MSPPs can deliver digital video over existing networks

Several industry trends are coming together to give service providers an opportunity to increase revenues from digital video transport services. The Federal Communications Commission has mandated an aggressive plan for TV stations to convert their broadcast signal format from traditional analog National Television Standards Committee (NTSC) to digital television (DTV) by 2006. DTV will ultimately pave the way for mass consumer deployment of high-definition television (HDTV). Emerging applications such as pay-per-view events, interactive TV, videoconferencing, post-production for filmmaking, distance learning, telemedicine and remote security monitoring are creating demand for digital video services.

Digital video is in line with the general trend in the consumer electronics industry, which is quickly moving to the digital equivalents of traditional analog devices, including the digital camera, digital camcorder, DVD player, and 3G wireless phone. It has different traffic characteristics than voice and data, and transporting all three across a single converged transport network can be challenging. Voice traffic maintains a constant rate and is sensitive to delay across the network. Data traffic runs at variable rates and is generally bursty in nature. Digital video traffic shares a number of voice and data characteristics, while consuming large amounts of bandwidth and requiring a deterministic network. In addition, video services often travel in one direction across the network, while voice and data services travel in both directions.

Previous attempts for video transmission across a MAN required a costly overlay network separate from the voice and data networks, because none of the current digital video interface standards were designed for WAN transport. As a result, initial approaches to digital video transport over SONET were problematic due to the inefficient manner that video was mapped into the standard SONET timeslots of STS-1, -3c, -12c, etc. Traditional solutions typically mapped digital video signals into ATM, which was then mapped into SONET. ATM was used to supply a virtual layer for providing finer bandwidth granularity to allow additional video channels to be transported over SONET.

The development of next-generation SONET technologies such as virtual concatenation (VC), link capacity adjustment scheme (LCAS), and generic framing procedure (GFP) gives service providers a new opportunity to market video services without replacing their metro SONET networks. Multiservice provisioning platforms (MSPPs) that support these technologies allow service providers to deliver digital broadband video, broadband data, video on demand (VoD), telephony, and other business services over converged networks.

Digital video requires enormous bandwidth. An uncompressed NTSC signal requires a transmission capacity of about 200 Mbits/sec. That pales in comparison to HDTV, which requires raw bandwidth in excess of 1.5 Gbits/sec. Advanced video compression schemes are available that can compress digital video signals to reduce their bandwidth requirements, while maintaining entertainment quality. The Motion Picture Experts Group (MPEG) has developed MPEG-2, the most widely accepted compression method. MPEG-2 is an ISO/IEC-13818-1 standard and defines the syntax and semantics of bit streams in which digital video is multiplexed and transported.

The asynchronous serial interface (ASI) is a standard developed by the European Digital Video Broadcasting (DVB) Standards Association to provide simple transport and interconnection of MPEG-2 streams from different manufacturers' equipment. Equipment supporting this widely accepted standard includes MPEG-2 encoders, receivers, multiplexers, servers, and quadrature amplitude modulation devices.

The digital video industry is working to provide additional compression algorithms that will further reduce digital video bandwidth requirements and make it possible for video transmission over broadband access technologies such as DSL and cable modem. MPEG-4 AVC, or ITU Recommendation H.264, a joint effort between the International Telecommunication Union-Telecommunication (ITU-T) and MPEG, improves compression efficiency to make it possible to deliver entertainment-quality video at transmission speeds of around 2 Mbits/sec.

While many next-generation technologies exist for optical transport, SONET still dominates public and to a large extent private networks.

Recent activity within ANSI T1X1.5 and ITU-T committees has focused on enhancing SONET standards to create a better method for transporting video. The publishing of new mapping, switching, and interface standards gives service providers an opportunity to offer emerging digital video services without replacing their metro SONET networks. The new VC, LCAS, and GFP standards give equipment providers and subsequently service providers a cost-efficient solution for transporting digital video over existing reliable SONET networks without significant network-upgrade expenditures.

VC provides a finer granularity of SONET bandwidth mapping for transporting various video and data interfaces (STS-1-nv, -3c-nv). VC is an inverse multiplexing technique that enables an arbitrary number of low-order (VT1.5) or high-order (STS-1/-3c) SONET channels to be bundled into one virtual concatenation group (VCG). Only the source and the destination nodes need to be VC-capable since virtual concatenation is transparent to intermediate nodes. Thus, when carriers want to introduce VC support into their network, only the transport nodes at the add and drop points have to be upgraded to provide this function. VC enables a service provider to offer services that don't map neatly into fixed SONET timeslots without wasting bandwidth. That is especially important for Ethernet and video services (see the Table).

LCAS provides a bandwidth on demand mechanism for capacity changes, enabling rapid service activation and upgrades without traffic interruption. It also allows auto removal and auto recovery of failed paths. LCAS provides a control mechanism and protocol that can dynamically increase or decrease the number of STS member links of the VCG to meet the bandwidth needs of an application, without affecting existing traffic. Signaling messages are exchanged in the overhead bytes of the synchronous payload envelope between the network elements. LCAS also provides a means of automatically removing member links that have experienced failure and recovering the member link that has cleared the failure condition. LCAS enables efficient service upgrades and improved response to network outages by eliminating the need for manual provisioning (see Figure 1), which helps service providers reduce the number of truck rolls, improve customer service, and lower operational costs.

