2.	Recommendation G.181

CHARACTERISTICS OF 1+1 TYPE RESTORATION SYSTEMS

FOR USE ON DIGITAL TRANSMISSION LINKS

1.	General

Transmission restoration functions are often implemented in the modern telecommunication network to improve the availability and quality of services, by minimizing the effects or potential effects of a transmission failure, and to make the maintenance operations easier.

The terminology and general principles of transmission restoration are described in Recommendation M.495. The functional organization for automatic transmission restoration is described in Recommendation M.496.

2.	Object of Recommendation

This Recommendation specifies the characteristics of equipment for 1+1 type transmission restoration systems (protection link switching) for digital transmission links, (see Recommendation G.701). The general arrangement of a system of this type is shown in Figure 1/G.181. It uses hybrid on the send side, splitting the input path into two output paths. On the receive side the two paths are supervised and are connected further by a switch automatically controlled by the received signals. The switch may additionally be operated manually or by some kind of remote procedure. The two transmission directions are handled independently.

This Recommendation refers to the equipment labelled as H (Hybrid) RSE (Restoration Switching Equipment) and RSCE (Restoration Switching Control Equipment).

This Recommendation does not cover the restoration systems fully embedded in transmission systems.

The hierarchical level at interface T is 2048 kbit/s. Other hierarchical levels are under study.

3.	Equipment specifications

Equipment H and RSE (see Figure 1/G.181) may be of the regenerative or non-regenerative type.

3.1	Interfaces

3.1.1	Transmission path interfaces (T)

For H and RSE equipment of the regenerative type the interfaces shall be as specified in Recommendation G.703. The intrinsic output jitter should be not greater than 0.05 UI (measurement filter bandwidth: 20 Hz to 100 kHz).

For H and RSE equipment of the non-regenerative type the interface characteristics are under study.

3.1.2	Control interface (X)

The control interface X is not at present standardized by CCITT. However in the future it may be specified as a Q interface (see Recommendation G.771).

3.2	Operational aspects

3.2.1	Transfer of the switched signals

For H and RSE equipment of the regenerative type the jitter transfer gain should be not greater than 0.5 dB (the frequency limits are under study).

For H and RSE equipment of the non-regenerative type the transfer characteristics are under study.

3.2.2	Response

Switching between the two paths occurs only on the receive side, as indicated in Figure 1.

One of the two paths may be the path with the primary right, e.g. path II/II'. If this path fails the switch is operated to path III/III'. After restoration of path II/II' the switch will automatically be set back to this path.

If the two paths have the same right the switch will remain in the last position even after restoration of a failed path. This is the preferred method.

Note - Paths II/II' and III/III' have the same performance under normal planning conditions of transmission routes and systems. The "method of the same right" reduces the frequency of switching and resynchronization by a factor of 2.

Switching to a failed path must be avoided.

The RSCE should operate the switch:

-	automatically according to the criteria expressed by Tables 1/G.181 and 2/G.181, based on AIS reception and loss of incoming signal or (as an option) on transmission quality (Note);

-	manually on command from the local manual control;

-	under a request coming from interface X.

The switching criteria require further study.

Note - As an option the switching can be based on transmission quality as defined in Recommendation G.821 and by agreement between administrations.

In this case the transmitted signals need to have a standard frame structure in accordance with Recommendation G.704 which for 2048 kbit/s should also include the CRC4 option.

It is recommended that the time required for the above response actions, that is the "Confirmation time" plus the "Restoration Transfer Time" should be less than 10 ms for terrestrial routes and 500 ms for satellite routes.

TABLE 1/G.181

Response criteria for the hybrid H at the transmit side

TABLE 2/G.181

Switching criterion for the RSE at the receive side