1 Introduction
LTE (LONg Term EvoluTIon, long-term evolution) is the latest standard of mobile communication system formulated by 3GPP (Third Generation Partnership Project Organization). In fact, LTE itself is also constantly evolving. The currently defined LTE is not a commonly understood 4G technology, but a 3.9G or quasi 4G standard, and its corresponding R8 standard has been officially frozen. It takes OFDM (orthogonal frequency division multiplexing), MIMO (multiple input multiple output) and other advanced physical layer technologies as the core, improves and enhances the 3G air interface technology, and can provide downlink 100Mbit under 2 & TImes; 2MIMO, 20MHz spectrum bandwidth / s and upstream 50Mbit / s theoretical peak rate (when using 4 & TImes; 4MIMO, the downstream peak rate can even reach 326Mbit / s); supports FDD (frequency division multiplexing technology) and TDD (time division multiplexing technology) two kinds of work Mode, and supports high-speed mobile up to 500km / h; R10 is an enhanced version of R8, and its theoretical peak rate has reached the level of 1Gbit / s in the downlink and 500Mbit / s in the uplink, so it is called LTE-Advanced / LTE-A This is the so-called 4G technology. Currently, R10 is under development.
LTE will be the world's most important wide-area broadband mobile communication system in the future. In the future, all 2G / 3G / 3.5G technologies will share the same path and evolve to the LTE / LTE-A stage. However, due to the different development of 2G / 3G networks of operators in different countries, the evolution route for LTE is different. At present, there are the following 4 LTE evolution routes:
(1) Evolution in the most traditional way. That is from 2G to 3G, and then to the long-term evolution of LTE. For example, Orange is a typical representative of this robust strategy. Its main idea is to strengthen its 2G / 3G development strategy, and LTE is its long-term evolution goal.
(2) Jumping evolution. That is, directly evolve from 2G to LTE stage. T-Mobile is a typical representative of this idea. Its main development strategy is to limit investment in 3G / HSPA, skip HSPA +, and go directly to LTE in 2010.
(3) Handover evolution between different systems (3GPP / 3GPP2). For example, Verizon started the commercial process from CDMA2000 / EV-DO to LTE in 2010.
(4) Evolution from TD-SCDMA to LTE. This is a specific evolution idea of ​​China Mobile. At present, it has clearly defined the big idea of ​​actively promoting the development of the TD-LTE industry.
In 2010, foreign telecom operators have started the first round of commercial use. North America (Verizon), Europe (TMO), Japan (DoCoMo) and other operators have all begun to deploy commercial networks; European FT, Telefonica, VDF have also announced 2011 The second round of commercial use will be launched this year. Relatively speaking, China's LTE is still in the technical start-up phase, and it will take about five years before large-scale commercial use of more than one million users. Especially in the past two years, huge investments have been made in 3G, so it is unlikely that LTE licenses will be issued again in a short time. Therefore, these two years will mainly focus on the construction of test, trial and trial commercial networks.
2 LTE requirements for bearer networks
2.1 New changes in LTE networks
The LTE radio access network is named Evolved UMTS Terrestrial Radio Access Network (E-UTRAN), and the core network is Evolved Packet Core Network (EPC). Compared with 2G / 3G networks, the most significant new changes in LTE networks are:
(1) The network structure is fully IP-based. The core network cancels CS (circuit domain), and the all-IP EPC supports 3GPP and non-3GPP technologies to achieve unified access to achieve fixed network and mobile convergence (FMC). EPC is mainly composed of MME (Mobile Management Entity), SGW (Service Gateway), PGW (Packet Data Gateway) and other major functional entities. Among them, MME is mainly responsible for all control functions of user and session management, which is equivalent to 2G / 3G SGSN control plane. SGW is mainly responsible for user plane data transmission, forwarding and routing switching, etc., which is equivalent to the 2G / 3G SGSN user plane; PGW is the anchor point for all external data network access, similar to the 2G / 3G GGSN function.
(2) Flattened network architecture. The traditional 3GPP access network is composed of two layers of nodes, Node B and RNC. However, the layer of RNC is omitted in LTE. The eNB (evolved NodeB) directly accesses the EPC equipment. Therefore, E-UTRAN is mainly composed of eNB. The purpose of this flat structure is to simplify the network structure and reduce the network delay.
