (msec) 2 ch audio 5 ch audio 10 ch audio Conclusion and Further Studies

5 First order calculated results presented in this paper yield optimal performance at 11Mbps rate rates for 2.4GHz networks, when using PCF for time bound services. However networks operating at 1Mbps were substantially degraded when handling voice traffic and not recommend for these types of services. Overhead delays due to the implementation PCF in the AP was not considered in the calculations. These factors along with the processing delay for handing security and encryption may have some effect on latency, and require further study. However further analysis is currently underway at Harris Semiconductor to determine the quality of service (QoS) for delivery of such services. Parameters under study include the variation in latency delay for single and multi-user networks as well as the effects of delays contributed to networks handling voice over IP traffic. Acknowledgements Special thanks to the participating members of the IEEE working group for their dedication in the development of a world-wide standard that incorporated mechanisms such as PCF for time bounded services. References 1 IEEE Std , Wireless Local Area Networks Standard, November Rappaport, Theodore, Wireless Communications, Principles and Practice, Prentice-Hall, Minoli, Daniel, Minoli, Emma, Delivering Voice over IP Networks, Wiley, 1998