WiMAX Architecture Transmitter Receiver
Fixed WiMAX has two types of architectures: Point-to-Point (P2P) Architecture and Point-to-Multipoint (P2MP) Architecture. In P2P architecture, there is one transmitter and one receiver. In WiMAX's application as a backhaul, this architecture is used, in which a WiMAX base station acts as a transmitter and another base station acts as a receiver as shown in figure 1 below. Due to P2P architecture connectivity as far away as 30 miles becomes possible. So P2P finds application in providing wireless backhaul between two locations, thus connects cellular towers and Wi-Fi networks to cover a large geographical area.
P2MP architecture is based upon IEEE 802.16.2004 standard. As the name suggests, in this architecture there is one transmitter and several receivers. P2MP is synonymous with distribution. One base station supports thousands of subscribers which might be similar or dissimilar.
As shown in figure, a WiMAX base station provides services to corporate branch offices, small businesses, residences and Wi-Fi hot spots. So WiMAX MAN uses P2MP architecture to locate base stations strategically so as to cover a large metro-area via microwave links.
[1] (2006, November, 12) “Wireless architecture: point-to-point and point-to-multipoint”
http://www.wimax.com/education/wimax/wireless_architectures
[2] (2005, April, 25) “Standards versus Proprietary Solutions- The Case for WiMAX Industry Standards”
http://www.alvarion.com
Wireless Architectures
by Carl Townsend — last modified 2006-08-14 08:45 PM
The following section will provide a simple overview of wireless concepts and nomenclature to help the reader understand how WiMAX works and will assist the reader in com-municating with the WiMAX industry. Wireless architecture: point-to-point and point-to-multipoint There are two scenarios for a wireless deployment: point-to-point and point-to-multipoint. Point-to-point (P2P) Point to point is used where there are two points of interest: one sender and one receiver. This is also a scenario for backhaul or the transport from the data source (data center, co-lo facility, fiber POP, Central Office, etc) to the subscriber or for a point for distribution using point to multipoint architecture. Backhaul radios comprise an industry of their own within the wireless industry. As the architecture calls for a highly focused beam between two points range and throughput of point-to point radios will be higher than that of point-to-multipoint products. Point-to-Multipoint (PMP) As seen in the figure above, point-to-multipoint is synonymous with distribution. One base station can service hundreds of dissimilar subscribers in terms of bandwidth and services offered. Line of sight (LOS) or Non-line of sight (NLOS)? Earlier wireless technologies (LMDS, MMDS for example) were unsuccessful in the mass market as they could not deliver services in non-line-of-sight scenarios. This limited the number of subscribers they could reach and, given the high cost of base stations and CPE, those business plans failed. WiMAX functions best in line of sight situations and, unlike those earlier technologies, offers acceptable range and throughput to subscribers who are not line of sight to the base station. Buildings between the base station and the subscriber diminish the range and throughput, but in an urban environment, the signal will still be strong enough to deliver adequate service. Given WiMAX's ability to deliver services non-line-of-sight, the WiMAX service provider can reach many customers in high-rise office buildings to achieve a low cost per subscriber because so many subscribers can be reached from one base station. Next Section
WiMAX Technical Information
The IEEE 802.16 Air Interface Standard is truly a state-of-the-art specification for fixed broadband wireless access systems employing a point-to-multipoint (PMP) architecture. The initial version was developed with the goal of meeting the requirements of a vast array of deployment scenarios for BWA systems operating between 10 and 66 GHz. As a result, only a subset of the functionality is needed for typical deployments directed at specific markets. A revision to the base IEEE 802.16 standard targeting sub 11 GHz is near completion with a publishing target date of July 2004. This revision will include the amendments from Task Group c, Task Group a, and Task Group d.
The IEEE process stops short of providing conformance standards and test specifications. In order to ensure interoperability between vendors equipment, the WiMAX technical working groups have completed the work for 10 to 66 GHz and has started work for the sub 11 GHz part of the standard. The working groups develop a set of system profiles, Protocol Implementation Conformance Statement Proforma, Test Suite Structure & Test Purposes, and Abstract Test Suite specifications for 10 to 66 GHz and sub 11 GHz, all according to the ISO/IEC 9464 series (equivalent to ITU-T x.290 series) of conformance testing standards.
