Construction Solution for 3G Transmission Network on Access Layer

The 3G system offers abundant mobile data services. With the implementation of High-Speed Downlink Packet Access (HSDPA), it becomes possible to support abundant 3G device multimedia data services. Therefore, flow and interaction services among 3G services increase greatly.

    With an exclusive service-priority-classification function, the RPR guarantees service transmission QoS.
The RPR helps implement statistical multiplexing and bandwidth convergence of Node B data services on the access layer. As a result, convergence and transmission layers only need to offer point-to-point data private line transmission and ensure transmission bandwidth for accessed 3G services.

    When RPR-embedded MSTP  is used to establish the transmission network of the 3G network access layer, it implements compatibility with TDM services of the original network, ensures a smooth evolution to the 3G network, guarantees QoS of the transmission of the UTRAN-layer services, and helps realize low-cost network construction for operators.

The 3G network consists of the Core Network (CN), UMTS Terrestrial Radio Access Network (UTRAN) and User Equipment (UE). According to the metro transmission network layered structure, the 3G transmission network is also divided into the access layer, convergence layer and core layer. Construction of 3G networks has become the hottest topic in the telecom industry. What characteristics do 3G services have? The answers are:

    (1) Heavy Transmission Bandwidth Requirement
    A 3G network offers multimedia services such as voice, data and video services with a high data flow rate of 144 kb/s-2 Mb/s. 3G supports various handsets, and implements global seamless connectivity. Compared to 2G and 2.5G systems, a 3G system, with the introduction of 3G specific services, brings a much larger requirement for transmission network bandwidth.

    (2) Variety of Services
    In addition to traditional voice services, the main 3G services are data related, such as multimedia, Internet, messaging, location and special services based on business and individual needs. With strong bursts, data services require broad bandwidth, as well as high bandwidth utilization and powerful multi-service process ability. Thus, the bearer network for 3G service not only needs TDM techniques to guarantee voice QoS, but also packet switching techniques to sufficiently utilize bandwidth and transmit efficiency. Desirable packet switching techniques include ATM and Fast Ethernet/Gigabit Ethernet(FE/GE).

    (3) Different QoS Requirements

• Voice services: minimum fixed latency, no buffer, balanced transmission, and guarantee of bit rate
• Flow services: minimum variable latency, allowed buffer, non-balanced transmission, and guarantee of bit rate
• Interaction services: proper variable latency, allowed buffer, non-balanced transmission, and no guarantee of bit rate
• Background services: large variable latency, allowed buffer, non-balanced transmission, and no guarantee of bit rate

1 Solution to Implementation of 3G Transmission Network on Access Layer
    3G services development brings two important requirements to the transmission network: support for
multi-service access and good scalability.

    During the primary stages of 3G network building, voice exists as the dominant service, with data service applications gradually growing at the same time. At this point, data services are mainly "voice" and "flow" applications, which won´t bring large traffic to the base station. As stated, voice and flow services have strict requirements for both latency characteristics and bit rate. Consequently, how to ensure service QoS is the most important issue to consider during the building of the transmission network.

    As for the solution at this primary stage when there is no large demand for service bandwidth, it is preferable to adopt mature technologies to transmit services in a transparent mode. At the same time, the QoS requirements and the costs of network construction should be taken into consideration.

    The existing Synchronous Digital Hierarchy (SDH) networks are used for the transparent transmission of 3G services. This solution will not only ensure the high quality of service transmissions, but also fully utilize available SDH equipment without reconstruction. Accordingly, a network can be built up quickly with low cost. This solution separates service and transmission layers, thus the network layers are very clear in concept, and easily managed.
As shown in Figure 1, the Inverse Multiplexing over ATM (IMA) E1 services of Node B (base station) have access to the transmission network via the transmission equipment E1 interface. These services are transparently transmitted to the core backbone node Radio Network Controller (RNC) with VC-12 as the bearer. These services may also be directly connected to the E1 interface of the RNC, again, via the transmission equipment E1 interface. Such a networking mode is relatively simple, and is preferred when RNC capacity is small.

    When there is large traffic accessing the RNC and a large number of E1 interfaces are necessary, the networking structure mentioned above would increase RNC cost and include the problem of laying cable. The solution is the VC-12 time slot cross-configuration function of the Multi-Service Transport Platform (MSTP) equipment, as part of the convergent layer of the transmission network docked to the RNC. The transparently transmitted E1 services are configured to VC-4 by the VC-12 time slot cross, and connected to the RNC via the channeled STM-1 interface. In this way, the pressure on RNC interfaces is greatly reduced and networking costs are decreased.

