1. Current industry transition from Fixed WIMAX to Mobile WIMAX

Currently, the wireless broadband space is getting ready to transition from the Fixed version of the WIMAX standard termed 802.16d-2004 to the Mobile form: 802.16e-2005. In recent news, the WIMAX Forum welcomed 35 companies to AT4 Labs in Malaga, Spain for its second Mobile WIMAX plugfest starting February 11. This event was to establish interoperability of equipment, and is the precursor to Mobile WIMAX certification testing that will happen in the 2nd half of 2007. Consequently, the industry should see certified Mobile WIMAX equipment in the marketplace by 4Q 2007.
 
2. What is QoS?

QoS refers to different parameters in the network that determine the types of traffic that can be supported, and the type of experience a user will have. For each application and for each customer, a different set of requirements is critical. Typical network parameters that determine QoS are bit error rate, jitter, latency, average data throughput and minimum throughput.

Bit errors and associated packet loss is damaging for voice communications since retransmission is not an option. Error detection and correction algorithms can be used to trigger frame repetition from the previous received bits, but you can only do a certain amount of that before the impairments become noticeable.

Jitter is caused when packets arrive at different times due to different queuing times or due to the different routes taken by the communications. Jitter is typically addressed through a memory buffer that stores early arriving packets, concatenates later arriving packets, and thus smoothes the voice arriving at the receiver.

Transmission over wireless also causes the problem of latency or delay. The individual factors that contribute to it are channel propagation delay, serialization delay, channel coding delay, and delays associated with MAC processing. At the network layer, we encounter forwarding and buffering delays, and last at the application layer we have packetization, coding/decoding, and look-ahead delays. For voice if these delays sum to 150 ms or more, the quality is impaired and noticeable by the user.

Each of these QoS parameters effect different types of traffic differently. For example, audio and video traffic is tolerant of bit errors, because both types of content have inherent redundant data. The caveat is that, these days most audio and video is first source coded or compressed which means that most of the redundant information is pared away; in this case bit and packet errors will affect it too. As far as jitter is concerned, it can cause difficulties because it can affect intra-frame or inter-frame synchronization which is necessary to decode the video signal. Data rates required for video depend on the kind of compression used; MPEG4 requires less than 2 Mbps while high-definition video requires anywhere from 6 to 18 Mbps, depending on the type of compression and frame rates chosen. Latency is less of an issue here unless the corresponding voice suffers different latency than the video; but jitter needs to be kept to below a few microseconds. As far as video conferencing is concerned, typical throughput required is upwards of 128 Kbps, and the latency and jitter needs to be low.

For voice communications what is most important is latency - it is unnerving to listeners to hear long and inconsistent pauses between sounds. Uncompressed voice requires 64 Kbps, but advanced vocoders can bring that down to as low as 2 Kbps. The codecs used for VoIP are called G.729 and G.723; G.729 operates at 8 Kbps, while G.723 used primarily for video telephony operates at around 6 Kbps.

Finally, as far as data traffic such as Web browsing or file transfer is concerned, jitter and latency are not important except maybe for graphics content; what is important is error rate since it directly affects the information.

3. Why is QoS important for Mobile WIMAX?

QoS is important in order to support different tiers of service for different customers. For business customers, guarantees on data rate, latency, jitter, and error rate are critical as far as Mobile WIMAX is concerned; otherwise the incumbent or a competitive provider will win the business. Realize that 802.16e is not coming into a vacuum; rather it is attempting to compete and coexist with other similar technologies such as 3G Cellular, WiFi, DSL, and Cable. And for applications such as enterprise video conferencing, high quality video streaming depends on the underlying network characteristics.

For residential customers it is important to remember that, cable companies as well as telecomm providers are starting to offer the triple and quadruple play and they are bundling services; it is into this space that Mobile WIMAX is being introduced. Voice and video content are intermixed with data in an end-to-end IP system such as Mobile WIMAX and each of these has different requirements for their transport. If the quality of the 16e connection is poor, or it affects mission-critical features such as voice, customers will churn and go with rival technologies.

Last, QoS is important because it enables the service provider to manage the network in the most efficient manner; this is a key issue in an area like wireless which is always ruled by scarcity of spectrum and capacity.

4. How is QoS accomplished for Mobile WIMAX?

QoS for Mobile WIMAX is handled at both the PHY and the MAC layers:

In terms of the PHY layer, Mobile WIMAX uses OFDMA which allows granular resource allocation and ensures that since the data is carried on multiple sub-carriers, the information can be decoded at the receiver even with channel impairments. In addition, FEC with interleaving, adaptive modulation (64 QAM to QPSK), and advanced antenna technologies offer high spectral efficiencies in both the downlink and uplink. All of these PHY enhancements allow high throughput which implies low latencies, and high SNR which implies low bit error rates. In addition, the Time-division-duplex form of Mobile WIMAX can dynamically allocate bandwidth on the forward and reverse link based on traffic needs. The last point worth noting in the PHY is that Mobile WIMAX offers adaptive burst profiles where parameters like modulation type, FEC, preamble and guard times can be adjusted on a frame-by-frame basis to best mate with existing channel conditions.

