synchronization techniques

8/12/2019 Synchronization Techniques

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Ethernet synchronization

ABSTRACT

INTRODUCTION

Due to the surge in data traffic requirements, service providers are migrating their backhaul to packetswitched networks. Packet switching offers statistical multiplexing, which is well suited to the burstynature of data traffic. However for many applications either frequency or time synchronization is

important, and these were lacking in packet networks. In recent times, two standards have evolved thatoffer frequency synchronization, time synchronization or both over a packet network. SynchronousEthernet or SyncE is a technique that uses the same links that carry data to carry frequency informationalso. 1588v2 is a technique/protocol to synchronize timing across various nodes to sub 1us accuracy.

NEED FOR ETHERNET SYNCHRONIZATION

The conventional transport networks have been built on TDM technologies. With the increase in thedemand for the data traffic, these networks have been adapted to carry data traffic. Enhancements suchas Virtual Concatenation, GFP and LCAs enabled these TDM networks to carry data traffic in the form ofEthernet. However this is not efficient for carrying large amounts of data As the proportion of the datatraffic continuously increased, carriers’ effort to migrate to a pure Ethernet network was hindered bythe lack of synchronization of the network elements. Synchronization is especially important in cellularnetworks during cell handoffs.

Improperly synchronized networks would suffer from several problems. These include call dropswhile switching from one cell to the next, interference between different frequencies and in MIMOarchitectures.

This synchronization is currently made available by the SDH Network where each signal carriesfrequency information and the receiver has the ability to extract this information and lock-on to thatfrequency. While migrating from SDH to an Ethernet network, a similar capability was required.

In traditional Ethernet, transmission happens on a line only when there’s data to be sent. Inaddition, Ethernet is a store and forward technology. Each packet is buffered for various operations likedestination lookup, before it is sent out on an Egress port. The amount of time for which it is bufferedis dynamic and depends on the congestion in the network. Thus, in an end-to-end session, it canintroduce a lot of jitter.

Some applications like mobile back haul and circuit emulation (transport of E1 traffic) requireaccurate frequency information to be sent across the network. Without synchronization there will be“bit loss” in circuit emulation and call drops in mobile backhaul.

SYNCHRONIZATION IN PACKET SWITCHED NETWORKS




VARIOUS METHODS OF SYNCHRONIZATION

The two widely deployed methods of synchronization are

1588v2:

1588v2 Precession Timing Protocol (PTP) enables sub microsecond synchronization of clocks by having amaster clock send multicast synchronization message frames containing timestamps. All 1588 technologyaware receivers correct their local time based on the received timestamp and estimate the one-waydelay from transmitter to receiver.1588v2 provides both time and frequency synchronization.

There will be a master clock generator in the 1588v2 network which serves as the primary clockreference for rest of the PTP elements. There may be intermediary clock amplifiers which serves as themaster in the downlink with the control information received in the uplink.

The control packets received are processed at layer 2 and layer 3 level thereby layer 1 is unaware of thetechnology. Since the clock information is recovered from these control packets these packets should

not be lost in the network due to congestion, network degradation etc. Careful network planning isrequired for the positioning of the master clock generator and the clock amplifiers for ensuring efficientsynchronization in the network.




ADVANTAGES OF 1588v2

1588v2 can distribute synchronization without the need for expensive GPS equipment or precisionoscillators on the equipment. Another big benefit is that 1588v2 does not operate at the Phy layer and

hence some of the legacy networks can be upgraded to provide synchronization. It also supports wall-clock synchronization. Wall clock synchronization is needed for applications that require the time of theday such as billing, trouble-shooting etc.

SYNC-E

Sync-E involves feeding of one network element in an Ethernet network with a Primary ReferenceClock and employing Ethernet PHY circuitry with well-engineered timing recovery circuitry to set up afully frequency Synchronized network. It provides access to a highly accurate and stable frequencyreference to the applications that requires it.

