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Network time protocol synchronisation for IPTV, IMS and femtocells

Posted: 16 Feb 2012     Print Version  Bookmark and Share

Keywords:Telecom networks  Network Time Protocol  NTP 

Telecom networks are going through a significant change both in terms of technology as well as in the services they support. Ethernet displaced TDM transport, and IP is the underlying service delivery protocol for IPTV, VoIP, multimedia services, and of course, the Internet. These packet-based technologies have placed new demands on network-wide timing. This article will discuss how these changes are creating new requirements for accuracy, security, and availability of Network Time Protocol (NTP) based time, and then presents what constitutes a true carrier-calls implementation for NTP synchronisation that is needed to support these changes.

Background
NTP is an Internet protocol for synchronising the clocks of computers and other equipment to a common time reference over a network. It is also a program that implements the protocol and controls the computer clocks. As originally conceived, NTP was designed to provide time to computer hosts, however, its use has grown and today it is the universally accepted method to synchronise system clocks in computers, servers, data communication equipment, and now, full service networks.

Historically, NTP has been deployed in the Internet, and then in telecom IT departments or datacenters for post-processing functions to support operational activities such as billing, access, authentication and accounting (AAA), and event log generation. Today, many uses for NTP have emerged, and it now is a critical technology that underlies carrier networks, services and operations.

IPTV
Internet Protocol TV (IPTV) is the end-to-end delivery of video and related content using Internet Protocol over a managed network to consumers (i.e., not over the Internet). NTP-based time is used for many critical processes in IPTV service delivery. The most fundamental is synchronisation of the IPTV headend and the set top box at the end points of the service delivery path. If their clocks are not aligned so that their data rates match, the set top box buffer will experience data overflow or data underflow. This result will lead to skipped frames and momentary freezing of skipping in the TV picture that are noticeable by the end user.

Two other examples are Conditional Access (CA) and QoS. To support these functions, the IPTV service infrastructure—and therefore the time synchronisation that supports it—is distributed across a hierarchy of locations that includes the video headend office, the video service offices, the access network, and the customer premises.

Conditional Access licences provide the basis for content usage and associated revenue generation. They are key to fulfilling digital rights management (DRM) agreements with the content owners as they prevent misuse and piracy that can steal revenue and lead to agreement violations.

Time on the systems that enforce CA licences needs to be accurate and secure, for example, to ensure that operators can only show content during specific periods (such as after a DVD's release date). CA licence keys also need secure time to prevent theft of service and alteration of the licence usage duration. High capacity carrier grade NTP servers are required to handle the large transaction volumes generated by potentially millions of IPTV subscribers.

Quality of Service measurement is another key function in end-to-end IPTV service delivery. Some unique aspects of QoS measurement for IPTV are the number of measurement points in the network, the need for real-time monitoring, and the collection of metrics inside the customer premise at both the residential gateway and the set top box. Accurate and distributed time references are needed to enable QoS measurements across hops in the network, or between.

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