Synchronous Digital Hierarchy, SDH

Listen to the network pulse

SDH (Synchronous Digital Hierarchy) technology is a transport traffic standard defining speed levels for a signal passing from a Synchronous Transport Module (STM), including a physical (optic) level required to support compatibility of various vendors’ equipment.   

The main traffic speed (STM-1) is 155.250 Mbit/sec. Other, higher speeds are defined as speeds multiple of STM-1:

  • STM-4 – 622 Mbit/sec,
  • STM-16 – 2488.32 Mbit/sec,
  • STM-64 – 9953.28 Mbit/sec.

The SDH technology presupposes using a Time-Division Multiplexing (TDM) method and time slice cross-switching, with SDH terminal equipment processing E1 data flows (2.048 Mbit/sec) and client equipment connected to SDH (built-in cross connectors are used for accessing Nx64К data flows). SDH multiplexers are the main network devices.

One important feature of an SDH network is the requirement for traffic time slice synchronization between all network components. Usually, a multiplexer can be synchronized with any incoming signal, reference timing signal, or its own clock pulse generator. The reference timing signal synchronization can be spread along a circuit containing up to 20 network components (G.803).

The synchronization source can be chosen automatically, by a software module, or it can be set manually by the operator.

Functionally, the SDH multiplexers have two interface sets, the user and the aggregate ones. The user interface set is purposed for connecting users, while the aggregate one is for creating linear inter-switch connections.

These interfaces make it possible to create the following base topologies:

  • “ring”,
  • “chain”,
  • “point-to-point”.

When building an SDH network, the ring topology is usually used having two loops. One of the loops is used for transferring timing and signal information, while the other one is purposed for carrying the main data traffic. A special reservation mechanism is provided for the cases when one of the loops fails. It is also possible to connect equipment under the point-to-point topology, but this will reduce the solution fault tolerance.

Centralized network control system provides full line and node (multiplexer) status monitoring. Using ring topologies makes it possible to switch lines automatically, in case of any fault occurring, to provide reserve paths. The SDH equipment can provide line and main equipment reservation, for switching automatically to reserve data paths in emergency situations, using a 1+1 reservation scheme.

This is the feature that increases the network reliability significantly, making it possible to service the network with no interrupts in servicing clients.

The SDH-based network is able to provide transport facilities for nearly every existing high-speed optic data transfer technologies (including ATM and POS).

The overall multiplexer network topology is built using basic components. Ordinarily, complex networks are built as multilevel structures where the first, equipment, level is supposed to provide user access. The level consist of “last mile” equipment and includes, most often, STM-1 multiplexers. The former is used for carrying user signals (most often, E1 and E3 signals) to the first level multiplexers. The “last mile” equipment consists, mainly, of optic modems converting electric signals into optical ones and vice versa. Multiplexers of this level collect user lines for subsequent data transfer. The next level may consist of STM-4 and STM-16 mltiplexers.

The main SDH technology advantages are as follows:

  • simple multiplexing/demultiplexing methods;
  • accessing low-speed signals with no need for multiplexing/demultiplexing all the high-speed line making it easy to connect client equipment and cross-switch data flows;
  • providing reservation mechanisms for the cases when communication lines or equipment fail;
  • possibility of creating “transparent” communication lines required for some solutions (e.g., for voice traffic transfer between automatic telephone exchange autonomic units or transferring telemetric data);
  • solution scalability;
  • compatibility with various vendors’ equipment;
  • comparatively low equipment prices;
  • quick equipment tuning and configuration.

Now about some disadvantages:

  • inefficient use of communication line bandwidth (because of line reservation and inability to allocate the bandwidth dynamically for various applications, lack of traffic prioritization mechanisms);
  • restricted network scalability.

The Projects that Are Already Implemented

Development and commissioning of the "Main Ring" digital transmission network and renovation of equipment for "Kiev Ring" network for UMC.

This project was implemented by PrioCom Company on the basis of the equipment produced by Alcatel.

11 multiplexers manufactured by "Alcatel" Company (OPTINEX™ line, 1660-SM type) were assembled and installed within the UMC "Main Ring" transmission network. This kind of equipment has made it possible to choose an optimum scheme of organization of the transmission network. Such a scheme, in its turn, has made it possible to implement the cheapest variant of the network, on the one hand, and to create the broadband multi-service medium for traffic transmission with the possibility to offer channels for data transmission (Ethernet, ATM, nxE1).

Development of transport SDH-network for "Kyivstar" with the help of the equipment manufactured by "ECI" Company.

In the course of development of the network, length of which exceeds 5,000 km, it was decided to utilize the equipment, which belongs to the XDM line (manufactured by "ECI Telecom" Company). This project was implemented on the basis of platforms: XDM-1000, XDM-500, XDM-100 and uSDM-1. Due to utilization of the single architecture and the same network interface cards, fact of utilization of the products belonging to the XDM line makes it possible to increase volume of the traffic to be transmitted, to widen the assortment of the services to be provided, as well as to achieve an essential economy in spending of the operators' funds in order to ensure operation of their networks. Main specific features of the proposed modular platform of XDM multi-servicing are as follows: the fully-connected matrix, which can not be blocked; possibility of allocation of Е1 flows from the levels up to STM-64; as well as modularity.

The created optical network for voice and data transmission may be easily transformed into the network based on DWDM technology in future.

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