ÿWPCL ûÿ2BJ|xÐ ` ÐÐÌÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿH øÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÌÐÐ °°°è ÐÁà1ÁÑ Âx„|ü@Ž ÑÐ Å@8Ø'@8Ø'Å ÐÓÓÃÃContents of Recommendation Q.522Äă ÁàAÁƒ Áà3ÁDIGITAL EXCHANGE CONNECTIONS,ƒ Áà0ÁSIGNALLING AND ANCILLARY FUNCTIONSƒ ÁàAÁƒ ÁàAÁƒ 1.ÁHÁGeneral 2.ÁHÁConnections through an exchange 3.ÁHÁSignalling and D©Channel handling 4.ÁHÁAncillary functions 5.ÁHÁControl functions associated with call handling Ð ø Ð6.ÂXHÂControl functions associated with maintenance and automatic supervisionÆÆ ÁàðDÁ*ƒ ÁàðDÁƒ ÁàðCÁ*     *ƒ ÃÃRecommendation Q.522ÄÄ Áàð6ÁDIGITAL EXCHANGE CONNECTIONS,ƒ Áàð3ÁSIGNALLING AND ANCILLARY FUNCTIONSƒ 1.ÁHÁÃÃGeneralÄÄ Ð 8 ÐÁHÁThis Recommendation applies to digital local, combined, transit and international exchanges for telephony in Integrated Digital Networks (IDN) and also to local, combined, transit, and international exchanges in an Integrated Services Digital Network (ISDN). The field of application of this Recommendation is more fully defined in Recommendation Q.500. 2.ÁHÁÃÃConnections through an exchangeÄÄ 2.1ÁHÁÃÃGeneralÄÄ ÁHÁThe characteristics of the connections detailed in this section refer to an established connection when it is made available to the users. ÁHÁAn exchange must be able to provide originating, terminating and internal exchange connections between input and output interfaces for telephony and other services as required. It may also provide transit connections. Ð Ð ÐÁHÁ©Âà  ÂA connection (if any) between an incoming and an outgoing circuit at interfaces to other exchanges/networks is called a transit connection.ÆÆ ÁHÁ©Âà  ÂA connection between channel(s) of a calling subscriber line at an interface for subscriber access and an outgoing circuit at an interface to other exchange/networks is called an originating connection.ÆÆ ÁHÁ©Âà  ÂA connection between an incoming circuit at an interface to other exchange/networks and channel(s) of a called subscriber line at an interface for subscriber access is called a terminating connection.ÆÆ ÁHÁ©Âà  ÂA connection between channels of two subscriber lines at interfaces for subscriber access is called an internal connection.ÆÆ ÁHÁAn exchange must be able to provide bidirectional connections between input and output interfaces for telephony and other services as required. ÁHÁAlso unidirectional connections may be required. 2.2ÁHÁÃÃBasic exchange connectionsÄÄ 2.2.1ÁHÁÃÃGeneralÄÄ ÁHÁThe requirements in this section primarily apply to digital local or combined exchanges. Applicability to digital transit or international exchanges requires further study. ÁHÁFour types of exchange connection have been identified to show the basic forms of connection and their associated information flows that a digital local or combined exchange may be required to handle in an ISDN. They have been based on originating/terminating connections established via interfaces for subscriber access as described in Q.512 to/from locations external to the exchange. Calls may be set up in either direction, i.e. subscriber to network or network to subscriber. ÁHÁThese diagrams are functional and not intended to represent any particular implementation. They illustrate the options which may be available for handling a given information type or service within a digital local or combined exchange. Although this approach leads to some duplication between the individual diagrams when considered from the connection point of view, the approach is a logical basis for