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INTERNATIONAL  TELECOMMUNICATION  UNION



CCITT	G.707
THE  INTERNATIONAL
TELEGRAPH  AND  TELEPHONE
CONSULTATIVE  COMMITTEE





GENERAL  ASPECTS  OF  DIGITAL
TRANSMISSION  SYSTEMS;
TERMINAL  EQUIPMENTS


SYNCHRONOUS  DIGITAL  HIERARCHY
BIT  RATES






Recommendation  G.707



Geneva, 1991





































Printed in Switzerland


FOREWORD
	The CCITT (the International Telegraph and Telephone Consultative Committee) is a permanent organ of the International Telecommunication Union (ITU). CCITT is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis.
	The Plenary Assembly of CCITT which meets every four years, establishes the topics for study and approves Recommendations prepared by its Study Groups. The approval of Recommendations by the members of CCITT between Plenary Assemblies is covered by the procedure laid down in CCITT Resolution No. 2 (Melbourne, 1988).
	Recommendation G.707 was prepared by Study Group XVIII and was approved under the Resolution No. 2 procedure on the 5 of April 1991.


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CCITT  NOTES
1)	In this Recommendation, the expression “Administration” is used for conciseness to indicate both a telecommunication Administration and a recognized private operating agency.
2)	A list of abbreviations used in this Recommendation can be found in Annex A.








ăITU1991
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PAGE BLANCHE
Recommendation G.707
Recommendation G.707
SYNCHRONOUS  DIGITAL  HIERARCHY  BIT  RATES
(Melbourne 1988, revised 1990)
	The CCITT,
considering
	(a)	that Recommendation G.702 specifies a number of digital hierarchy bit rates for 1544 kbit/s and 2048 kbit/s based digital networks;
	(b)	that the various hierarchy levels specified in RecommendationG.702 are interconnected by means of digital multiplexing employing justification methods;
	(c)	that synchronous digital multiplexing and a related synchronous digital hierarchy offer advantages such as:
–	simplified multiplexing/demultiplexing techniques;
–	direct access to lower speed tributaries, without need to multiplex/demultiplex the entire high speed signal;
–	enhanced operations administration and maintenance (OAM) capabilities;
–	easy growth to higher bit rates in step with evolution of transmission technology;
	(d)	that the synchronous digital hierarchy rates need to be chosen such that they allow the transport of digital signals:
–	at hierarchical bit rates as specified in Recommendation G.702;
–	at broadband channel bit rates;
	(e)	that Recommendation G.708 specifies the network node interface (NNI) for the synchronous digital hierarchy;
	(f)	that Recommendation G.709 specifies the synchronous multiplexing structure;
	(g)	that Recommendations G.707, G.708, and G.709 form a coherent set of specifications for the synchronous digital hierarchy and network node interface (NNI),
recommends
	(1)	that the first level of the synchronous digital hierarchy shall be155520 kbit/s;
	(2)	that higher synchronous digital hierarchy bit rates shall be obtained as integer multiples of the first level bit rate;
	(3)	that higher synchronous digital hierarchy levels should be denoted by the corresponding multiplication factor of the first level rate;
	(4)	that the following bit rates should constitute the synchronous digital hierarchy.

ANNEX
(to Recommendation G.707)
Alphabetical list of abbreviations used
in this Recommendation
NNI		Network-node interface
OAM		Operations, administration and maintenance
SDH		Synchronous digital hierarchy