(3) Two new interfaces are introduced. One is the flexible networking method that supports the S1 interface, the so-called S1-flex function. S1 is the interface between eNB and MME / SGW. In 2G / 3G network, RNC can only connect to one core network element, but S1-flex allows one eNB to connect to multiple MME / SGW POOL (pool) to achieve load balancing, Redundancy, etc. Each eNB supports up to 16 S1 interfaces; another important change is the X2 interface, that is, the distributed interface between adjacent eNBs, which is mainly used for mobility management (handover) and interference suppression of neighboring cells. Each eNB can define 32 For an X2 interface, the number of neighboring base stations during actual deployment is determined by coverage.
2.2 LTE requirements on the bearer network
(1) Transmission bandwidth. The access bandwidth of LTE base stations has been significantly improved compared to 3G networks. According to empirical estimates, taking the TD-LTE S111 station type as an example, the transmission bandwidth of the eNB is at least about 440 ~ 590M. If the S222 station type is adopted, the transmission bandwidth will be doubled, that is, around 880 ~ 1180M, which is almost 10 ~ 20 times the bandwidth of the 3G base station.
(2) The changes in the LTE network structure also have a greater impact on the bearer network. The introduction of S1flex and X2 interfaces breaks the original 2G / 3G converged networking architecture, which requires the bearer network to have flexible services on the original basis. Dispatching ability.
(3) LTE has stricter requirements on QoS and delay than 2G / 3G. LTE will provide more types of data services (multimedia, video, interactive, etc.) than 3G. 3GPP specifies the end-to-end delay and packet loss rate requirements of various services in detail, so the bearer network is required to have strong QoS scheduling Capability; 3GPP recommends one-way delay of S1 / X2 interface is 2 ~ 15ms, NGMA (Next Generation Mobile Alliance) requires that the maximum end-to-end delay of the bearer network is less than 10ms, and the special area should meet the requirement that the delay does not exceed 5ms.
LTE will achieve deep coverage, and the number of eNB nodes will be 2 ~ 3 times the number of existing base stations. The size of the bearer network should meet the needs of this group of large networks; LTE will coexist with 2G and 3G networks, requiring the bearer network not only to have IP grouping Service carrying capacity, and the need for unified access to TDM and IP multi-services should also be considered; of course, carrying IP also requires the network to ensure high reliability, LTE protection requirements are similar to the protection of 2G / 3G bearer networks, and failover is less than 50ms; At the same time, LTE also has requirements for frequency synchronization and time synchronization.
3 LTE bearer solution and analysis
Since EPC is a core network that is fully IP-based, the IP backbone network is the only bearer network between the main core network elements. The IP backbone network here is CN2 for China Telecom and an IP private network for China Mobile. Therefore, there is no suspense in the core network bearer solution. IP over WDM / OTN is the best backbone layer solution to meet the current and future development trends. Therefore, the E-UTRAN bearer solution has become the research focus of the LTE bearer network.
In general, the E-UTRAN bearer network is also referred to as the LTE backhaul network, and mainly refers to the metropolitan area transport / bearer network part (only a small amount of traffic needs to pass through the IP backbone network). The solution should meet the requirements of LTE high bandwidth, flexible service scheduling, large network, multi-service bearing, high reliability and QoS, low latency, time synchronization and other aspects. Based on these needs, the industry proposes multiple solutions from different angles.
3.1 End-to-end PTN solution (see Figure 1)
Figure 1 Full PTN solution
This is a plan adopted by China Mobile on a large scale. To further divide, the end-to-end PTN solution has two implementation methods: the first is a pure L2 method, the existing PTN does not need to support complex L3 protocol, and the L2 channel is used from top to bottom; the second method requires the core layer equipment upgrade to support L3VPN .
The device is the simplest to implement in pure L2 mode, and has low requirements for operation and maintenance personnel. The static LSP is used to solve the S1flex and X2 distributed interface requirements. However, the defects are mainly waste of label resources, because this method must go from base station to multi-homing Multiple LSPs are pre-established between the gateways. Although the use of multi-segment PW / LSP nesting can save the problem of label waste to a certain extent, in general, the flexibility of business scheduling is poor; the introduction of L3 in the core layer can better solve the defects of pure L2, This saves label resources and enables flexible scheduling of services, that is, the core layer L3VPN implements other scheduling services such as S1 flex or X2.
3.2 PTN plug-in CE router solution (see Figure 2)
Figure 2 PTN plug-in CE solution
This solution is still based on PTN, which is actually an extension of the end-to-end L2 PTN solution. That is, PTN still maintains the end-to-end L2 function, and the newly introduced CE is responsible for the L3VPN function to implement dynamic IP services such as S1 flex and X2. It should be noted that CE here refers to the customer edge router (CuSTomer Edge Router).