the further consideration of the more detailed issues arising from the impact of the ISDN on a digital local or combined exchange. ÁHÁIt is not intended to imply that every digital local or combined exchange should necessarily have the capability to handle all these types of connection. ÁHÁOther types of connection and variants of these basic exchange connections may be feasible in an ISDN and are the subject of further study. ÁHÁThe signalling and control aspects of these connections are covered in ÀÀÀÀ 3 and 5 of this Recommendation. 2.2.2ÁHÁÃÃExplanatory information on the exchange connection diagramsÄÄ ÁHÁThe functions associated with the groupings shown on the types I©IV exchanges connection diagrams are covered in section 3. © Key                           ÃÃÀÀÄÄ  Ãà              ÄÄ  information other than                           ÃÃÀÀÄÄ                   separate signalling                           ÃÃÀÀÄÄ                              ©©©©©©©©©©©©©©©  separate signalling(s) © Information flows Ð À ÐÂHH©Âà  ÂpÃÃ1ÄÄ = packet data information different from customer originated packetized data;ÆÆ Ð ` Ð Ð  ÐÂHH©Âà  ÂsÃÃ1ÄÄ, sÃÃ2ÄÄ, sÃÃ4ÄÄ, sÃÃ5ÄÄ = signalling information different from the signalling associated with customer terminals;ÆÆ Ð ` Ð Ð H ÐÁHÁ©Âà  Âpacket switching interworking functions may be provided at otherÔ w.Ô exchanges in an ISDN or at the point of access to a separate packet switched network.ÆÆ 2.2.3ÁHÁÃÃType 1 exchange connection (Figure 1/Q.522)ÄÄ ÁHÁThis connection is used to transport telephony and associated voice services. ÁHÁThis connection is characterized by (see Recommendation I.340, Table 2, A4,A5, A7, A8): ÁHÁ©Á  ÁÃÃinformation transfer attributesÄÄ ÁHÁÁ  Ámode:          circuit ÁHÁÁ  Árate:          64 kbit/s ÁHÁÁ  Ácapability:    speech © 3.1 kHz audio ÁHÁÁ  Áestablishment: switched © semi permanent ÁHÁÁ  Ásymmetry:      bidirectional © symmetric ÁHÁÁ  Áconfiguration: point©to©point uniform ÁHÁÁ  Ástructure:     8 kHz integrity ÁHÁ©Á  ÁÃÃaccess attributesÄÄ ÁàHAÁÀ À    user access    À À  network access       ƒ ÁàH-ÁÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ ƒ ÁàHEÁaccess channel      À À                   À À                       ƒ ÁàHIÁand rate            À À                   À À                       ƒ ÁàHMÁÀ À                   À À                       ƒ ÁàH<Áinformation         À À       B/64        À À digital circuit with  ƒ ÁàHDÁÀ À                   À À access to analogue/   ƒ ÁàHEÁÀ À                   À À digital switched      ƒ ÁàHJÁÀ À                   À À network               ƒ ÁàHMÁÀ À                   À À                       ƒ ÁàH:Ásignalling          À À     D/16 © 64     À À digital circuit (sÃÃ1ÄÄ)  ƒ ÁàHMÁÀ À                   À À                       ƒ ÁàHCÁaccess protocol (1) À À                   À À                       ƒ ÁàH;ÁLayer 1          À À For further study À À Q.702, others         ƒ ÁàHAÁLayer 2          À À       I.441       À À Q.703, others         ƒ ÁàH=ÁLayer 3          À À       I.451       À À Q.704, Q.714, Q.764,  ƒ ÁàHJÁÀ À                   À À others                ƒ       (1) Only for switched services ÁàHNÁƒ ÁHÁ©Á  ÁÃÃother attributesÄÄ ÁHÁÁ  ÁFor further study. ÁàHGÁFIGURE 1/Q.522ƒ ÁàHNÁƒ ÁàHAÁÃÃType 1 exchange connectionÄă 2.2.4ÁHÁÃÃType II exchange connection (Figure 2/Q.522)ÄÄ ÁHÁThis connection is used to transport circuit switched services such as data, telephony, multiple subrate information streams multiplexed into 64 kbit/s by the user, transparent