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 telecom-
munication network to improve the availability and quality of services, by minimiz-
ing 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 trans-
mission 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 proce-
dure. The two transmission directions are handled independently.



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



	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 speci-
fied 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 RecommendationG.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 accor-
dance 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