rc whitepaper next generation access network-1 20110224

Whitepaper Next Generation Access Network 1/4

Raisecom Technology Co.,Ltd February 2011

www.raisecom.com // Whitepaper

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1. General Situation for Access and Unbalanced

Development between Access and Backbone

The new requirements and new services provisioning are both never drastically hungered to be developed like today. In the past two dozens of years, telecom access for the key accounts was dominant by TDM technology from a very low speed asyn interfaces to E1/T1, STM1/OC3 which almost covered all over the world network. Just in the past 5 years, Ethernet is becoming more and more popular to access the customers, but mainly for residential customers. As matter as fact, backbone was shifted gradually to IP network with very high bandwidth and hard QoS requirements several years ago. And many mobile operators have deployed an IP/MPLS network, but only for signaling between MSC and some media gateway connection. PWE3 technology is emerging to add fuel to the fire. It seems all the services possibly will shift to IP backbone one day, no matter of voice, video, data, etc. However, this TDM access still keeps a little bit long time for the reason of bad reputation and less diagnostics function for current Ethernet access network, even though Ethernet backbone is ready for operation. So, business connection for the enterprise market, financial customers and voice service are still running on the TDM network mostly.

2. Evolution of Mobile Network Enabled Ethernet Access to

Play an Important Role

3G and LTE deployment broke this digamma since legacy infrastructures and connections cannot meet the request for the huge increment on the bandwidth requirement. High speed data is the key application for 3G and LTE whose station needs at least 30-45Mbps for the backhaul. Only Ethernet access may offer the enough bandwidth for the service provisioning and flexible scalability. In addition, hard QoS, service protection should be requested for the voice application and the most important issue with regard to clock offering are requested Ethernet access to be same to or much more selected to TDM network.

3. Wireless Architecture and Performance

The voice and data applications only depending on circuit switched such as PDH/SDH backhaul from BTS to BSC. Sometimes, DSL channel is also used for the backhaul on 2G network. Data rate is just 9.6kbps.


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Figure.1

Figure.2 In this stage, voice application still is based on circuit switched. Data application is scalable to 19.2k GPRS (typical), 64K EDGE (typical), 120K UMTS (typical), 800K HSDPA (typical). The most important is packet network is already introduced into core network. However, the backhaul is still based on circuit switched or ATM network. So the data rate is still limited by access network resource. And the initial R99 version 3G technology started from ATM technology and it is very tough to upgrade and develop more since very high cost is involved for the infrastructure.

Figure.3


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This is the next generation access network for 2G, 3G and LTE. To meet the different technologies and different versions of 3G, the next generation access network should support not only hard QoS, service protection ITU-T G.8031/2, T-MPLS encapsulation to establish tunnel accomplished by VLAN mapping into LSP as well as PWE3 for circuit emulation service over packet switched network. The voice service could be circuit or packet based and data is moving to all packets network from backhaul to core. The data rate is from 2M to 50Mbps.

4. The Importance of Synchronization in Mobile Network

Figure.4

In backhaul, the need for synchronization is focused in 3 main areas 1. Radio framing accuracy 2. Handoff control (no dropped call handovers) 3. Backhaul transport (no data slips) Typical mandated accuracy targets: GSM and W-CDMA: 50 ppb frequency Phase accuracy: 2.5 S between BTS/NodeB

5. How to Offer Synchronization in Different Types of Mobile

Network

A number of techniques have been established for the distribution and acquisition of synchronization. Each approach has its own characteristics and dependencies as shown below:


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Table. 1

Approach Characteristics Dependencies

External synchronization

Clock receives from an external clock. E.g GPS satellite

Access to GPS receiver or local timing device with BITS capability.

Line synchronization Physical layer technique: clock is extracted from the received synchronous data stream.

A suitable line (e.g. PDH/SDH) must be available to provide timing with received data.

Timing over packet (adaptive clock recovery, IEEE 1588v2)

ToP uses a stream of timestamped message (packets) sent to nodes needing synch. to allow the local clock to be synchronized.

Impacted by variations in network transmit delay. 1588v2 requires an external grand master device.