access to a PSPDN (see section 5.2.2). ÁHÁThis connection is characterized by: ÁHÁ©Á  ÁÃÃinformation transfer attributesÄÄ (see Recommendation I.340, ÁGÀ!HÁÁH8"IÁTable 2, A1, A2) ÁHÁÁ  Ámode:          circuit ÁHÁÁ  Árate:          64 kbit/s ÁHÁÁ  Ácapability:    unrestricted digital information ÁHÁÁ  Áestablishment: switched © semi©permanent ÁHÁÁ  Ásymmetry:      bidirectional © symmetric ÁHÁÁ  Áconfiguration: point©to©point © uniform ÁHÁÁ  Ástructure:     8 kHz integrity ÁHÁ©Á  ÁÃÃaccess attributesÄÄ ÁàHAÁÀ À     user access     À À  network access       ƒ ÁàH,ÁÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ ƒ ÁàHFÁaccess channel      À À                     À À                       ƒ ÁàHIÁand rate            À À                     À À                       ƒ ÁàHMÁÀ À                     À À                       ƒ ÁàH<Áinformation         À À        B/64         À À digital circuit with  ƒ ÁàHDÁÀ À                     À À access to analogue/   ƒ ÁàHEÁÀ À                     À À digital switched      ƒ ÁàHJÁÀ À                     À À network               ƒ ÁàHMÁÀ À                     À À                       ƒ ÁàH:Ásignalling          À À      D/16 © 64      À À digital circuit (sÃÃ1ÄÄ)  ƒ ÁàHMÁÀ À                     À À                       ƒ ÁàH<Áaccess protocol     À À      see type I     À À see type I            ƒ ÁàH;ÁÀ À   (For signalling   À À (For signalling accessƒ ÁàHAÁÀ À        only)*       À À to digital switched   ƒ ÁàHIÁÀ À                     À À network)*             ƒ * Further study is required for information transfer protocol or access to    PSPDN. ÁHÁ©Á  ÁÃÃother attributesÄÄ ÁHÁÁ  ÁFor further study. ÁàHGÁFIGURE 2/Q.522ƒ ÁàHNÁƒ ÁàHAÁÃÃType II exchange connectionÄă Ô w.ÔŒ 2.2.5ÁHÁÃÃType III exchange connection (Figure 3/Q.522)ÄÄ ÁHÁThis is a connection used to transport packetized data information between an information channel on a digital subscriber access and a 64 kbit/s digital circuit which has an access to: ÁHÁa)Âh   Âa remote ISDN Packet Handling Functional Grouping;ÆÆ ÁHÁb)Âh   Âa remote ISDN Packet Switching Interworking Functional Grouping;ÆÆ ÁHÁc)Âh   Âa public packet network (see section 5.2.3).ÆÆ ÁHÁThis connection is characterized by: Ð  ÐÂHH©Âà  ÂÃÃinformation transfer attributesÄÄ (see Recommendation I.340, Table 2, A10, A11)ÆÆ Ð 8 Ð ÁàÐLÁƒ ÁàÐ@ÁÀ À         Option a          À À      Option b and c        À Àƒ ÁàÐ'ÁÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀƒ ÁàÐBÁmode            À À circuit                   À À packet                     À Àƒ ÁàÐ0Árate            À À 64 kbit/s (Further study) À À 64 kbit/s (Further study)  À Àƒ ÁàÐ2Ácapability      À À unrestricted digital      À À unrestricted digital       À Àƒ ÁàÐ@ÁÀ À   information             À À   information              À Àƒ ÁàÐ+Áestablishment   À À switched © semi permanent À À switched © semi permanent  À Àƒ ÁàÐ0Ásymmetry        À À bidirectional symmetric   À À bidirectional symmetric    À Àƒ ÁàÐ6Áconfiguration   À À point©to©point            À À point©to©point             À Àƒ ÁàÐ6Ástructure       À À 8 kHz                     À À service data unit integrityÀ Àƒ ÁàÐKÁÀ À                           À À                            À Àƒ ÁHÁ©Á  ÁÃÃaccess attributesÄÄ                       À À    user access    À À          network access           ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ   access channel and À À                   À À                                     rate                À À                   À