From a MAC point of view, QoS is accomplished through defining different "Service Flows" which are a set of data packets with the same QoS requirements assigned to it. An SFID (Service Flow ID) is assigned to each existing service flow; this serves as the identifier for the information flow between a Base and a Subscriber station. The service flow parameters can be managed through MAC messages in order to control traffic quality in a dynamic way. And this method of controlling QoS can be done on both the uplink and downlink. Once the base station and CPE establish a connection, the outbound MAC assigns packets to service flows, and the service flow is associated with QoS logical connections and determines the order in which the packets are sent out over the air-interface. These service flow parameters can be dynamically changed to accommodate changes in user demand. As opposed to priority-based QoS schemes, this method better enforces SLAs (service level agreements) through average and peak data rates, latency, and jitter for different kinds of traffic and for different tiers of customers.

The base station MAC works with the radio system to guarantee QoS while maximizing throughput. In turn the CPE MAC schedules packets from the connection queues into the transmission buffer so that, the CPE can transmit packets when the base station allocates the necessary bandwidth in different frames. Uplink and downlink frames can be allocated different choices of STC (space time coding), AAS (adaptive antenna system), and MIMO (multiple input multiple output) based on how the optional features in the standard have been implemented.

The Mobile WIMAX standard supports a range of data services with varying QoS requirements. For example, UGS (unsolicited grant service) is designed to support real-time service flows that sometimes generate fixed size data, such as T1/E1 and VoIP without silence suppression. 

In order to do a Streaming Audio or Video application, the QoS category is Real-Time Polling Service (rtPS) where the QoS specifications are Minimum Reserved Rate, Maximum Sustained Rate, Maximum Latency Tolerance, and Traffic Priority. In the case of ertPS (extended real-time polling service), the base station provides unsolicited unicast grants, thus removing the latency caused by bandwidth requests. nrtPS (non-real-time polling service) is a service class that offers unicast polls in order to ensure that the service flow receives request opportunities even during network congestion. Finally, in order to do Data transfer or Web browsing, the QoS class is Best-Effort service, and the QoS specifications are Maximum Sustained rate and Traffic Priority.

How is different broadband content such as voice, data, video, and graphics sent efficiently over the Mobile WIMAX wireless channel? That's accomplished by the MAC which has the following properties:

• Fast Data Scheduling: The scheduler is located at the base station in order to make quick decisions regarding the type of traffic being transported, its specific transport requirements, as well as existing channel conditions. Channel information is received through fast CQICH (channel quality indication channel) feedback, which enables the MAC to choose the most appropriate coding and modulation method. This is combined with HARQ (hybrid ARQ) to provide robust QoS requirements. In terms of QoS for the Uplink, the MAC needs timely and frequent information on channel conditions. The CPE requests uplink data bandwidth through the ranging channel, piggyback requests, and through polling mechanisms.

• Dynamic Resource Allocation: Through MAP (media access protocol) messages, the MAC supports frequency and time allocation on a frame-to-frame basis on both the downlink and the uplink. Thus the amount of resource in each allocation can range from one slot to the entire frame based on traffic requirements and channel conditions. In addition, each connection is associated with a single data service and assigned with a set of QoS parameters; thus QoS can be assigned on a very granular basis.

• Frequency Selective Scheduling: This means that the MAC can support QoS in sub-channels in order to mitigate channel impairments such as attenuation or fading. For example, with PUSC (partially used sub-channel) permutation, since the sub-channels are somewhat randomly distributed across the frequency band, all the sub-channels will be of similar quality.

5. How is QoS different for Mobile WIMAX compared to Fixed WIMAX?

QoS for fixed access is a whole lot less complex since it only involves stationary communications with the same base station. And even for limited portability, all that needs to happen is that the SLA needs to be transferred across Access Points during handoff. But with full mobility, there has to be guarantees for seamless QoS even while in the middle of a handoff. This becomes especially critical in the case of delay sensitive traffic such as voice and network-based gaming, and jitter-sensitive traffic such as video. Mobility may pose problems if base stations and mobile users have wireless interfaces from different manufacturers; this is because, each time a move is made, the mobile station has to authenticate and associate with the new base station.

