Recommendation Q.554
TRANSMISSION CHARACTERISTICS AT DIGITAL INTERFACES OF A DIGITAL EXCHANGE
1	General
	The field of application of this Recommendation is found in Recommendation Q.500.
	The signals taken into consideration are passed through the following interfaces as described in Recommendations Q.511 and Q.512 and Figure 1/Q.551.
–	Interface A is for primary rate digital signals at 2048 kbit/s or 1544 kbit/s.
–	Interface B is for secondary rate digital signals at 8448 kbit/s or 6312 kbit/s.
–	Interface types V are for digital subscriber line access.
	Interface types V may appear remote from the exchange through the use of digital transmission facilities. When this occurs, there should be no impact on transmission parameters other than delay.
	Detailed transmission characteristics of the digital interface ports are given in § 2 of this Recommendation.
	§ 3 covers the requirements for transmission characteristics of the half–connections between the digital interfaces and the exchange test points. The half–connection comprises an input connection (the one–way 64 kbit/s path from the interface to the test point) and an output connection (the one–way 64 kbit/s path from the test point to the interface) as defined in Recommendation Q.551. Requirements are given for the input connection and the output connection characteristics and the two are not necessarily identical.
	The overall characteristics of a connection through the digital exchange involving two interfaces can be obtained by suitably combining the values for the characteristics of the two half–connections. For example, the overall characteristics of the connection between a Z interface and the A interface are obtained by combining the Z interface half–connection characteristics given in § 3.3 of Recommendation Q.552 with the A interface half–connection requirements given in § 3.1 of this Recommendation.
	Where bit integrity is maintained on the digital half–connection and the error performance requirements are met, the digital half–connection will add no impairment to the voice–band performance of a complete connection through the switch (with the exception of delay). For this reason the digital half–connection requirements do not include the conventional voice band parameters.
	(The cases where bit integrity is not maintained are for further study.)
	The values given are to be considered as either “design” or “performance objectives” according to the explanation of the terms given in Recommendation G.102 (Transmission performance objectives and recommendations) and the particular context. These objectives should be met under the most adverse specified timing and synchronization conditions as given in Recommendation Q.541, § 3.
2	Characteristics of interfaces
	This section covers requirements for interfaces A, B, V.
	These interfaces must meet the requirements for timing and synchronization given in Recommendation Q.541, § 3.
2.1	Interface characteristics common to digital interfaces
	The general characteristics of the interfaces A, B, V2, V3, V4 are given in Recommendations G.703, G.704, G.705, G.706, Q.511 and Q.512.
2.2	Interface characteristics at interface A
	The physical and electrical characteristics of interface A are given in §§ 2 and 6 of Recommendation G.703.
2.2.1	Jitter and wander tolerance at the exchange input
	Jitter and wander tolerance is the ability of the exchange to accept phase deviations on incoming signals without introducing slips or errors.
	The jitter/wander tolerance at input A should comply:
–	with Recommendation G.824, § 3.1.1, for the A interface at 1544 kbit/s;
–	with Recommendation G.823, § 3.1.1, for the A interface at 2048 kbit/s.
	This specification may not be applicable to inputs used solely for synchronization purposes (i.e. for deriving the internal timing of the exchange).
2.3	Interface characteristics at interface B
	The physical and electrical characteristics of interface B are given in §§ 3 and 7 of Recommendation G.703.
2.3.1	Jitter and wander tolerance at the exchange input
	Jitter and wander tolerance is the ability of the exchange to accept phase deviations on incoming signals without introducing slips or errors.
	The jitter/wander tolerance at input B should comply:
–	with Recommendation G.824, § 4.2.2, for the B interface at 6312 kbit/s;
–	with Recommendation G.823, § 3.1.1, for the B interface at 8448 kbit/s.
	This specification may not be applicable to inputs used solely for synchronization purposes (i.e. for deriving the internal timing of the exchange).
2.4	Interface characteristics at interface V1
	The functional characteristics of the basic access digital section between the V1 and T reference–point are defined in Recommendations Q.512 and I.AA. The characteristics and parameters of a digital transmission system which may form part of the digital section for the ISDN basic rate access are given in Recommendation I.AB.
2.5	Interface characteristics at other V–type interfaces
	The other V–type interfaces will have transmission characteristics of the A and B interfaces as given in §§ 2.2 and 2.3 above.
3	Characteristics of 64 kbit/s half connections
	This section covers the essential digital characteristics of 64 kbit/s half connections. Where these requirements are met, the digital half connection will add no impairment to the voice band performance of a complete connection through the exchange (with the exception of delay). The voice band performance of digital half connections may therefore be interpreted by assuming that ideal send and receive sides (see Recommendation G.714) are connected to the digital inputs and outputs respectively. The digital half connection requirements also ensure that any connection through the exchange using a pair of digital half connections will provide acceptable performance for non–voice 64 kbit/s digital services.
3.1	Half connection characteristics common to all digital interfaces
3.1.1	Error performance
	The design objective long–term mean Bit Error Ratio (BER) for a single pass of a 64 kbit/s connection through an exchange between the digital transmission/switching interfaces should be 1 in 109 or better. This corresponds to 99.5% error–free minutes assuming that the occurrence of errors has a Poisson distribution.

3.1.2	Bit integrity
	Bit integrity will be maintained if called for to support 64 kbit/s non–telephony services.
	Note – It is understood that to meet this requirement, digital processing devices such as m/A law converters, echo suppressors and digital pads must be disabled for non–telephony calls requiring bit integrity. The means of disabling these devices has yet to be determined. (See Recommendation Q.551, § 1.2.6.1.)
3.1.3	Bit sequence independence
	No limitation should be imposed by the exchange on the number of consecutive binary ones or zeros or any other binary pattern within the 64 kbit/s paths through the exchange.
3.1.4	Absolute group delay
	The requirements for absolute group delay are given in § 3.3.1 of Recommendation Q.551.