À                                                         À À                   À À                                     information         À À                   À À                                                         À À                   À À                                     options a, b, c     À À  B/Further Study  À À 64 kbit/s digital circuit, Note 1                       À À                   À À                                     signalling a, b, c  À À     D/16 © 64     À À 64 kbit/s digital circuit, Note 1                       À À                   À À                                     access protocol     À À                   À À                                       options a, b      À À For further study À À For further study                     option b          À À For further study À À X.75 © X.25                       Ð à ÐÃÃNote 1ÄÄ © Information and signalling may be carried by the same circuit (in©band signalling). ÁHÁ©Á  ÁÃÃother attributesÄÄ ÁHÁÁ  ÁFor further study. ÁàÀJÁFIGURE 3a/Q.522ƒ ÁàÀQÁƒ ÁàÀ>ÁÃÃType III exchange connection option a)Äă ÁàÀJÁFIGURE 3b/Q.522ƒ ÁàÀQÁƒ ÁàÀ>ÁÃÃType III exchange connection option b)Äă Ô w.ÔŒ ÁàÀJÁFIGURE 3c/Q.522ƒ ÁàÀQÁƒ ÁàÀ>ÁÃÃType III exchange connections option c)Äă ÃÃNote 1ÄÄ © These links may pass through the 64 kbit/s circuit switch. This ÁI°"JÁrequires further study. ÃÃNote 2ÄÄ © SÃÃ2ÄÄ, optional. 2.2.6ÁHÁÃÃType IV exchange connection (Figure 5/Q.522)ÄÄ ÁHÁThis connection is used to transport message type data, for example packetized data messages or teleaction information messages (see section 5.2.4). ÁHÁIt consists of a message/packet type connection via a D channel on a digital subscriber access to an access port to either: ÁHÁa)Âh   Âa remote ISDN Interworking Functional Grouping; orÆÆ ÁHÁb)Âh   Âa public packet network.ÆÆ ÁHÁThis connection is characterized by: Ð  ÐÂHH©Âà  Âinformation transfer attributes (see Recommendation I.340, Table 2)ÆÆ Ð ` Ð ÁHÁÁ  ÁSee type III, options b and c. ÁHÁ©Á  Áaccess attributes ÁàAÁƒ Áà4ÁÀ À    user access    À À    network access     ƒ Áà ÁÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀƒ Áà6Áaccess channel and À À                   À À                       ƒ Áà>Árate                À À                   À À                       ƒ Áà@ÁÀ À                   À À                       ƒ Áà9Áinformation and     À À                   À À                       ƒ Áà;Ásignalling          À À                   À À                       ƒ Áà)Áoption a and b      À À  D/Further Study  À À 64 kbit/s digital     ƒ Áà4ÁÀ À     (Note 1)      À À circuit (Note 2)      ƒ Áà@ÁÀ À                   À À                       ƒ Áà8Áaccess protocol     À À                   À À                       ƒ Áà+Áoption a          À À For further study À À For further study     ƒ Áà.Áoption b          À À For further study À À X.75 © X.25           ƒ ÃÃNote 1ÄÄ © Signalling in©band or out©band. Ð Ð ÐÃÃNote 2ÄÄ © Signalling in©band or out©band for option a or, in©band for option b. ÁHÁ©Á  ÁÃÃother attributesÄÄ ÁHÁÁ  ÁFor further study. ÁàHHÁFIGURE 4/Q.522ƒ ÁàHOÁƒ ÁàHBÁÃÃType IV exchange connectionÄă 2.3ÁHÁÃÃFunctions associated with attributes shown in section 2.2ÄÄ ÁHÁThe following information is to clarify which functions may be associated with the attributes shown on the types I©IV exchange connection diagrams described in ÀÀ 2.2. 2.3.1ÁHÁÃÃLayer 1 functions (see note)ÄÄ ÁHÁThis functional grouping includes: ÁHÁ©Á  Ádigital line/exchange termination interface functions. 