Influence of LTE technology on PTN
China and the rest of the world are entering the brand new 4G/LTE era at full speed. As a mobile return technology, PTN faces severe challenges even after years of evolution and maturity. Some of these technological changes are huge enough to trigger a PTN revolution. This article takes into account the deployment of macro base stations and micro base stations, and reviews the changes and challenges faced by the LTE return technology compared with the current 3G deployment.
First, we will introduce the 2G and 3G infrastructure currently deployed. The current PTN network is the result of switching from 2g to 3G instead of SDH. 2G/3G base station traffic is aggregated by the aggregation device located in the base station and transmitted to the center point named RNC (RF network controller) or MSC (mobile exchange center. The traffic is further processed at this center and forwarded.
A key aspect of 2G/3G backhaul is the hub or star topology. The traffic of the base station is aggregated by the switch or router at the base station, and then transmitted back to the center through a point-to-point connection. Such point-to-point connections can be easily implemented on L2 Networks (such as PTN) Using Pseudo-line technology.
Carriers turn to LTE technology to cope with increasing mobile traffic. Obviously, the backhaul network needs to improve performance to adapt to the data transmission rate of LTE. However, in addition to higher performance, the LTE return technology also needs to solve other problems.
Figure 1 LTE return Network
From Layer 2 to Layer 3
LTE uses a flat IP structure, where RNC or central controller no longer exists. The traffic from the base station does not need to flow to the same center point. Instead, the LTE base station or eNodeB can send traffic to multiple access gateways. The base stations can also transfer traffic to each other. The 3G hub model has been replaced by the IP network format topology. The result is that the backhaul network needs to be upgraded from L2 PTN to L3 PTN to support routing.
After a layer-3 network is introduced, more functions are required for the return device. For example, VPN, OAM, and synchronization can be implemented on both the layer-2 and layer-3.
There are various ways to deploy LTE. You can create a new LTE base station or install it on an existing base station tower together with 2G and 3G base stations. In addition, 3G base stations can be upgraded to LTE through RF head and software. No matter in this way, operators are faced with the coexistence and co-management problem between 4G base stations and existing 2G and 3G base stations. If the same base station aggregation device supports both LTE backhaul and 2G/3G backhaul, the operator's capital expenditure can be greatly reduced. To achieve this, such base station aggregation devices must be able to support multiple business functions. The Convergence device must have Ethernet ports and services required to support LTE backhaul. In addition, TDM interfaces and TDM pseudo-line functions based on T1/E1 must be provided for 2G/3G backhaul. Multi-service functions are crucial to maximize the investment income of operators.
LTE micro Base Station
The introduction of the micro-base station concept also has a huge impact on backhaul. Many LTE operators are considering deploying several (from two to ten) Micro base stations around an LTE macro base station. These micro base stations can increase throughput and coverage. In this mode, the traffic of the macro base station is returned by the aggregation device at the base station. The traffic of the micro base station is transmitted back to a nearby macro base station or base station aggregation device in the most convenient way. Microwave and broadband connections are two promising technologies used for micro-base station backhaul.
LTE micro-base stations bring more traffic and users to the backhaul network. This not only improves the performance of the backhaul network, but also adds many specifications. For example, the number of users, the size of the forwarding list, and the number of OAM sessions must be elastically expanded to meet the needs of the increase in the number of micro base stations.
Due to the shortage of IPv4 addresses for mobile users and base stations, LTE micro-base stations have also accelerated the deployment of IPv6 in China.
For equipment suppliers, they need to develop new devices with more abundant resources and comprehensive IPv6 functions.
Synchronization and IEEE 1588
IEEE 1588 and synchronous Ethernet are two important synchronization technologies used for mobile backhaul. Synchronous Ethernet is used for frequency synchronization, while 1588 is used for phase or Todd (current time) synchronization.
Obviously, the change in LTE return brings about time and synchronization challenges.
A large number of LTE micro base stations or 1588 client devices impose a huge burden on 1588 servers or the master clock. LTE macro base stations or base station aggregation devices require boundary clock capabilities to reduce server load by 1588.
Introducing L3 to the backhaul network adds more latency and latency changes to the transmission of 1588 packets, which degrades the performance by 1588. The support for the border clock or transparent clock is crucial for the switch or router that receives the LTE return.
IPv6/UDP server 1588 is facing more challenges. To load 1588 on IPv6 and UDP, The 1588 implementation on the switch or vro must be able to update the UDP checksum in real time. This may mean that the device must be updated on the hardware.
Broadband access in the backhaul Network
LTE micro-base stations require the most popular wireless or wired connections for backhaul. Microwave and broadband access media such as DSL or PON have become obvious candidates.
Assume that a DSL or PON link exists between the micro base station and the 1588 server. The micro-base station runs the 1588 SLAVE mode and tries to restore the CTO from the server. This was originally a very basic configuration, but unfortunately this configuration is invalid. The reason is that the latency between DSL and PON is very large, and worse, the latency between the two is still asymmetrical between upstream and downstream. This will definitely impede 1588 normal operations.
Carriers are searching for solutions that support 1588 of broadband access. At present, there are several ways to solve this problem. We need to add special functions in the design of PON or DSL to support 1588 of the operation.
LTE and SDN (Software Defined Network)
The large-scale deployment of LTE, especially the LTE micro base stations, will also bring huge difficulties for operators to manage and operate the backhaul network. The operator is eager to find a solution that simplifies network management and reduces operating costs. SDN uses a centralized control plane and provides an open standard method to configure and manage the data plane of the return device. Therefore, SDN is a very promising technology in the return field.
To support SDN, the device supplier must support protocols such as OpenFlow in its control plane and its network management system. In addition, you must update the data plane to make it compatible with OpenFlow.
LTE has a significant impact on the backhaul technology, such as PTN, including the flat IP structure, introduction of micro base stations, and coexistence of 3G/LTE base stations. Operators are searching for a brand new return device optimized for LTE to reduce capital investment. At the same time, new network management technologies including SDN are used to save operation expenses. PMC's solution to these challenges is WinPath4, a processor tailored specifically for LTE.