There is a view that this solution is suitable for large-scale networking and operation and maintenance optimization, so it is likely to be one of the mainstream of LTE bearer applications, but the main problem is that the introduction of CE routers may bring new hidden troubles. If the CE fails, it will cause The entire mobile network is paralyzed, so using a pair of CEs can solve this problem to a certain extent.
3.3 End-to-end router solution (see Figure 3)
Figure 3 Full router solution
From the core to the entire network, dynamic routing protocols are used to carry IP services, and PWE3 pipes are used to carry traditional services such as TDM / ATM. Obviously, this solution has natural advantages in handling LTE dynamic services, and has end-to-end flexible service scheduling capabilities, without the need to establish many channels in advance like L2. However, the biggest problem with the full router solution is the relatively high cost of network construction, the large power consumption of equipment, and the limitation of the scale of networking. In addition, this large-scale dynamic network has the highest requirements for operation and maintenance, and the current network protection and OAM capabilities of routers are weaker than PTN equipment. Therefore, this solution is suitable for small-scale networks and has poor applicability to such large-scale networks of domestic operators.
3.4 Router + IAN / PTN / CE hybrid networking solution (see Figure 4)
Figure 4 Router + PTN / IAN / CE
The core / aggregation layer of this solution uses routers, and the access layer uses IAN / PTN / CE, where CE refers to a carrier-grade Ethernet switch. It can be further expanded into 3 ways:
(1) Router + PTN mode.
(2) Router + CE mode.
These two methods are essentially that the core / aggregation layer supports L3 routing, and the access maintains the static L2 function. Only PTN (MPLS-TP) and CE (PBT or VPLS) adopt different technical systems.
(3) The core / aggregation layer uses routers, and the access layer uses IAN. IAN stands for Integrated Access Node Equipment, which is a newly defined data equipment combining MPLS-TP and IP / MPLS functions by China Telecom. This method is more suitable for operators with rich IP metropolitan backbone network resources. On the basis of implementing LTE service bearers, it can also achieve comprehensive bearing of high-quality premium services such as private lines and multicast for large customers.
At present, for router + CE, CE's ability in OAM is relatively weak; adopting router + PTN solution, there are some problems of end-to-end management, interoperability of heterogeneous networks, and the need for data communication and The transmission professional jointly maintains the wireless backhaul network; using the router + IAN solution, the LTE backhaul network is networked by end-to-end data communication products, and unified by the data communication professional operation and maintenance, so this solution is for fixed network resource-rich operations Business is more attractive.
4 Conclusion
There are currently several solutions for LTE backhaul networks, such as end-to-end PTN, PTN + CE (customer edge router), end-to-end router, router + IAN / PTN / CE (Carrier Ethernet Switch). Among them, the first two solutions are mainly based on PTN, and the latter two solutions are mainly based on data equipment. In terms of technical feasibility, these solutions can meet the requirements of the LTE backhaul network. The main difference lies in the starting point of L3 routing. In fact, the reason for introducing the L3 function is to provide flexible service scheduling capabilities in a suitable location. The ideal situation is to not only hope that the network has high flexibility and reliability, but also to be simple and easy to maintain, but these two aspects are often It is more difficult to unify. Generally, a smaller-scale network has relatively greater flexibility, and the network size of domestic operators is often very large, so it is more important to pay attention to reliability and maintenance capabilities. Therefore, when considering the choice of LTE bearer solution, it is necessary to find a suitable balance point. Due to different network resource conditions, different operators often have different options.
From the current domestic development situation, PTN-based solutions have achieved large-scale commercial use, so the maturity is the best; router + IAN-based data network-based solutions are more innovative and suitable for rich IP cities The operator of the domain network resources, but the actual engineering applications are few, and the maturity needs to be further verified.
Although LTE is still in large-scale commercial use in China, major mainstream network equipment providers have already launched related bearer network products. Fiberhome Communications, as a well-known domestic information communication product and solution provider, its PTN products have successively served the construction of mobile bearer networks in many provinces and cities in China, such as Heilongjiang, Liaoning, Shandong, Hebei, Hubei, Hunan, and Jiangsu. In the process of transition from traditional optical network products to packetization, FiberHome will continue to take customer needs as its own responsibility and bring more comprehensive solutions to the telecom operators in transition through continuous technology and product innovation to meet different future applications. The demand for LTE bearer network construction in this scenario.
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