2.3.2ÁHÁÃÃLayer 2 functions (see note)ÄÄ ÁHÁThis functional grouping includes: ÁHÁ©Á  Álayer 2 D Channel protocol handling (LAP D). Ô w.ÔŒ2.3.3ÁHÁÃÃ64 kbit/s circuit switching functionsÄÄ ÁHÁThis functional grouping includes: ÁHÁ©Á  Á64 kbit/s switching stage(s). 2.3.4ÁHÁÃÃSignalling handling and exchange control functionsÄÄ ÁHÁThis functional grouping may include: ÁHÁ©Á  Álayer 3 D Channel protocol for signalling (see note); ÁHÁ©Á  Áfunctions related to circuit switched connection control; ÁHÁ©Á  Ásignalling functions for common channel signalling; ÁHÁ©Á  Ásignalling interfaces with "packet handling function"; ÁHÁ©Á  Áfunctions related to packet switched connections; Ð  ÐÂHH© Âh   Âsignalling interface with "packet switching interworking function".ÆÆÐ ` Ð Ð H ÐÃÃNoteÄÄ © The term "layer" refers to Open Systems Interconnection as applied to CCITT signalling systems as defined by Recommendations X.200 and I.112. 2.3.5ÁHÁÃÃPacket switching interworking functionsÄÄ ÁHÁThis functional grouping may include: ÁHÁ© Âh   Âsignalling interface with "packet handling function" and "signalling handling and exchange control function" allowing call packets to be routed to/from the appropriate subscriber terminals;ÆÆ ÁHÁ©Á  Árouting functions; ÁHÁ©Á  Áfunctions such as compatibility checking; ÁHÁ©Á  Áprotocol conversion; ÁHÁ©Á  Ánumbering interworking; ÁHÁ©Á  Ácontrol function (note). 2.3.6ÁHÁÃÃPackets handling functionÄÄ ÁHÁThis functional grouping may include: ÁHÁ©Á  Álayer 3 D Channel protocol for packet calls; ÁHÁ©Á  Ápacket level multiplexing for outgoing calls; ÁHÁ©Á  Ápacket level demultiplexing for incoming calls; ÁHÁ© Âh   Âsignalling interface with the "signalling handling function" and subscriber terminals via layer 1 and 2 functional block;ÆÆ ÁHÁ© Âh   Âcontrol functions for packet switched connections (note);ÆÆ ÁHÁ© Âh   Âsome or all the functions associated with packet switching (e.g. internal packet calls).ÆÆ ÃÃNoteÄÄ © Clarification of control function would need further study. ÁHÁIn the case where the packet switching interworking function is not present in the local exchange, the local exchange contains the minimum functions necessary to enable it to communicate with the packet switching interworking function. The protocols to carry out this minimum function require further study. 2.4ÁHÁÃÃBit rate of a connection through an exchangeÄÄ 2.4.1ÁHÁÃÃBasic bit rate for circuit switched connectionsÄÄ ÁHÁThe exchange should be able to make circuit switched connections between channel timeslots with the basic bit rate of 64 kbit/s. The channel timeslots to be connected are contained in primary or higher order frame structures appearing at the digital interfaces of the exchange or derived from analogue channels appearing at the analogue interfaces, or from individual digital interfaces for subscriber access. ÁHÁSwitching at rates other than 64 kbit/s is for further study. 2.4.2ÁHÁÃÃBasic bit rate for message/packet switched connections Type IVÄÄ ÁHÁThe bit rate of a message/packet connection Type IV will depend on a number of factors including the bit rate of the subscriber terminal equipment, the bit rate of the D channel and the bit rate capacity of the connection to the appropriate network. 2.5ÁHÁÃÃServices offered at bit rates less than 64 kbit/sÄÄ ÁHÁServices requiring less than 64 kbit/s for a connection should be switched as 64 kbit/s connections. 