IEEE 1588v2 Sets A New Paradigm for Packet Synchronization
The Precision Time Protocol allows the use of packet networks like Ethernet for mobile backhaul.
Before telecommunications carriers can leverage cost-effective Internet Protocol (IP) networks to handle the increased bandwidth expected from 3G and 4G services, they need to know the issues surrounding time synchronization and the solution provided by IEEE1588v2. Officially named IEEE 1588-2008, this standard addresses network-based timing and synchronization.
Synchronization is a fundamental technology building block of today’s service provider networks. The performance of the traditional wired T1/E1, Sonet, and synchronous digital hierarchy (SDH) networks is based upon constrained clock control theory and phase-locked loop (PLL) design. This type of synchronization signal is a closely bounded frequency. The synchronization signal delivery system used in today’s practice is based on the physical layer (PHY) of the transmission system providing a clock signal.
Electronic circuits discipline the signal to provide the frequency accuracy, stability, phase movement control, wander filtering, and edge jitter (phase noise) levels specified in volumes of standards documents. Each “end customer service” has a specific set of performance requirements detailed in these documents. The various international standards bodies define this performance, which is well understood.
There are two mobile wireless network synchronization schemes. The first uses the same technology as a wired network. This standard is used to provide frequency-division duplex (FDD) radio-based mobile wireless network synchronization signals for ingress/egress of data and accurate radio frequency. For example, WCDMA FDD is the most popular method of radio air interface used in GSM systems.
The second is synchronization of time-division duplex (TDD) radio-based mobile wireless networks. This radio technology requires frequency accuracy, phase alignment, and, in certain cases, time alignment between all basestations in the cell network. CDMA, cdma2000, Mobile WiMAX 802.16e, and Long-Term Evolution (LTE) are all examples of TDD radio systems. The traditional wired “circuit” oriented synchronization signal delivery system cannot be used because there is no phase or time relationship between signal termination points on the clock distribution network.
The four requirements for mobile wireless network synchronization depend on having an inter-basestation aligned timing reference. This is essential to guarantee transport channel alignment for handoff and guard band protection. 3GPP-specified FDD systems require frequency accuracy better than 50 parts per billion (PPB). The 3GPP TDD systems require an inter-basestation time alignment of 2.5 µs to IS-97’s 10 µs in addition to the 50-ppb frequency accuracy. The IEEE 802.16e mobile WiMAX requirements are 20 ppb of frequency accuracy and 1 µs of phase alignment. Ensuring the fulfilment of these requirements reduces the call drop rate and improves the quality of services by decreasing packet loss. This is how it works today and will continue to work when new backhaul techniques are implemented.
So what is the big deal if this technology is so well understood and bounded by standards? T1/E1 circuit-based technologies are dedicated to the individual service they are delivering. Each service end point bears the cost of the entire circuit path 24 hours per day, 365 days a year. Considering the scale of billions of circuits worldwide needed for 3G/4G services, the costs of service delivery present a significant barrier to profitably providing higher-bandwidth service offerings.
For example, these circuits can consume up to 40% of the total operating expense for a mobile wireless operator. Cell-phone customers pay for this in their bill. This market dynamic, plus the competitive nature of the industry, is driving the move to the shared resource of packet-based Ethernet backhaul. The cost per megabit ratio comparison of a T1/E1 circuit versus Ethernet-based packet backhaul averages 6 to 1.
With these metrics, it is easy to understand the motivation of the service provider to migrate to the cheaper Ethernet transport. The question of how to outfit routers and switches with IEEE1588v2-based semiconductor master and slave semiconductors remains. The provision of synchronization for future mobile networks will pose new challenges triggered by the following factors:
- • New mobile networks will offer new content-rich services (e.g., IPTV, VoIP), demanding higher capacity and faster backhaul links.
- • New mobile network technologies such as mobile WiMAX and LTE are hungry for bandwidth. They are also capable of a much higher degree of radio spectrum efficiency than today’s systems. This is key, particularly for licensed spectrum usage.
- • The PHY transport for traditional systems will be packet-based, most likely using Ethernet. Carriage of voice and data traffic in the 3GPP UMTS R5 and above specifies use of IP for data and that compressed voice be carried via VoIP versus being carried over ATM today.
- • There will be an increased number of cell sites due to the need for a higher signal-to-noise ratio to support the increase in bandwidth delivered to the users. Increasing the bit density of the radio channel to pack more traffic requires more nodes and different timing distribution techniques.
- • Ethernet is the only candidate for transport technology for future backhaul networks given the lower cost of the facility and switching equipment. Ethernet is inherently asynchronous.
The migration toward a packet-based transport network poses the challenge of providing the level of synchronization requirements defined in the 3GPP, 3GPP2 (LTE), and IEEE 802.16e (WiMAX) specifications.
The transmission of the clock information over a packet network eliminates the need for alternative mechanisms, such as GPS or prohibitively expensive oscillators placed at the receiving nodes. This provides significant cost savings in network equipment as well as in ongoing installation and maintenance. This synchronization solution transmits dedicated timing packets, which flow along the same paths with the data packets, reducing the cost of synchronization and simplifying implementation.
Master and slave semiconductors based on the IEEE1588v2 Precision Time Protocol (PTP) are available now to carry the synchronization data. The IEEE1588v2 protocol is fully compatible with all Ethernet and IP networks. Additionally, the protocol is designed to enable a properly designed network to deliver frequency and phase or time with precision rivalling a GPS receiver.
An IEEE1588v2 protocol implementation can supply FDD and TDD radio systems and CES-based (circuit emulation services) transport systems with the synchronization signals they require. This greatly reduces the costs of clocking all wireless basestation equipment.
- Although IEEE1588v2 systems add a small amount of additional traffic to the network load, they have several advantages. First, they work in the data path, the most redundant and resilient part of the network, resulting in “always on” operation. Next, multiple transmission paths reduce redundant clock system costs. They also use a single synchronization session for all basestation traffic.
- Furthermore, they support legacy systems in mobile networks, where Circuit Emulation Service is employed to carry both TDM traffic (2G) and ATM traffic (3G). They support any generic packet-based transport (such as IP RAN). And, they feature configurable packet rates for network conditions to maintain accuracy (e.g., packet rate less than 1 pps to greater than 100 pps)
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© 2013 Penton Media Inc.
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