Cross-Canada fiber-optic network integrates Sonet and SDH protocols

Cross-Canada fiber-optic network integrates Sonet and SDH protocols

To combine its offshore North American and terrestrial Canadian calling traffic, service provider Teleglobe implemented a coast-to-coast fiber-optic ring network that provides multimedia applications, automatic survivability and Sonet/SDH compatibility

francis gagnon

northern telecom

To upgrade its telecommunications services, Teleglobe in Montreal--the overseas telecommunications provider for Canada--decided to integrate its offshore calling traffic and terrestrial domestic

traffic on a reliable, high-capacity fiber-optic Synchronous Optical Network (Sonet). The telecommunications company engaged Stentor in Ottawa, ON--Canada`s alliance of major telephone companies--to deliver the network. In turn, Stentor selected Northern Telecom in Mississauga, ON, to supply the fiber-optic equipment.

The coast-to-coast Canada network project resulted in a 10-year, $250-million contract between Stentor and Teleglobe for the lease of fiber-optic bandwidth from Stentor. The eastern segment of the Teleglobe network, deployed across eastern and central Canada to Toronto, ON, went into service in November 1994 (see Fig. 1). The extension of the network to the west coast of British Columbia was completed in October 1995 (see Fig. 2).

Spanning the width of Canada, the network travels 3800 miles through eight provinces and is probably the longest survivable fiber-optic network in the world. Its deployment passes through the operating territories of eight Stentor member telephone com panies--Maritime Telephone and Telegraph Ltd., the New Brunswick Telephone Co. Ltd., Quebec Telephone, Bell Canada, Manitoba Telephone System, Saskatchewan Telephone, Alberta Gov ern ment Telephone and BC TEL in British Columbia.

By installing a Sonet/ Synchronous Digital Hierarchy (SDH) fiber-optic ring network, Teleglobe is delivering reliable and cost-effective voice, video and data communications, competitively positioning the company as a leading long-distance carrier, interconnecting to European and Pacific Rim countries and reserving bandwidth to accommodate future growth. Among the communications services available are traditional telephone service, virtual private network, global Internet and transit, Asynchronous Transfer Mode (ATM) data, Integrated Services Digital Network, and international toll-free telephony.

Compatibility accomplished

In addition to network survivability, Teleglobe needed to carry both Sonet and SDH traffic without requiring translation between the two protocols. European communications traffic arrives on the Cantat-3 submarine fiber-optic cable at Pennant Point, NS. It is then routed to four Teleglobe sites in the provinces of Ontario, Quebec and British Columbia.

Based on protocol translation work performed at the Stentor lab in Montreal in collaboration with Teleglobe engineers, a direct interface was achieved at the transport layer. The SDH signals originate from a Marconi SDH multiplexer and feed into a Northern Telecom 2.5-gigabit-per-second OC-48 S/DMS Transportnode in Montreal. These SDH signals are transported transparently over the Sonet network, without the need for protocol translation, thus increasing system reliability.

This dual transport is achieved because the frame structure of 155-megabit-per-second SDH STM-1 signal is identical to that of a 150-Mbit/sec Sonet STS-3c signal. As a result, the SDH STM-1 payload (VC-4) can be transported transparently over the Sonet network through an OC-3c interface. The section overhead bytes of the SDH equipment also need to be compatible with the transport overhead bytes of the Sonet equipment at both the definition and the implementation levels. Modeling studies and field tests confirmed that Sonet/SDH compatibility could be implemented in the Teleglobe fiber-optic network.

Running rings

Some of the largest bidirectional line-switched rings in the world are linked across Canada to form the backbone network for Teleglobe`s integrated Sonet/SDH system. This network uses an innovative configuration featuring large, bidirectional line-switched rings, some as long as 1365 miles, interconnected via redundant gateways using drop-and-continue capabilities.

These bidirectional rings are also interconnected via redundant gateways to provide matched node service; if a catastrophic event affects the primary gateway, the traffic is automatically transported via the secondary gateway to the other ring.

In this manner, the company`s Transportnode configuration provides redundant routing across inter-ring boundaries. In the outbound direction, drop-and-continue routing is used within a primary gateway network element to simultaneously pass an existing 52-Mbit/sec STS-1 or STS-3c signal to both the neighboring ring and a remote OC-48 shelf configured as a secondary inter-ring gateway. For transmissions in the opposite directions, the primary inter-ring gateway incorporates a service selector that chooses either the primary or secondary input from the adjacent ring.

The service selector contained within the primary gateway switches from the primary input signal to the secondary input based on standard line and path alarm conditions. Because the Northern Telecom OC-48 matched-node feature supports both line and path analyses, inter-ring services are protected against a wider range of faults than they would be in systems providing only line layer analysis.