2.6ÁHÁÃÃServices offered at bit rates greater than 64 kbit/sÄÄ 2.6.1ÁHÁÃÃGeneralÄÄ ÁHÁServices requiring more than 64 kbit/s for a connection are through© connected as a multiple of 64 kbit/s connections. They are called multi©slot connections. ÁHÁThe exchange may be required to establish the following types of multi© slot connections (see Recommendation I.340): ÁHÁ©Á  Á2 x 64 kbit/s connections; ÁHÁ©Á  Á6 x 64 kbit/s connections to provide a HÃÃOÄÄ channel; ÁHÁ©Á  Á24 x 64 kbit/s connections to provide a HÃÃ11ÄÄ channel; ÁHÁ©Á  Á30 x 64 kbit/s connections to provide a HÃÃ12ÄÄ channel. ÁHÁIt should be noted that an n x 64 kbit/s connection can seriously affect the blocking probability of an exchange and the network, particularly if all n timeslots are routed in a defined order in the same multiplex. The ability to handle multi©slot traffic will be influenced by the traffic loading of the exchange at any instant and the number of circuits available on the required route. 2.6.2ÁHÁÃÃ2 x 64 kbit/s connectionsÄÄ ÁHÁThis connection type is characterized by the following attributes (see Recommendation I.340). ÁHÁThe attributes are for further study. ÁHÁA 2 x 64 kbit/s connection is established in response to signalling information received on the subscriber line or on an inter©exchange link. ÁHÁThe exchange should maintain restricted differential time delay between the two timeslots involved in the connection. The precise definition of "restricted differential time delay" is still to be formulated, but the intention is to ensure that the time delay between the individual slots forming a 2 x 64 kbit/s connection is not excessive, for example as could occur if the two channels were routed out of the exchange on diverse physical routings. ÁHÁThe exchange aspects of meeting this requirement require further study, but will include the need for the exchange to be capable of recognizing the signalling information on incoming inter©exchange circuits/subscriber accesses which indicates that there is an association between two incoming 64 kbit/s channels and to ensure that the two channels are handled by the exchange in a uniform manner. 2.6.3ÁHÁÃÃSwitching of 6 x 64 kbit/s connections (HÃÃOÄÄ Channel)ÄÄ ÁHÁThe 64 kbit/s timeslots which form an HÃÃOÄÄ Channel are transmitted over the same primary multiplex system within the same frame. This is valid for bothÔ w.Ô the subscriber line and the inter©exchange links. ÁHÁThe requirement for 6 x 64 kbit/s connections can be satisfied by establishing 6 separate 64 kbit/s semi©permanent connections, each of which would be set up to preserve the sequence with the other slots forming the       6 x 64 kbit/s connection. 2.6.4ÁHÁÃÃSwitching of 24 or 30 x 64 kbit/s connection (HÃÃ1ÄÄ Channels)ÄÄ ÁHÁThis is for further study. 2.7ÁHÁÃÃMode of establishmentÄÄ 2.7.1ÁHÁÃÃCircuit switched connections, Type I, Type II and Type III option a)ÄÄ ÁHÁCircuit switched connections are set up at any time on demand in response to signalling information received from subscribers, other exchanges or other networks. 2.7.2ÁHÁÃÃPacket switched connections, Type III options b) and c)ÄÄ ÁHÁFor further study. 2.7.3ÁHÁÃÃMessage/packet switched connections, Type IVÄÄ ÁHÁThese connections are set up on demand subject to any D Channel priority/flow control restrictions that may be applicable. 2.7.4ÁHÁÃÃSemi©permanent connectionsÄÄ ÁHÁThe exchange should have the capability of establishing semi©permanent connections which pass through the exchange switching network. ÁHÁOther features of semi©permanent connections, e.g. grade of service, the need for an out©slot signalling channel associated with the connection, etc. are for further study. 