The PHY of the master node element is fed with the clock information through the BITS interface. Mostlythe BITS interface is connected to a GPS. The frequency of the clock of the PHY of other nodes isderived from the incoming traffic signal from the master node in a way similar to SDH. ESMC are thecontrol packets transmitted by the node elements to convey the clock information to adjacent nodes.The SSM (Synchronous Status Message) is encapsulated in the ESMC. Upon reception of the ESMC protocolpackets in the upstream the clock of the PHY gets tuned to the best clock available, inferred from theESMC packets and transmits the information of the newly clocked info in the ESMC packets downstream.When an Ethernet switch is informed of an upstream synchronization failure condition, the switch cantake appropriate action, such as selecting an alternate synchronization source. SYNC-E enables all the




nodes in the entire network to be in frequency synchronized in a cascaded manner. This technique oflocking the slave clock with the Master is analogous to the synchronization techniques implemented inthe TDM network.

ADVANTAGES OF SYNC-E

The advantage of using Synchronous Ethernet, as compared to methods relying on sending timinginformation over an asynchronous packet network, like 1588v2, is that it is not influenced by congestionor other dynamic conditions in the network. 1588v2 handles lots of these packets in software and thebuffering and delay can vary with varying network conditions. Hence 1588v2 cannot equal theperformance levels of synchronous Ethernet. Applications that requires carrier class synchronization inthe network that is relatively noisy has to employ Sync-E.

APPLICATION OF SYNC-E FOR MOBILE BACKHAUL

Lack of synchronization in PSN can trigger failures in many different areas of the network, including

dropped calls, interference between channels, slow handover between cells, and speech clipping (loss ofspeech segments). Hence synchronization becomes an important requirement for delivering good qualityof experience to the end customer.With the implementation of synchronization techniques such as SYNC-E the previously discussed hurdlescan be overcome.

Below is the network diagram which has implemented SYNC-E and established frequency stabilizationand thus provides high quality service to the end users.




SYNC-E and 1588v2 COMPLEMENT EACH OTHER

1588v2 (PTP) and Synchronous Ethernet should not be regarded as being competing technologies. In

reality they complement each other. There are applications that require wall-clock, and not (just)accurate frequency for which 1588v2 will be the right choice. 1588v2 is made difficult by therequirement that these protocols simultaneously lock on to frequency and time. When the network issynchronous, provided by SYNC-E, the wall-clock distribution protocol can assume accurate frequency,and its job becomes much easier. This hybrid approach significantly reduces time estimation error. Edgeelement which requires 1588v2 can interoperate with the backhaul network employing SYNC-E. Judiciousdesign of the network employing both SYNC-E and 1588v2 can result in a network with greater reliabilityand cost effectiveness.




DELIVERING SYNCHRONIZATION SERVICES TO 2G/3G BTS and NodeBs

As was discussed previously, synchronization is a key requirement to support mobile applications and tomeet critical system needs related to minimization of air interference, facilitation of handover betweenbase stations, and fulfilling regulatory requirements. Various mobile technologies stipulate that theradio signal must be generated in strict compliance with frequency/time/phase accuracy requirements,as illustrated below.

Thus in the case of 2G GSM, CDMA2000, WCDMA and Femtocell technologies, the BTS/NodeB clock needs

to be only frequency synchronized to the radio controller within certain limits in order to ensure glitchfree performance. Tejas Carrier Ethernet equipments will source synchronization information through a“BITS IN” interface that connects to the Grandmaster Clock located at the radio controller site. Theinformation is then transported over the Tejas CE network to the node that hands-off this traffic in asuitable format to the Access network.

Application Frequency Phase Time

CDMA2000 ±50 ppb±10µs(±3µs

Preferred)

GSM ±50 ppb

WCDMA ±50 ppbTD-SCDMA ±50 ppb ±3µs

LTE (FDD) ±50 ppb±5µs for

MBMS

LTE (TDD) ±50 ppb ±3µs

WiMAX (TDD) ±50 ppb ±1.5µs

FemtoCell ±250 ppb




However, the real complexity in delivering synchronization to the cell site arises from the diversity oftechnology types that are commonly found in the access part of these networks, as shown in the figurebelow. In this section, we consider various application scenarios and demonstrate how the Tejasapproach to packet synchronization effectively solves the synchronization hand-over problem in eachcase.