Figure 1. Link capacity adjustment scheme features a control mechanism that can dynamically increase or decrease the number of STS member links of the virtual concatenation group to meet the bandwidth needs of an application. This bandwidth on demand capability enables rapid service activation.

GFP allows different types of traffic to be mapped into SONET tributaries or virtually concatenated pipes for transport across SONET networks. It is defined in ITU-T G.7041/Y.1303 for framing or mapping different protocol streams into virtually concatenated STS-1 or STS-3c payloads within a SONET frame. Compared with other framing procedures such as point-to-point protocol and X.86 that employ high-level data-link control, GFP has a more deterministic overhead and lower processing requirement.

Figure 2. Frame-mapped generic framing procedure is used for mapping packet- or frame-based protocols such as Ethernet and IP within SONET tributaries.

Two types of GFP mapping formats exist: frame-mapped GFP (GFP-F) and transparent GFP (GFP-T). GFP-F is normally used to encapsulate packet/frame-based protocols such as Ethernet and IP (see Figure 2). GFP-T is optimized for protocols that utilize 8B/10B line coding, including DVB-ASI, Gigabit Ethernet, and major storage protocols. 8B/10B encoding provides an equal transition density of 1s and 0s in the data stream to ensure that timing can be recovered from the data (see Figure 3).

MSPPs that support VC, LCAS, and GFP provide a cost-effective way to consolidate voice, data, and video services onto the existing SONET-based network infrastructure. They combine the transport capabilities of multiple add/drop multiplexers, the switching/grooming capacity of a small digital-crossconnect system, and next-generation TDM, IP, video, and storage interfaces into compact platforms.

Figure 3. Transparent generic framing procedure is used for mapping services that utilize 8B/10B line coding such as Gigabit Ethernet, digital video broadcasting-asynchronous serial interface (DVB-ASI), and many storage protocols within SONET.

For unidirectional digital video broadcast, MSPPs support efficient transmission of compressed (DVB-ASI) video-signal formats directly over SONET links between hubs. Since VoD and near-VoD applications require a return path, digital video can be transported over Ethernet over SONET interfaces to leverage reliable and existing SONET networks. On-demand digital video signals are carried from the video server over the IP/Ethernet/SONET network into the remote hub for video distribution into homes.

In addition to point-to-point applications, broadcast applications for multinode video distribution via SONET are also fully supported. With the robust broadcast capabilities of SONET, adding another protocol layer such as IP to provide broadcast functionality is not necessary. SONET is widely used to broadcast video signals from a cable TV provider's headend location to remote distribution hubs. In this application, incoming video signals are duplicated to multiple output ports and sent to several distribution hubs. The drop and continue function of SONET is used to extract the video signal from the main ring and direct it to other feeder rings while keeping a copy of the video signal going around the main ring. Typical deployments use OC-12 (622-Mbit/sec) and OC-48 (2.5-Gbit/sec) rings for the feeder network, with an OC-48 or OC-192 (10-Gbit/sec) ring configured as the main ring. Both video transport modes are shown in Figure 4.

Figure 4. SONET supports both point-to-point and broadcast digital video links. For unidirectional digital video broadcasting, multiservice provisioning platforms support efficient transmission of compressed video-signal formats directly over SONET links between hubs. For broadcast video, incoming video signals are duplicated to multiple output ports and sent to several distribution hubs. The drop and continue function of SONET is used to extract the video signal from the main ring and direct it to other feeder rings while keeping a copy of the video signal going around the main ring.

SONET provides STS path-level protection for video signals to support uninterrupted transmission. The MSPP at the headend sends two copies of the video signal across diverse paths around the ring. If any MSPP on the ring detects a failure in the main path of the video signal, the MSPP will initiate a switch to the alternative path for protection. This switch is completed within 50 msec.

The development of next-generation SONET technologies gives service providers the option of offering video services without replacing their metro SONET networks. MSPPs added to these SONET infrastructures, allow service providers to create digital video transport networks that feature the same carrier class robustness that service providers—and their customers—expect.

William Yue is senior product planner and David Gutierrez is product manager at Fujitsu Network Communications (Richardson, TX). They can be reached at [email protected] or [email protected].

ITU-T G7041, Generic Framing Procedure (GFP), 2002.
ISO/IEC DIS 13818, Information Technology - Generic Coding of Moving Pictures and Associated Audio Information Systems.
ITU-T G.7042, Link Capacity Adjustment Scheme (LCAS) for Virtual Concatenation Signals.
ITU-T G.707, Network Node Interface for the Synchronous Digital Hierarchy.
ANSI T1.105.01-199x, Draft ANSI Standard, SONET Automatic Protection Switching.
ANSI T1.105-1991, Digital Hierarchy - Optical Interface Rates and Formats Specifications.
ETSI (CENELEC) EN 50083-9 - Cabled Distribution Systems for Television, Sound and Interactive Multimedia Signals, Part 9: Interactive for CATV/SMATV headends and similar professional equipment for DVB/MPEG-2/MPEG-4 transport streams (DVB Blue Book A0101), Annex B, Asynchronous Serial Interface, 1998.