Synchronous Ethernet

Uses Ethernet physical layer to distribute clocking analogous to the SDH/SONET architecture.

All the intermediate nodes must support synchronous Ethernet for end to end transmission.

The above techniques are the currently available to deploy into mobile network. In 2G network, the line synch. has been deployed to distribute clocking to each station since it is the most cost effective and stable clock distribution way and it is very suitable for large scale deployment. External synchronization is known as 2 main methods. One of them is receiving signal from GPS satellite. The other is using local BITS clock device. However the second method is not used for large scale deployment since it is too expensive for the physical clocking resource to afford for the deployment. Another reason is showed below:

Table. 2

Resource Accuracy Lasting time less than 1S

PRC/LPR (Cesium) 210-12 115 days G.811 clock 110-11 17 minutes Rubidium atom 510-11 3.4 minutes

It is obvious external BITS clock couldnt preserve the accuracy for long time. So, such synchronization is mainly suitable for the backup application once the primary clock resource is failed. GPS satellite is usually deployed for the reason of high accuracy to be the


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reference as primary clock resource. Another reason for the large scale deployment is CDMA technology no matter of CDMA 2000, WCDMA or TD-SCDMA needs precise timing information besides clocking information requested in GSM network. GPS is the only way to provide precise timing information since IEEE 1588v2 PTP was not available at that time. Timing over packet is known from PWE3 technology at the beginning. There are 2 methods: adaptive method (G.8261 9.3) and differential method (G.8261 9.2) are used to extract the service clock and reconstruction. However the dependencies of performance are quite different for these 2 methods:

Table. 3

Adaptive method Differential method

Poor performance because of very sensitive to PDV:

Based on time stamps of packets

Random PDV Some require the common PRC in intermediary PTN

Low frequency PDV Better performance, limited by :

Routing changes Any phase noise on the reference clock will cause phase noise on the reconstructed TDM signals.

Congestions Congestions and delay of time stamps will degrade performance

Systematic PDV

Due to reasons of high requirement for the clock resource of differential method, adaptive method is often deployed to recover the clock and reconstruction when TDM service is emulated over packet switched network. And it is easy to find good performance of transportation and backhaul Ethernet is the KPI to acquire the accurate clocking information with adaptive method. Another method for timing over packet, PTP (precision timing protocol) is emerging recently to meet the pure Ethernet environment to distribute the timing and clocking information end to end. 3G and LTE service provide huge data application and 3G NodeB will be much density than 2G deployment for this reason. To distribute timing and clocking like 2G is the way to reduce the big cost for large deployment. Only IEEE 1588v2 can meet this requirement. Another issue as mentioned above is GPS is not so cost effective for clocking and timing distribution, since GPS antenna installation requests complexity (e.g. clear outer environment towards 120of antenna and repeater is needed if the feeding cables is longer than 100m ) and failure rate is very high to reach 15% approximately. IEEE1588v2 is coming dedicated for this application. Compared


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to the other timing over packet methods, IEEE 1588v2 will not waste much bandwidth, but the each node of whole network backbone and access should be all support this precision timing protocol to enable the timing information to transmit and recover to the connected 3G NodeB.

Figure.5

6. Factors Impacted IEEE 1588v2 Performance Potentially

and Results Analysis

The list of factors impacted timing accuracy theoretical 1. The number of hops 2. Asymmetry of time delay 3. Timing connections switch over 4. The performance of transmission degraded and temp changed 5. The traffic loading changed 6. Frequency synch. There are some graphics showed below for the testing result in the lab.


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Number of hops

Figure.6

Timing changed 252ns (-120.ns- 131.5ns) within 9 hours in 10 hops PTN devices in the whole network. (shown in figure6)

Figure.7

Timing changed 253ns (-61.1ns- 192ns) within 9 hours in 20 hops PTN devices in the whole network (as shown in figure7).

Figure.8

Timing changed 266ns (-239.3ns 26.8ns) within 4 hours in 30 hops PTN devices in the whole network (shown in figure8). From the above records of testing in the lab, the timing accuracy is not too much related to the number of the hops.