Scenario A: Carrier Ethernet PSN in the Access and Aggregation

This is the simplest network scenario for synchronization delivery. In this case, Tejas recommends thattiming frequency from the PRC source at the controller site be delivered over the end-to-end CarrierEthernet network using the SyncE algorithm. Besides providing an accurate and highly stable frequency

reference, an important benefit of using SyncE, as compared to methods relying on sending timinginformation over an asynchronous packet network, like 1588v2, is that it is not influenced by congestion orother dynamic conditions in the network.




Scenario B: PDH/SDH Microwave in the Access and Carrier Ethernet PSN in the Aggregation

Several large Carriers today have extensive deployments of PDH and SDH microwave equipments in theaccess to support their 2G and 3G networks. When these Carriers upgrade to WiMAX or LTE, they wouldlike to continue using these networks for carrying voice and TDM services while offloading data traffic tothe new PSN network.

Since regular Carrier Ethernet equipments do not provide TDM interfaces or a BITS clock interface,achieving an accurate frequency hand-off from the PSN-based Aggregation network proves to becomplex. Tejas however solves this issue in two ways –




i) BITS interface: Tejas Carrier Equipments support “BITS OUT” interfaces that can be connectedto BITS inputs of the PDH/SDH microwave equipments. However, if the existing microwaveequipment does not feature an external timing input, this feature cannot be used.

ii) Retimed E1 interface: Tejas Carrier Equipments can also deliver frequency synchronizationusing a re-timed E1 signal. In this case, the outgoing E1 traffic signal contains the traffic datacoming from the traffic input and the timing coming from the synchronization input. Re-timingmay also be used when the original E1/DS1 signal is affected by excessive levels of wander.

iii) STM-1 interface: Tejas Carrier Ethernet equipments can also deliver frequency synchronizationthrough STM-1 interfaces. This is also useful if the last mile to the cell site uses an SDH fiber ringin a few locations, besides supporting synchronization delivery over hybrid microwaveequipments.

Scenario C: IP Microwave in the Access and Carrier Ethernet PSN in the Aggregation

To support the emerging need to transport increasing amount of Ethernet/IP traffic, a new type of

microwave equipment has arrived in the Carrier market. Unlike current generations of “HybridMicrowave” products that encapsulate Ethernet packets within TDM/SDH frames, “IP Microwave”equipments map IP/Ethernet traffic directly on the radio frames. “IP Microwave” equipments pose freshchallenges when it comes to delivering synchronization data to the cell sites.

i) “All-IP” Packet Microwave: “All-IP” microwave equipments are capable of transporting bothTDM and data traffic over microwave. TDM traffic is supported using E1/DS1 interfaces on theequipments and circuit emulated over the packet microwave network. These equipmentstypically support SyncE for packet synchronization and hence transport of synchronization can behandled seamlessly from the PSN Fiber-based Aggregation to the PSN Microwave-based Access,all the way to the 2G/3G cell site. Hence, the solution is similar to Scenario “A’ except for thefact the use of microwave as the transmission medium in the Access instead of fiber.




ii) “IP-only” Packet Microwave: “IP-only” microwave equipments are used in greenfield networks(e.g., several WiMAX networks) that have been designed to deliver only packet services to end

users. In this case, since there is no support for TDM traffic, SyncE feature is not typicallyprovided by the packet microwave equipment. Tejas can then transfer synchronizationinformation using the IEEE 1588v2 protocol, whereby an IEEE 1588v2 Grandmaster (connected toa PRC source) at the controller site communicates with 1588v2 slaves at the remote destinationsite using the PTP protocol. However, a 1588v2 based implementation require careful pre-deployment planning to ascertain that network performance (especially PDV or jitteraccumulation) remains within reasonable bounds under varying network loads and trafficpatterns. Depending on the nature of the network, external clock regenerators may be used atchallenging sites.

CONCLUSION

Ethernet synchronization techniques enable service providers to migrate to pure packet networks whileleveraging higher bandwidth efficiencies of packet and without compromising on the quality ofexperience to the customer. Tejas Carrier Ethernet equipments support multiple packet synchronizationtechniques and interface types that together help in delivering end-to-end frequency/time/phasesynchronization in both greenfield and brownfield networks.

synchronization techniques

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