    In addition, data from Node B may be accessed from the transmission network via the STM-1 interface, and transparently transmitted to the RNC of the core backbone node with VC-4 as the bearer, which may also be connected to the RNC interface directly via the STM-1 interface of the transmission equipment. Such a networking mode is simple, but requires a major channel on the transmission network, and may be adopted when Node B operates with large traffic.

    When Node B has a large amount of traffic, services there may directly access the transmission network via the ATM STM-1 interface. The ATM of MSTP may be used to establish the VP-RING, in order to bear and transport accessed ATM services from Node B. Use of such a method prevents these services from occupying 100% of the VC-4 bandwidth, and thus fully capturing network bandwidth resources. However, it requires connecting the ATM layer of the services from the base station to the transmission network for configuration purposes. This increases both the difficulty of management and configuration of the transmission network and the cost of network construction
or reconstruction.

2 ZTE´s Solution to 3G Transmission Network on  Access Layer
As described above, various types of 3G services have different QoS requirements, while the QoS requirements also have an impact on the network.

    First, proper latency minimization (average level) requires considerable bandwidth resources, which means that the network must have enough transmission bandwidth to reliably distribute to a large number of connections. The longer the allowable latency is, the less the required network resources are.

    In addition, another factor is the buffer for bit streams and packets. Temporarily storing the data stream may minimize the usage of valuable network resources. However, if the data is allowed to delay too long, the number of buffered messages will increase.

    Therefore, delays would lead to a rapid increase in the resources required for buffering, which would make users complain about network quality and perhaps unsubscribe.

    Capital, i.e. cost and investment, is a general factor affecting the demands on network resources and buffering.
Figure 2 indicates that different QoS requirements have different latency requirements, and that both very good and very bad QoS are relatively expensive and in fact, not good choices.

    The 3G system offers abundant mobile data services. With the implementation of High-Speed Downlink Packet Access (HSDPA), it becomes possible to support abundant 3G device multimedia data services. Therefore, flow and interaction services among 3G services increase greatly. At this time, the requirement for network bandwidth is much more than during network construction primary stages.

    Therefore, if the service transparent transmission is maintained, it will cost too much to greatly upgrade the rate of the network. In addition, new technical measures need to be employed to optimize network construction, not only guaranteeing QoS during service transmission, but also modulating network construction costs.

    After comprehensive consideration regarding the QoS requirements of service transmissions and the low-cost requirements of network construction, ZTE Corporation believes that Resilient Packet Ring (RPR) technology could be introduced into the access and transmission layers of  Node B services on the UTRAN layer. Consider the illustration in Figure 3 as an example. The MSTP is used to establish a 622 M ring on the access layer. The ring offers bandwidth of 1 x 155 M for compatibility with originally accessed PSTN or 2G TDM services, and simultaneously provides service path protection. The other 3 x 155 M is used, via the embedded RPR ring, to supply
bi-directional 900 M bandwidth (approximately) for data service access. Additionally, the RPR ring offers protection for those services.

    With an exclusive service-priority-classification function, the RPR guarantees service transmission QoS. VLAN ID is used to distinguish different types of Node B accessed services. The connection of voice and signaling data services on the control layer (with high priority), adopts RPR A-type service configuration. Accordingly, with the
pre-reserved bandwidth and the rapid ring-pass transfer mechanism of RPR, these services are transported on time and the requirements of low latency, and transmission at a certain bit rate, are both met.

    With regard to ordinary data services such as Internet services, the RPR C-type service configuration would be distributed. Since these services are burst data, the fairness algorithm of RPR allows some nodes to "over occupy" network bandwidth in order to transmit services with over-configured bandwidth. This configuration is allowed when network bandwidth has margins, and guarantees the data services of every node access to the network when it is busy. In this way, network bandwidth is resilient and economically utilized.

    Moreover, the RPR helps implement statistical multiplexing and bandwidth convergence of Node B data services on the access layer. As a result, convergence and transmission layers only need to offer point-to-point data private line transmission and ensure transmission bandwidth for accessed 3G services.

    When RPR-embedded MSTP  is used to establish the transmission network of the 3G network access layer, it implements compatibility with TDM services of the original network, ensures a smooth evolution to the 3G network, guarantees QoS of the transmission of the UTRAN-layer services, and helps realize low-cost network construction for operators.

Manuscript received: 2005-06-20