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Asymmetry of time delay

Testing result: The tested output timing accuracy in the normal configuration is 35.3 ns. The tested output timing accuracy is 151.6ns when add 60m length fiber cord in one of the directions. Testing analysis: 60m fiber cord will bring 240ns time delay and time deviation is half of time delay 120ns which is almost same to the real tested result. So, it could be offset by network management if the fiber distance of bi-direction has big difference.

Timing connections switch over

Fiber link switch-over: Change range of timing and frequency is almost same to 26ns (shown in figure9)

Figure.9

Clock cross-connection module switch-over: change range of timing and frequency is almost same to 13ns (as shown in figure10).

Figure.10


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Time resource switch-over: change range of timing and frequency is almost same to 6ns (as shown in figur11).

Figure.11 To be summarized, the time deviation brought by switch-over protection is complied with mobile network requirement.

The performance of transmission degraded and temp changed

When we insert bit errors into the PTN lines, the output time range is changed just 110ns. It is almost same accuracy to before. The figure has shown the testing result below:

Figure.12 During the temp changing, there is about 40ns adjustment. It is no clear relations between time adjustment and temp change (in the figure13).

Figure.13


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In general, timing accuracy is influenced very little by transportation performance degrade and temp change.

The traffic loading changed

Traffic load has impacted for transparent mode of IEEE 1588v2. It brought about 200ns deviations when adding 80% traffic load at the point of start and end (as shown in figure14).

Figure.14

PDV has impacted heavily for transparent mode of IEEE 1588v2. T=0 540s, time jitter is 1 10S; time accuracy is 3S. T=540 1050s, time jitter is 2.5 12.5S; time accuracy is 6S. T=1050s - , time jitter is 25 125S; time accuracy is 60S. It is tested and shown below:

Figure.15


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Frequency synchronization

How does it impact timing when frequency synchronization is lost? Take down external clock of first network element and make it preserved. The last element output of frequency wonder is 5E-9. The last element time accuracy is 100 200ns change range (Shown in figure16).

Figure.16

When the first element is forced to be free random, the last element output of frequency wonder is 3.8E 8. The last elements timing synchronization Lost (as shown in figure17).

Figure.17

When the timing is working very closely with frequency, the frequency loss will result in timing synchronization loss. To be summarized in all, it is almost satisfied with requirement of timing accuracy


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and the most cost effective solution in new mobile network deployment with PTN and IEEE 1588v2. However, it should be better to rebuild a dedicated PTN network to transport only for mobile subscribers because traffic load will impact more based on the tested result.

7. The Most Practical Way to Deploy the Next Generation

Access Network for Mobile Operator

Unfortunately, it looks not practical way for all the carriers since the backbone is not easy to upgrade from pure IP/MPLS network to PTN and IEEE 1588v2 complied. And even though all nodes could be upgraded, it exists the interoperability issues for this protocol. So, the most practical way is building a next generation access network dedicated for mobile backhaul. All access rings can synchronize with each other. Backbone just delivers the traffic and doesnt handle timing and clocking. Each access ring can connect 10 or more NodeBs and at one point demarcation device could utilize the existing GPS receiver as external synchronization to provide the clock source for this ring. Demarcation device connects 1pps and 10Mbps and ToD interface to GPS receiver and translate into IEEE1588 timing frame to distribute into this access ring. And each NodeB will receive timing information from service interface along with traffic.

Figure.18

Many incumbent carriers provide mobility service for long time and mobile network is hybrid with 2G and 3G. So, PWE3 technology could be deployed in demarcation device to connect base station with multiple E1/T1 lines with CESoPSN or SAToPSN protocol and distribute clocking information to base station. However, the whole network architecture and equipments and clock resource are same for this hybrid 2G and 3G mobile operator. Till now, new TDM lines deployment is waste in the coming days for your future mobile network. We


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should disregard to operate 2 independent backbone and backhaul network for 2G and 3G service, but use one next generation access network to meet all the voice, data, fixed and mobility requirements.

Figure19

Contact: Tim Kwun Marketing Director [email protected] #28 (building 2), Shangdi 6th Street, Haidian District, Beijing 100085, China

rc whitepaper next generation access network-1 20110224

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