Wireless VoIP System Design Considerations

1. Wireless VoIP System Design Considerations Date: 2005-09-19 Authors: Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)

2. Introduction • Wireless VoIP systems based on dot11 have been designed and deployed in the enterprise for the past few years • In this presentation we will share the design criteria used in these deployments • Techniques are described which enhance the user experience • We discuss requirements for next generation wireless handheld devices Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)

3. Design Criteria • From a customer satisfaction perspective the key metrics and criteria in order of importance are: • Talk time, standby time, battery life • Call quality • Seamless handovers • Coverage, range • Capacity Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)

4. Talk Time, Standby Time, Battery Life • Like other wireless telephony systems, the amount of talk time and standby time between a battery charges is critical for positive user experience • Many hours of talk time and many days of standby time allow the user to be un-tethered from the charger • U-APSD in 11e allows for substantial power saving in the handset in a distributed fashion making for easy deployment • At a high level, the handset initiates the VoIP packet exchange with the AP. • During a voice call, the handset comes out of sleep mode every speech frame interval and sends the speech frame to the AP • This action indicates to the AP that the handset is awake and responds by sending any buffered packets down to the handset • The handset waits to receive the packets and then goes back to sleep for the duration of the speech frame interval • Back of the envelope calculation: • Packet exchange takes 1-2 msec, so for a 20msec speech frame interval the wireless part of the handset is powered down over 90% of the time Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)

5. Dropped calls or poor call quality will also frustrate the end user To improve quality, design links with PER better than 10% Rate shifting is also important for maintaining good call quality Within the coverage region, higher data rates are used The data rate starts shifting down to lower data rates in the transition region. Data rate shifts down to the lowest rate and the call is maintained down to the minimum received signal level Size the cells such that boundaries have margin in receive sensitivity so calls are not dropped at the edge Diversity transmitters and/or receivers significantly increase robustness of the link Call quality Minimum Received Signal Level AP AP AP AP AP AP AP Transition Region Minimum Coverage Level Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)

6. Users will be frustrated by wireless VoIP service if calls are dropped when transitioning from one cell to another A simple solution is to design adjacent cells with overlap in coverage Calls must be maintained with good quality by current AP with plenty of time for transition to neighboring APs Seamless Handovers AP AP AP AP AP AP AP Transition Region Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)

7. Coverage / Range • Coverage is similar to call quality in that it is improved by similar techniques, such as diversity • The trend in carpeted enterprise is towards dense deployments for high network capacity, so range is not a primary concern • However, VoIP deployments in other environments with lesser network capacity requirements, such as in small office or home office, will be more focused on enhanced range and coverage with few AP’s • In these environments absolute range may be improved by diversity or coding techniques and longer preambles for improved acquisition and synchronization at lower SNR Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)

8. Capacity • The end user only notices capacity limitations when not able to place or receive a call • Current 11ag based deployments can reasonably support 30 calls per channel • IP and dot11 MAC overhead play a large role in the achievable capacity • 11n preambles will be necessarily longer than 11a preambles for training and signaling • However, the impact on call capacity will be small as seen on chart in next slide • Modest downstream aggregation in the MAC will more than make up for impact of longer preambles with short VoIP speech frames • The chart on the following slide shows the substantial improvement in capacity even by only aggregating two speech frames Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)

9. Min data rate, max codec rate (solid red line) Longest packet time on air Increasing preamble from 20 to 40usec drops the capacity by 5% Two destination downstream aggregation recovers capacity loss (red dot) Max data rate, min codec rate (red line) shortest packet time on air Increasing preamble from 20 to 40usec drops the capacity by 17% Two destination downstream aggregation recovers capacity loss improving capacity by 30% (blue dot) VoIP Call Capacity Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)

10. Parameters for Capacity Calculation • Time on air computation based on • DIFS, backoff, packet, SIFS, ACK • Bi-directional packet flow • UDP and dot11 MAC overhead • Codec: • 20msec period • 64kbps and 4kbps • Aggregation • Two packets aggregated • Downstream aggregation only Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)

11. Next Generation Wireless Handheld Devices • To properly define the 11n standard for effective use with handheld devices, we need to understand the following: • Applications • Environments and usage scenarios • Device Requirements Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)

12. Applications • It has been presented in the past that applications for handheld devices will include VoIP, internet anywhere, and video streaming. • In order to quantify the benefit of algorithms or compare competing algorithms, we must define the details of these applications specific to handheld devices. • VoIP • What codec (bit rate, period)? • Are there capacity requirements (number of simultaneous calls)? • What target PER? • What target minimum receive sensitivity? • What target data rates? • Internet anywhere • What are the throughput requirements? • Video streaming? • What target application bit rates? • Are there capacity requirements (number of simultaneous streams)? Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)

13. Environments and Usage Scenarios • Enterprise • Dual mode handset will allow for a handoff between a cellular system and the in-building WLAN VoIP network, providing coverage for cellular dead spots indoors. • The current push in enterprise is high density deployments with smaller cells. • The important design criteria is aggregation of multiple VoIP packets to multiple receivers for improved capacity. • Home or small office • Achieve full home coverage with one access point (or few access points in small office) • 11n handsets provides VoIP telephony bridging on DSL or Cable throughout the home • Will these be dual-mode devices or a specialized version of a WLAN-based “cordless phone”? • Hotspot • Offload capacity from cellular system to WLAN • Fill in coverage holes in cellular system (e.g. train station) Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)

14. Device Requirements • Minimize power consumption • Size limitation • Reduce cost • Multiple air-interfaces Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)

15. Techniques to Address Requirements • Aggregation • Multi-receiver aggregation improves capacity for VoIP application. • Power save • To maintain long battery life of handheld devices, the power save algorithm must be integrated with multi-receiver aggregation. • Extended range • To extend the range for SOHO applications, we must consider that for indoor applications 10dB of gain is required to double the range. • Investigations should include • STBC, TxBF, advanced coding to extend data detection • Preamble design to extend acquisition and synchronization Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)

16. Summary • We described the criteria for designing wireless VoIP systems and equipment in the enterprise • The most important to the user is the talk time and standby time of the handheld device, no dropped calls, and good call quality • With respect to system performance, modest aggregation substantially improves call capacity making the impact of longer 11n preambles negligible • For next generation wireless devices, improvements should include aggregation integrated with power save for enterprise and extended range for SOHO applications Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)