Overview of IEEE 802.16
The IEEE 802.16 Working Group has developed point-to-multipoint broadband wireless access standard for systems in the frequency ranges 10-66 GHz and sub 11 GHz. The standard covers both the Media Access Control (MAC) and the physical (PHY) layers.
A number of PHY considerations were taken into account for the target environment. At higher frequencies, line of sight is a must. This requirement eases the effect of multipath, allowing for wide channels, typically greater than 10 MHz in bandwidth. This gives IEEE 802.16 the ability to provide very high capacity links on both the uplink and the downlink. For sub 11 GHz non line of sight capability is a requirement. The original IEEE 802.16 MAC was enhanced to accommodate different PHYs and services, which address the needs of different environments. The standard is designed to accommodate either Time Division Duplexing (TDD) or Frequency Division Duplexing (FDD) deployments, allowing for both full and half-duplex terminals in the FDD case.
The MAC was designed specifically for the PMP wireless access environment. It supports higher layer or transport protocols such as ATM, Ethernet or Internet Protocol (IP), and is designed to easily accommodate future protocols that have not yet been developed. The MAC is designed for very high bit rates (up to 268 mbps each way) of the truly broadband physical layer, while delivering ATM compatible Quality of Service (QoS); UGS, rtPS, nrtPS, and Best Effort.
The frame structure allows terminals to be dynamically assigned uplink and downlink burst profiles according to their link conditions. This allows a trade-off between capacity and robustness in real-time, and provides roughly a two times increase in capacity on average when compared to non-adaptive systems, while maintaining appropriate link availability.
The 802.16 MAC uses a variable length Protocol Data Unit (PDU) along with a number of other concepts that greatly increase the efficiency of the standard. Multiple MAC PDUs may be concatenated into a single burst to save PHY overhead. Additionally, multiple Service Data Units (SDU) for the same service may be concatenated into a single MAC PDU, saving on MAC header overhead. Fragmentation allows very large SDUs to be sent across frame boundaries to guarantee the QoS of competing services. And, payload header suppression can be used to reduce the overhead caused by the redundant portions of SDU headers.
The MAC uses a self-correcting bandwidth request/grant scheme that eliminates the overhead and delay of acknowledgements, while simultaneously allowing better QoS handling than traditional acknowledged schemes. Terminals have a variety of options available to them for requesting bandwidth depending upon the QoS and traffic parameters of their services. They can be polled individually or in groups. They can steal bandwidth already allocated to make requests for more. They can signal the need to be polled, and they can piggyback requests for bandwidth.
Mesh
Mesh architecture captures the high reliability envisioned by the early Internet pioneers. Mesh potentially offers advantages to the service provider including negating the need for a separate backhaul network and improved reliability. The reliability function of a mesh network is notable in that if one node fails, backhaul traffic could be routed around that failed node minimizing the service disruption to only those subscribers directly served by that failed base station. There is a lot of industry buzz around mesh networks and some success in the Wi-Fi market, however, mesh technology has yet to materialize in either the WiMAX access or the millimeter wave markets. As illustrated in Figure 2 below (current Wi-Fi mesh products, for example), a pure mesh network architecture links every base station. That is, every base station radio that services the access network pulls double duty as a backhaul radio. For the WiMAX service provider planning a high capacity, high reliability network, a mesh architecture may not be the best choice of architectures. The chief detraction to this strategy is that valuable spectrum that should be used for subscriber access is being used for backhaul. Some industry analysts speculate that backhaul needs might consume 50% of available access spectrum. A pure WiMAX mesh network remains an engineering challenge. At the time of this writing, no WiMAX mesh product has been commercially deployed. A complex routing technology must be built into the WiMAX radio. Many vendors claim that the marriage of a WiMAX radio and router at a WiMAX base station might adequately function as a WiMAX mesh. Few service providers have the in-house resources to do the necessary research and development to produce a WiMAX mesh solution.
This brings the discussion the "meshing" backhaul radios such as the millimeter wave products that compliment WiMAX network in their gigabit plus throughput and multiple kilometer ranges. The advantage of this would be that a) the backhaul would use spectrum separate from that being used for WiMAX access and b) the same advantages of meshing that apply to existing Wi-Fi mesh enhancing reliability by routing traffic around ailed links would apply. Figure 3 below illustrates a meshed backhaul network.