6. How was QoS handled in allied technologies such as Cable, DSL, 3G, and WiFi?

When compared to other last mile technologies such as Cable, WiMAX's built in PHY and MAC provisions are better able to support QoS. It is not possible to do reservations of bandwidth on cable networks, and it is hard to commit to certain bit rates for enterprise customers. In cable, each video channel is given a fixed amount of bandwidth which cannot be made available for other purposes. This has advantages in that, there are no issues like jitter or insufficient bandwidth and this is the reason we can expect excellent video quality on cable connections. But things got more complex when cable operators started to offer broadband data. The operators assigned only certain fixed channels for data, and access was contention-based which meant that it was not possible to offer any kind of QoS or SLAs. And as far as voice is concerned, cable operators initially offered circuit-switched voice, but now most have migrated to VoIP wherein voice is allocated channels that are separate from data.

DSL service providers can do voice communications well - both circuit-switched as well as VoIP, since their traditional business was in offering toll-quality voice. But the story for data over DSL is not as rosy. Though the last mile to the consumer is a dedicated connection, and even the backhaul is usually on the provider's ATM network, once past the central office the data goes over the Internet. At this point, the transmission can degrade to Best-Effort. And as far as video-over-DSL is concerned, the earlier DSL technologies did not have sufficient bandwidth, and that's why DSL providers partnered with satellite entities for the video portion of the triple-play. 

QoS capabilities in 3G wireless are more limited than those in Mobile WIMAX. First of all, the system was not designed to be all-IP; rather IP is overlaid on the underlying circuit-switched layer. This can create queuing inefficiencies. At the PHY layer, cellular does not offer OFDM and adaptive modulation. At a service level, 3G does support different classes such as Background Class, Conversational Class, Interactive Class, and Streaming Class through priority based methods, but these kinds of methods can lock out certain types of traffic during periods of high usage.

Finally as far as QoS in WiFi is concerned, though WiFi offers multiple levels of priority and polling, there are no methods to reserve bandwidth; this will have an impact on latency sensitive traffic such as voice. In traditional WiFi, the MAC is contention based and thus there is no provision for priority or QoS. But with the new standard 802.11e - the MAC layer will be changed to allow priority mechanisms for say voice or video but note: the QoS guarantees will still only be statistical. Basically only traffic (not locations) can be prioritized, and higher priority traffic is assigned shorter deferral times, so it has a greater chance of grabbing the next opportunity to transmit. Voice and network control traffic are given the highest priority followed by video, Best-Effort data, and background data. Moreover, WiFi's QoS operates on a distributed architecture where the operation of the MAC is coordinated between Access Points and Subscriber Stations (SS), while WIMAX operates on a centralized architecture which allows the base station to have complete control of all the SSs in its network. Another disadvantage that WiFi QoS suffers is that the MAC uses acknowledgements which results in delays and overhead. In addition, the channel size is fixed, unlike in WIMAX where the channel size is changeable. One last point to note on this topic is that QoS on WIMAX could face the same complications as WiFi if the provider uses unlicensed frequencies; in this case it is much harder to ensure QoS parameters.

7. How will QoS be implemented in actual Mobile WIMAX deployments?

Operators will implement QoS on their networks based on the SLAs signed with their customers. This can include multiple variables like whether it is an enterprise customer or residential, the types of traffic to be supported, the physical location, the time of day, and so on. The operator will have to enforce policy by individual user and user group, in order to ensure that overall system QoS and SLA objectives are being achieved. And in terms of managing consistent policy across different service providers, standard IETF (Internet Engineering Task Force) mechanisms will be used.

In terms of vendors, WIMAX certification for the first phase does not mandate QoS provisions, thus there will be wide variations in early Mobile WIMAX equipment hitting the marketplace. Though the majority of vendors' solutions are software upgradeable from Fixed to Mobile WIMAX, some vendors are doing a better job in terms of handling QoS; this is because the technology is still relatively new. Some vendors deliver up to 16 separate SSIDs per CPE - each one has a different service, with different information rates, latency and jitter requirements. Others use service recognition and multiple classifiers for generating different service profiles and SLAs with QoS guarantees. Last, some vendors integrate VoIP capabilities to allow for seamless toll-quality voice with built-in QoS capabilities.

8. Summary

In conclusion, as far as QoS and Mobile WIMAX are concerned, the things that every company needs to determine are:

1. What are the applications running on their network, and what are its QoS needs?

2. How many of these applications need to run concurrently?

3. How many users can access these applications, and will they accept the associated costs?

4. Will users subscribe to these applications in a temporary or a permanent fashion?

5. Are end-user devices going to be primarily fixed, portable, or fully mobile?

6. If mobile, what will the typical speeds be, and how sensitive will the applications be to handoffs?

7. What architecture - both hardware and software - across the entire system from networks to subscriber stations can meet these QoS requirements?