The service selector also uses a delay technique to allow other protection switches to complete their functions. This delay enables other restoration mechanisms, such as low-speed interconnection protection and ring protection, to be used. Based on the assumption that any protection switch activity in all interconnected rings does not propagate, and knowing that all protection switches must be detected and completed within the Sonet specifications, the delay is set to equal the longest protection switch activity.

Traffic exchanges between interconnected rings occur at the tributary level through Sonet tributary interface formats, such as STS-1, OC-3, OC-12 or STS-12. Users have the option of provisioning STS-1 rate signals or STS-3c concatenated payloads facilitating the end-to-end transport of both conventional circuit-switched trunks and ATM services carried in the STS-3c payload. Although the STS-3c circuits can transport international SDH STM-1 signals, four STS-3c circuits can also be provisioned for 600-Mbit/sec STS-12c capability.

Extended matched nodes

Path analysis also allows use of Sonet systems on the inter-ring links, thereby permitting a wide separation between interconnecting gateways. This concept, also known as extended matched nodes, is applied in the Stentor network, where the matched nodes are located far apart. The matched nodes are linked by Sonet linear systems that maintain the reliable restoration capability required by the network. The extended matched nodes configuration is primarily used in the western part of Canada, where the length of cross-links (south to north) spreads out geographically.

The extended matched nodes configuration is more economical to implement than using multiple rings to make connections (see Fig. 3). To keep costs down without reducing network reliability, Stentor considered using OC-48 0:1 linear systems to interconnect the extended matched nodes. However, this architecture would have required the service selector contained within the primary gateway to switch from the primary input signal to the secondary input based on the signal-degrade-threshold crossing alarm in addition to the line/path alarm indication signal.

At the time, this feature was undefined by Bellcore GR-1230 standards. Therefore, Northern Telecom and Stentor evaluated various alternatives and concluded that reconfiguration of existing OC-48 0:1 linear systems with a new 0:1 linear system into a 1:N linear system would circumvent the issue of signal degradation.

In addition, Stentor had another key network requirement to address: Carry 48 STS-1 signals in a fully survivable configuration across the network. Northern Telecom`s implementation of matched node technology for bidirectional line-switched rings uses the working channels between the primary and secondary gateway nodes--defined in GR-1230 as drop-and-continue on working. However, this approach allows for only 24 STS-1 signals to fully survive across the network. To carry 48 fully survivable STS-1 signals, the secondary feed needed to use the protection channels between the primary and secondary gateway nodes--defined in GR-1230 as drop-and-continue on protection (DCP)--was deployed. The standards for DCP, also undefined at that time, have been finalized this year.

Technology innovations

To transport 48 STS-1 signals, Northern Telecom implemented an innovative concept employing a third matched node (see Fig. 4). This node permits the transport of 48 STS-1 signals fully survivable across the network and still uses the working channels on the secondary feed. The node also ensures protection against a double-failure occurrence, whereas DCP can only protect against a single failure. The third matched node overcame the limitation of only 24 STS-1 signals being protected by becoming the secondary gateway of both original gateways.

Because the Northern Telecom Transportnode OC-48 ring provides flexibility in primary/secondary inter-ring gateway assignments, Stentor reduced by half the number of Teleglobe circuits affected in a gateway node failure: A network element is provisioned as a primary gateway for half of the inter-ring traffic and a secondary gateway for the other half of the traffic.

Another innovative technology deployment involves the use of optical line amplifiers by BC TEL. The company deployed Northern Telecom bidirectional 1533- and 1557-nanometer optical line amplifiers to relieve nondispersion-shifted fiber plant exhaustion and increase the aggregate transmission capacity of a fiber by a factor of two.

Centralized control

Before the eastern portion of the Sonet network was put into service in October, 1994, and before the entire cross-Canada network was activated a year later, Stentor tested under various failure scenarios the network configurations used on this project in its Montreal laboratory. After network installation, the system design included a national network operations center and a centralized surveillance center in each province. In this way, each Stentor member telephone company could maintain the integrity of its portion of the network.

Remote maintenance, provisioning, monitoring and operations support are essential elements of the Teleglobe network. Stentor Canadian Network Management and Stentor Owner Companies are using a combination of transaction language-based, operation support system software and Northern Telecom S/DMS network element management systems for network control. Remote access to the network through the element management systems allows for rapidly activated services and ensures constant network equipment status.

In fact, when a 1995 mud-slide in British Columbia knocked out a fiber-optic cable on the cross-Canada telecommunications network, the outage could have turned into a financial disaster for Teleglobe. However, the carrier of all overseas telecommunications services from Canada benefited from a fiber-optic Sonet/SDH network that self-restored within milliseconds. Moreover, its customers were unaware that a cable failure occurred. u

Francis Gagnon is broadband transport marketing manager at Northern Telecom in St.-Laurent, QC, Canada.

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