At the time of this writing, no mesh millimeter wave backhaul product is available on the open market. Millimeter wave vendors state their products are intended as bridges and not routers. That is, in Internet-speak, they link two points point-to-point (bridging) and do not perform a routing capability necessary to support meshing. Figure 4 below illustrates the technical difference between a bridge and a router.
Just as no true mesh product has arrived in the WiMAX market, no mesh product is shipping in the millimeter wave backhaul market.
Ring Of the backhaul architectures discussed above, the ring is the most popular among millimeter wave applications. The ring architecture is best known from SONET ring architectures popular in fiber optic networks. Figure 5 below illustrates a simple ring architecture.
The success of the SONET ring architecture lay in its consecutive point network technology. The logic in consecutive point technology is that the data flow in a network is either clockwise or counter clockwise in a circular pattern. If one link in the consecutive point network were to fail, the intelligence in the network immediately senses this and reroutes traffic in the opposite direction thus minimizing the impact of the failure on that link. Figure 6 below illustrates a consecutive point network.
As a ring network grows, there is the potential for ever more subscribers (regardless of the percentage of the total number of subscribers) to be without service in the event of the failure of one link on the network. A ring or consecutive point network's availability (reliability) is enhanced by architecting it as a Figure 8. This way, in the event of failure on one link, the percentage of subscribers without service is further minimized.
The "gotchas" of Backhaul: QoS and Availability The two chief detractors from QoS are latency and jitter. Latency and jitter can multiply when traffic must route over multiple hops. That is, there is a "cost" for traffic for every additional hop it goes through. VoIP and video (potentially the bread and butter for the WiMAX service provider) are particularly vulnerable to latency and jitter induced when the packet stream must pass through the WiMAX base station and a number of backhaul radios until reaching the fiber point of presence (POP) connecting to a fiber backbone that is optimized to reduce latency and jitter using such technologies as multi protocol labeling system (MPLS). Latency occurs in both the wireless portion (over the air) and in radios and routers in a network. Ergo, the fewer hops a subscriber's packets have to make from the home or office to their destination (and vice versa).
Â
In engineering a wireless backhaul network, the network architect should minimize the number of hops from subscriber to fiber POP. As Figure 8 above illustrates, latency is cumulative across any network. There is latency both in over the air links as well as in the wireless radio and across the IP backbone. Too many hops over too many links could add unacceptable levels of latency and jitter detracting from the QoS for VoIP and video. High Availability Perhaps the simplest means of ensuring the highest availability and reliability in a network is to build in redundancy where ever possible so that there is no single point of failure. In a wireless backhaul network, this is accomplished by doubling the radios in the network such that if one radio (link) goes down, its backup radio can immediately take up the load. While this may double the total price tag for backhaul radios, it is much less expensive than alternative Â
It should be noted that having redundant radios in the backhaul network will do little good if the internet backbone provider servicing the backhaul network suffers a network outage. For that reason, the service provider should plan for multiple IP backbone service provider connections on their network. Not only does this provide a disaster recovery solution, it also presents the WiMAX service provider the opportunity to shop IP backbone providers for pricing and advantageous service level agreements (SLA). The WiMAX service provider can load balance between multiple IP backbone service providers for the optimum mix in pricing, SLA (reliability/availability), and geography (distance between fiber points of presence - POP). Conclusion
This article explored the challenges of architecting a wireless backhaul network to support a WiMAX access network. That network must be able to support the bandwidth demands both current and future for both residential (think IPTV at 19 Mbps per TV) and businesses (multiple megabits per employee per business). Any WiMAX access network must be carrier grade and completely and favorably with the PSTN's alleged "five 9s" of reliability. Network architecture must first be planned with an eye to providing the most "fail safe" backhaul network possible. This is accomplished by planning to minimize the percentage of subscribers affected by a network outage as much as possible. As the network must support time sensitive application such as VoIP and HDTV video, the backhaul network must offer the optimum in QoS in the form of minimizing latency and jitter which is best accomplished by minimizing the number of hops between the fiber POP and subscriber.
We provide a professional essay writing service that thousands of our customers use as an effective way of improving their grades, improving their research and saving them lots of time.
