Broadcast and cable providers are struggling to adapt as the public’s media viewing habits shift from traditional services to fixed and mobile IP streams. For both fixed and mobile service providers, success will hinge on an infrastructure equipped to deliver the raw bandwidth and quality of service necessary to ensure a seamless digital multimedia experience. As we’ll see, the IEEE (News - Alert) 1588v2 precision timing protocol, and products that support it, will play an essential role in building and managing timing-aware IP networks. Without accurate timing and synchronization, calls drop and video streams are disrupted. In addition, delivering mobile broadband services in dense, complex urban environments, or inside buildings, will require operators to enhance 4G/LTE network coverage and capacity with small cell clusters.
The Great IP Migration Continues
According to a recent Reuters (News - Alert) article, the cable industry is losing subscribers who opt for less expensive and more targeted broadband services, which deliver streaming multimedia from both from the carrier’s content partners and third-party over-the-top services. Typical of this trend is Comcast (News - Alert), which reportedly lost 117,000 video customers in the third quarter of 2012. While revenues from traditional media services continue to erode, some carriers are going on the offensive by offering premium-level services capable of supporting multiple channels of streaming multimedia, a strategy that requires nearly all carriers to upgrade their infrastructures.
Mobile service providers face even bigger challenges as their customers’ viewing habits shift from the big screen to the screens of smartphones and 4G-enabled tablet devices. This growing demand for wireless video dramatically multiplies the need for bandwidth in the mobile network – both at the base stations and the backhaul links that feed them. As a result, wireless operators must rely increasingly on IP/Ethernet-based backhaul to cost effectively upgrade existing network capacity.
Today, many small cell deployments are done purely to improve coverage in spots where the nearby cell tower coverage may be weak. However, as the number of 4G/LTE (News - Alert) device users increases, operators will have to rely on small cells to support the bandwidth demand growth cost effectively. While small cells promise a better capex and opex model than today, they also pose a new challenge for accurate timing and synchronization, which is essential for delivering voice and video quality. Many carriers are using GPS for delivering timing and synchronization today. But small cells sitting on the top of lampposts and traffic signals won’t have good line of site to GPS satellites. They will also be more susceptible to jamming and snooping at or close to street level. Hence, an alternative based on 1588 to the current timing and synchronization model is imperative, when carriers are looking to support a cost-effective model to support 4G/LTE bandwidth demand driven by multimedia applications.
It is important to note that delivering quality of multimedia experience to users requires a certain level of 1588 timing and synchronization accuracy. This is typically determined by the carrier’s wireless technology accuracy requirements. Normally such requirements are defined by the difference in the primary reference clock in the core of the network and recovered clock at a base station or other endpoint (i.e. the synchronization error). Multimedia streams require that modern 4G networks have end-to-end time-of-day synchronization requirements ranging from 1.5usec to as low as 0.5usec. If the timing derived by two base stations differs by more than the specified amount, a mobile user will drop his or her call or video stream when moving from one base station coverage area to the next. In a real-world cellular system, part of this timing budget is consumed by the latency variations in RF links and other timing variations inherent in various parts of the network. Another portion of the timing budget is consumed by the interference mitigation techniques used to combat the effects of multipath reflections and noise found in the urban corridors and indoor environments where small cell networks are used.
As a result, the synchronization error budget available to the small cell base stations and networking equipment in a typical 4G system shrinks to roughly 160ns. Assuming a timing packet may have to traverse as many as six nodes between a network node generating the master clock and the base station recovering the clock, each node’s timing variation budget must not exceed 20ns.
PTP to the Rescue
Developed primarily to meet the needs of packet-based backhaul networks, the PTP standard builds on synchronous Ethernet, which brought frequency synchronization to the packet world, but did not provide the necessary time-of-day synchronization required for LTE and LTE-A. To fill this gap, PTP carries time of day information (also known as timestamps) directly within the data packets. The packets carrying the timestamps flow along with the rest of the data traffic in the network from networking equipment that generates the timestamps (also known as primary reference clock) all the way to base station equipment where these timestamps are used to recover the original time using 1588.
Look for “PTP Inside”
Network equipment supporting 1588 can be implemented using either boundary clocks, transparent clocks or both. A node using BC regenerates the timing based on the timestamps that it receives, while TC node can simply forward the incoming timestamps after correcting for any error it may introduce.
As a result, TC is simpler and cheaper to implement compared to BC. Manufacturers of IP-based backhaul equipment can implement the TC function using inexpensive 1588-compliant PHYs or switch chip sets. As a result, Sprint (News - Alert) customers may roam into an AT&T network, while streaming their Netflix movie, but expect uninterrupted service regardless of whose network space they’re in. This requires a service provider to guarantee user experience over another operator network. It requires additional feature support by the network equipment in the form multi-operator management and measurement capabilities. Fortunately, solutions for timing-aware Ethernet backhaul do exist and will ultimately enable carriers to adapt and accommodate this wireless multimedia explosion.
Uday Mudoi is director of marketing at Vitesse Semiconductor (www.vitesse.com).
Edited by Stefania Viscusi