Assuring QoS With Precise Timing and Synchronization in IP-Based Wireless Networks

GSM mobile operators are switching from T1/E1 networks to Ethernet/IP in order to support deployment of high-speed data services and reduce infrastructure cost. Despite its many benefits, this change eliminates the timing signals that base stations currently rely upon to hold the required carrier frequency accuracy of +/- 50 parts per billion (ppb). If base stations drift outside the 50 ppb limit, mobile handoff performance decays, resulting in dropped calls and impaired data services.

The introduction of Ethernet as a backhaul transport technology for wireless GSM/UMTS base station traffic will cause synchronization issues that operators will need to address. Ethernet backhaul will break the physical layer time division multiplexing (TDM) synchronization distribution chain to the base stations and will require solutions for both the legacy base stations that are synchronized via T1/E1 connections and base stations of the future that will have Ethernet interfaces.

Mobile operators have four options to maintain carrier frequency accuracy. The first three alternatives apply to legacy base stations using T1/E1 over circuit emulation services (CES) backhaul and the fourth applies to next-generation packet-based backhaul. The first alternative is placing global positioning system (GPS) based retimers at each base station to buffer incoming traffic and clock it back out at the required primary reference source (PRS) level of accuracy. A second alternative is to use GPS-based retimers to provide synchronization and timing for T1/E1 through the aggregation hub. A third alternative is to deploy packet-based synchronization utilizing the emerging IEEE 1588 precision time protocol (PTP) to provide timing to the T1/E1 through the aggregation hub. The fourth alternative, which applies to next-generation base stations using Ethernet backhaul, is to use PTP to deliver timing signals direct to base stations.

Effect of timing and synchronization on QoS
Dropped call rates caused by failed handoffs are a major contributor to customer churn. Very precise frequency synchronization is required to avoid dropped calls. Poor synchronization between base stations results in dropping calls during handoffs. When the handoff between two base stations occurs, the mobile rapidly must switch to the target frequency of the new base station. The inability of the mobile transceiver to react quickly enough to frequency synchronization errors between base stations will result in a dropped call.

To facilitate call handoff and maximize efficient bandwidth usage, precision timing and synchronization is required between base stations. CDMA addresses this challenge by providing a GPS synchronization receiver in every base station to provide a common time-of-day reference for call handoffs. Most GSM base stations deployed today recover their synchronization from the leased line backhaul. This works very well as long as the backhaul facility provider maintains accurate and stable synchronization.

Providing timing over the backhaul
In the early days, the backhaul transport was comprised of a tightly controlled network that provided reliable transport timing signals from a stratum one PRS. Each node between the base station and PRS would track the PRS clock signal, maintaining its long-term accuracy. Of course, jitter and wander would be added in transit, but these would be filtered out at each transit point. Such a network provides the accurate and stable frequency reference required to ensure successful call handoff. This reference is used to calibrate or "discipline" a precision quartz oscillator that provides internal timing signals and RF carrier reference for the base station.

With the introduction of high-speed data services, operators are facing a need for increased backhaul capacity. When base stations carried just voice traffic, a single T1/E1 connection typically provided more than enough backhaul bandwidth. Today, many mobile operators are planning to transition to alternative backhaul mechanisms to reduce costs and increase bandwidth. Without a well-synchronized backhaul feed to lock to, the oscillator frequency will drift out of specifications within a few months. As this happens, dropped calls will increase. Regular, costly service calls may be required throughout the network to manually calibrate oscillators in base stations.

Even base stations that still make use of a T1/E1 backhaul are beginning to experience levels of degradation that reduce QoS and increase customer churn. CESs, such as encapsulating a T1/E1 line over asynchronous transfer mode (ATM), introduce delay variations that can cause synchronization issues at the base station. These links are very sensitive to traffic loading, and as traffic is dynamic, timing flows may experience delay variations in outgoing queues that cannot be filtered appropriately. TDM networks also may degrade timing signals because of ambiguities in SONET pointer adjustments that introduce nondeterministic wander and jitter. The accumulation of such errors can effectively cut a base station off from its source of synchronization.

Three alternatives to improve timing performance
GSM operators have three main alternatives to regain control of their timing and synchronization in order to reduce dropped calls and improve QoS. The first alternative relies upon putting a local source of synchronization at each base station to deliver an accurate clock reference (see diagram below).

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Local source of synchronization stabilizes timing signal.

One approach is to install a GPS receiver at each base station and couple it to a retimer that is placed on the backhaul feed directly before the base station. The timing signal received by the base station is reclocked to be precise and stable. GPS-based retimers commonly are used today to address troubled sites with high dropped call rates.

The disadvantage of GPS-based retimers is that they involve a substantial cost and implementation burden. First of all, there is the need to equip each base station with a GPS receiver that involves a significant capital outlay. With several million base stations in the world, the required investment is substantial. Another concern is that the existing GPS satellite infrastructure is not designed to interface with GPS/UMTS so the integration and implementation costs also will be substantial.

