Solution to WiMAX transmission

WiMAX is the IEEE 802.16 based wideband radio standard. It may provide the fixed, portable and fully mobile radio wideband connections, and even the-last-one-mile radio wideband access without direct line-of-sight basestation. WiMAX network structure is shown as Fig. 1.

Fig. 1  WiMAX network structure

CPE is the user part of WiMAX system. It includes general mobile equipment (MS) and general fixed equipment (SS). Access Service Network (ASN) consists of basestation (BS) and access gateway (ASN-GW). Connection Server Network (CSN) is composed of router, AAA server, HA, EMS, DHCP Server, Internet gateway equipment, etc.

The transmission network offers service transmission solution for ASN and CSN. ASN has many basestations distributed widely. The service transmission (usually called Backhaul) from basestation to AGW is the emphasis in the solution to WiMAX transmission. CSN has few equipments distributed centralizedly. Service transmission between CSN equipments is relatively simple. This article focuses on the demand and scheme of Backhaul transmission.

• Interface type and bandwidth demand

WiMAX basestation is designed with N×FE, GE or N×STM-1 interfaces. Most of them are N×FE interface.

The basestation channel bandwidth is mostly 5/10MHz, but 20MHz may be used in future. 1 or 3 sectors are generally available. If one omni-directional basestation is configured with 1×10MHz radio channel, the typical value of transmission bandwidth is 30~40Mbps. If one 3-sector basestation is configured with 3×10MHz radio channel, the typical value of transmission bandwidth will be 120Mbps.

• Basic demands of Backhaul transmission
• P2P transmission

WiMAX Backhaul service is the P2P service from basestation to AGW, so the intermediate node does not need the routing function. The transmission network should create specific connection-oriented service to facilitate service protection and E2E OAM.

• Convergence and statistical multiplexing

WiMAX adopts all-IP structure. Basestation and AGW mainly adopts FE/GE interfaces, and the services include broadband access, video, VoIP, etc. Such services as VoIP and video have low delay and low jitter. Broadband data service has large peak flow, bandwidth change has large dynamic range and burst service often appears. Therefore, the bandwidth should be assured for services with low delay and low jitter, e.g., VoIP, but the data service transmission network must have the convergence and statistical multiplexing functions which can share the bandwidth to improve the transmission efficiency and lower the cost.

• Connection-oriented and E2E OAM

The reliable connection-oriented service may be created between basestation and AGW to offer E2E OAM and minimize maintenance cost.

• Protection and reliability

WiMAX transmission network must have the carrier-class reliability and the availability of 99.999%. When a fault occurs to the network, the service can perform the timely switching and the protection switching time should be smaller than 50ms.

• Service-aware and DifferServ

In the transmission network the dispatching of different service types and service flows supports DifferServ, DSCP, TOS identification, etc. It offers the assured bandwidth for prior transmission of realtime services (e.g., VoIP), and provides other priorities or best-effort transmission for such services as email and broadband access to the Internet.

• Clock synchronization

The radio network has two levels of clock synchronization: frequency synchronization and clock synchronization. WiMAX basestation is the TDD system, and the OFDM system has high requirements for frequency accuracy and synchronization. The frequency accuracy should be 0.04ppm, and the deviation of timing reference 1pps should be smaller than 2us. The basestation is configured with GPS to obtain clock reference and proper locating, so it is unnecessary to get the clock out of service interface.

• Low cost

The carriers put much attention to low Total Cost of Ownership (TCO) of network construction. Backhaul has nodes distributed the most widely, the largest coverage area and the most complex application environment in the WIMAX transmission network. It is sensitive to CAPEX, but only low cost may make the network more competitive. And network OAM facilitation, compatibility with the existing network, and investment protection in equipment replacement and evolution must also be considered.

• Solution to WiMAX transmission

It may be found in the analysis of transmission demands that when WiMAX network is established, MSTP is still the best solution to WiMAX Backhaul service transmission. MSTP has found wide commercial application across the global, which proves that MSTP is the suitable solution to Backhaul carrying in the 2G/3G transition. MSTP-based WiMAX transmission solution is shown in Fig. 2.

Fig. 2  WiMAX transmission solution

The solution adopts the access-layer + convergence-layer structure to increase network coverage area and access point quantity. Access point consists of edge access point and common access point. Convergence point consists of edge convergence point and central convergence point. Central convergence point is directly connected to ASN-GW.

Access ring adopts chain or medium-sized/small ring structures (no more than 8 points) to improve network reliability and survivability. Edge access equipment usually adopts STM-1/4 level. Whether ring or chain structure is adopted depends on the fiber. Access equipment usually adopts STM-4 level and should have the capability of the upgrade to STM-16. Access point should adopt compact and low-consumption equipment for easy installation and maintenance.