A second alternative is to use GPS-based retimers to provide synchronization and timing for T1/E1 through backhaul aggregation hubs used in many networks. Restoring accurate synchronization and timing at the aggregation hub allows for reliable synchronization distribution from the hub down to the base stations over the T1/E1 feeds.

Packet-based protocols
A newer approach involves the use of packet-based protocols to provide reliable timing signals over the IP network. The most popular of these protocols is the Network Timing Protocol (NTP), which is the de-facto time synchronization standard for IP networks. NTP consists of a protocol for communicating time between clients and servers and complex statistical algorithms to improve the accuracy of the estimated delay and offsets. But NTP is not appropriate for the GSM base station timing applications because its current “best-effort” implementation is not deterministic, so it can’t provide carrier-class reliability.

Much effort is now under way to provide a Layer 2 packet-based solution to the GSM timing and synchronization problem. A Layer 2-based solution has the enormous potential advantage of being able to leverage the existing timing infrastructure in the master switching station while avoiding the need for a major investment in each base station. PTP (IEEE 1588) appears to provide the answer. This standard originally was developed to support industrial LAN applications such as distributed motors and servos that need to be accurately time-synchronized. This protocol now is being upgraded for use in telecom wide area networks.

How PTP (IEEE 1588) works
PTP (IEEE 1588) utilizes clocks configured in point-to-multipoint configuration. Master clocks can be installed in existing building integrated timing systems (BITS) located in master switching centers, and simple slave devices can be installed in remote base stations (see diagram below).

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PTP (IEEE 1588) utilizes existing BITS infrastructure to deliver timing and sync over the Internet.

The master clocks send messages to its slaves to initiate synchronization. Each slave then responds to synchronize itself with the master. Incoming and outgoing PTP packets are time-stamped at the start of frame of the corresponding Ethernet packet. The protocol then exchanges information between the master and slave using PTP message protocol.

The protocol is used to calculate the offset and network delay between time stamps, apply filtering and smoothing, and adjust the slave clock phase and frequency. This sequence is repeated throughout the network to pass very accuracy time and frequency synchronization. PTP networks automatically configure and segment themselves using the best master clock (BMC) algorithm. The BMC enables hot-swapping of nodes and automatically reconfigures the network in response to outages or network rearrangements.

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PTP (IEEE 1588) timing and synchronization provided over circuit emulation services.

PTP (IEEE 1588) can be used to provide timing and synchronization to base stations using both legacy T1/E1 and next-generation Ethernet backhaul (see diagram above). Where T1/E1 backhaul is used, a PTP translation device would connect to the aggregation hub that delivers T1/E1 service to the base station. This alternative can be combined with a GPS-based retimer into a single robust solution. For next-generation networks, PTP is used to provide timing and synchronization over the Ethernet backhaul to the base station.

Leveraging the existing timing infrastructure
The opportunity exists for most mobile operators to leverage their existing investment in a Building Integrated Time System (BITS) or Synchronization Supply Units (SSU) that are used by most carriers to provide the timing needs of today’s MSCs. Most MSCs have or are in the process of implementing powerful new synchronization supply units such as the SSU2000 from Symmetricom Inc. The SSU2000 main shelf provides up to 160 1+1 protected outputs, and expansion shelves can be added to increase the capacity to more than 1,200 outputs. The SSU2000 system also provides full visibility and manageability of all input and output ports and can be modified and controlled through multiple interfaces including Ethernet. As an "intelligent" network element, the SSU2000 provides multiple signal monitoring and alarm inputs and full remote management and provisioning, allowing network managers to monitor, manage and provision from a centralized location.

The ability of PTP (IEEE 1588) blades to integrate with the existing BITS/SSU architecture provides important cost and performance advantages. Additional blades can be added in an expansion shelf or remote master to easily deliver increased capability. The BITS/SSU architecture provides inherent redundancy based on multiple input references and also delivers a high level of holdover stability. The integrated platform offers the highest available levels of performance, accuracy and security. Integrating PTP servers with the existing BITS/SSU also dramatically reduces management requirements. The PTP blades can share a GPS reference with the existing BITS/SSU platform, providing substantial cost savings.

Maintaining high QoS through the transition to IP
Next-generation BITS/SSU platforms add support for PTP (IEEE 1588) protocols to deliver timing over packet networks to remote base stations.

IP represents the future of the mobile industry because it provides the bandwidth needed to deliver new data-centric services as well as cost savings. Yet customers are unlikely to be willing to tolerate any reduction in QoS as new IP networks are deployed. Further, the cost of customer churn as a result of QoS reductions from increased dropped calls can rival the savings achieved by moving to IP. Mobile operators need to ensure that they can support the synchronization accuracy needed to avoid dropped calls and maintain QoS. Operators now have a choice of several alternative solutions that can restore stability to critical timing signals essential for maintaining synchronization, and independent of whatever backhaul technology is selected (see chart below).

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The optimal long-term solution is to deliver timing signals over the IP network. PTP (IEEE 1588) leverages the existing BITS infrastructure to economically meet the timing and synchronization requirements of next-generation base stations.

Gurdip Jande is senior vice president of marketing for Symmetricom Inc.

Symmetricom Inc. www.symmetricom.com

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