Edge convergence point converges services in the access ring and then sends them to central convergence point. Finally the service will be converged to ASN-GW through central convergence point. On one hand, convergence ring may make the network hierarchy clear and the network manageable, maintainable and extendable. On the other hand, it can greatly alleviate ASN-GW port pressure.

Convergence node usually employs STM-16 equipment. When the traffic increases later, CWDM may be used to expand service capacity, or STM-16 equipment is upgraded to STM-64.

In the figure there are many access means in the edge access network. In addition to mainstream MSTP access, some transmission techniques like microwave and data network may be considered in the area where there is no fiber. However, they are not better than MSTP in the cost, bandwidth, reliability, OAM and so on, so they are just assistant and supplementary to MSTP access.

• Advantages

The solution adopts MSTP and CWDM equipment, so it has the following advantages:

• E2E transmission and powerful OAM

In the solution the service of below 2GE employs MSTP to create E2E connection, and the service of above 2GE employs CWDM to perform E2E transparent transmission. The maintenance staff can still adopt the traditional maintenance method to perform network OAM. The transmission network may implement high-efficiency and transparent transmission, and fulfill the carrier-class protection & fault recovery.

• High reliability and protection

MSTP equipment common boards have 1+1 hot backup and service boards have N+1 protection. The network has several mature and standard protection modes. The protection switching time is within 50ms and the network reliability is up to 99.999%.

• Convergence, statistical multiplexing and bandwidth sharing

ZTE’ MSTP product has EOS and RPR functions. EOS convergence rate is up to 48:1. It may meet the requirements in the WiMAX service convergence, and implement logic isolation and bandwidth sharing of different service flows through such techniques as VLAN and QinQ. Logic isolation ensures service security, and bandwidth sharing increases transmission efficiency and saves transmission cost. And ZTE is also very experienced in the RPR commercial application. RPR dual rings may work at the same time to support powerful statistical multiplexing function, which will greatly improve bandwidth utilization rate and finally save the cost. RPR can implement the statistical multiplexing via MAC layer to make high efficiency, but its cost is far lower than IP layer.

• QOS and DifferServ

ZTE deepens RPR technology so that the embedded RPR can process IP-layer priorities DSCP and TOS. According to different priorities, access services may be configured automatically to four RPR classes: Class A, A0, B and C.

Fig. 3  Service-aware and DifferServ

• Large-capacity transparent transmission of CWDM

Due to low cost, large capacity, transparent and easy scalability, CWDM is the best solution for access layer and convergence layer when there are few fibers. The maximum transmission capacity of a pair of fibers may be 45Gbps (that is, 18 wavelengths×2.5Gbps). The transmission bandwidth of WiMAX basestation is 30～40Mbps in the state of one-sector and 10MHz-carrier, and 120Mbps in the state of three-sector and 3×10MHz-carrier. CWDM equipment will find applications in such situations.

• Uniform transmission network and low cost

MSTP may access several types of services, e.g., data service and TDM service. If 2G network and its own transmission network are available when WiMAX network is created, the WiMAX network transmission capacity may be expanded through the original transmission network. When 2G and WiMAX basestations are at the same location, the same MSTP equipment can be used to access WiMAX services. When 2G and WiMAX basestations are at different locations, new MSTP equipment can be added to access WiMAX services. If the original SDH network can not expand Ethernet service capacity or it is limited to expand capacity, new MSTP equipment may replace SDH equipment.

When 2G and WiMAX use the same transmission network, MSTP Ethernet interface is used for WiMAX service transmission and E1 interface for 2G voice service transmission. On one hand, the original equipment investment is protected and the expansion cost is saved. On the other hand, only one team will work for the maintenance and management of transmission network, so the maintenance cost will be saved.

• Complying with WiMAX development trend

WiMAX future evolution and user rise will lead to increasing bandwidth demands, so bandwidth reservation and equipment upgrade should be considered to ensure sustainable development of WiMAX transmission network.

ZTE built a WiMAX network for a famous carrier in Asia Pacific. Fig. 4 shows the locations and ground object types of some WiMAX basestations. Fig. 5 is the solution to WiMAX network transmission. In the solution the original transmission network is replaced by and added with some MSTP access rings or chains.

Fig. 4  Locations and ground object types of WiMAX basestations

Fig. 5  Application case of WiMAX service transmission

The solution adopts mature EOS and embedded-RPR technologies to assure QoS and implement DifferServ and high-efficiency statistical multiplexing. The current and future transmission demands were considered in the network, so the solution was approved by the carriers.