2.8ÁHÁÃÃBit sequence independenceÄÄ ÁHÁSee Recommendation Q.554. 2.9ÁHÁÃÃBit integrityÄÄ ÁHÁSee Recommendation Q.554. 2.10ÁHÁÃÃOctet sequence integrityÄÄ ÁHÁSee Recommendation Q.9. 2.11ÁHÁÃÃ8 kHz (structure) integrityÄÄ ÁHÁSee Recommendation I.140. 2.12ÁHÁÃÃBit patterns generated by the exchange in idle channel timeslotsÄÄ ÁHÁAt interfaces A and B, the following patterns are recommended for the idle condition, where the left©most digit is the polarity digit. ÁHÁ01111111 for 1544 kbit/s systems ÁHÁ01010100 for 2048 and 8448 kbit/s systems. ÁHÁAt other interfaces the bit pattern generated in idle channel timeslots is for further study. ÁHÁThe patterns should not be used as an indication of the idle or barred conditions of a channel since this information should be derived from the control or signalling functions. ÃÃNoteÄÄ © These patterns are slightly different from the quiet code produced by external test equipment and used as an auxiliary signal for noise and crosstalk measurements on digital exchanges (see Recommendations Q.551, ÀÀ 1.2.3.1, Q.552, ÀÀÀÀ 2, 2.2.3, 3.1.4 and Q.553, ÀÀÀÀ 2.1.1.2, 3.1.4). 2.13ÁHÁÃÃError performanceÄÄ ÁHÁSee Recommendation Q.554. 2.14ÁHÁÃÃIn©call rearrangementÄÄ ÁHÁIn©call rearrangement is the rearrangement by the exchange of the established connections across the switchblock in a more efficient manner. ÁHÁWhen it is provided, it is essential that the requirements for error performance, Quality of Service, etc. be met (see Recommendation Q.543). 2.15ÁHÁÃÃTransmission performance characteristicsÄÄ ÁHÁSee Recommendations Q.551©4. 3.ÁHÁÃÃSignalling and D©Channel handlingÄÄ 3.1ÁHÁÃÃGeneralÄÄ ÁHÁThe exchange should be capable of interworking with other exchanges using signalling systems indicated in Recommendation Q.7, and for local or combined exchanges with user equipment on digital access lines (e.g. terminals and PABXs) using the signalling procedures in Recommendations I.430, I.431, and Q.920 (I.441), Q.930 (I.451)©Series of Recommendations. ÁHÁFor a local or combined exchange interworking with user terminals or analogue subscriber access lines should be accomplished using nationally recommended signalling procedures. ÁHÁ64 kbit/s signalling channels entering the exchange via a multiplex structure may be connected through the exchange as semi©permanent channels. 3.2ÁHÁÃÃSignalling associated with exchange connections Types I©IVÄÄ ÁHÁThis section applies to local or combined exchanges only. 3.2.1ÁHÁÃÃGeneralÄÄ ÁHÁDetails of the exchange connections Types I©IV are given in ÀÀ 5. ÁHÁFor internal and originating connections, the call set up signalling information will be received from the subscriber. ÁHÁFor terminating and transit connections, the call set up signalling information will be received from the appropriate network or separate signalling network. ÃÃNoteÄÄ © Receipt of call set up signalling information may be affected by the involvement of supplementary services. 3.2.2ÁHÁÃÃBasic connections including Type I exchange connectionÄÄÔ w.ÔŒ ÁHÁThe exchange should carry out the functions defined in the following signalling systems. 3.2.2.1Á  ÁOn the subscribers side: ÁHÁa)Âh   Âanalogue line signalling systems as defined nationally; andÆÆ ÁHÁb)Âh   Âthe defined digital subscriber access signalling system(s) if digital subscriber accesses are provided (see Recommendations I.430 and Q.920, Q.930©Series of Recommendations).ÆÆ 3.2.2.2Á  ÁOn the network side ÁHÁOne or more of the signalling systems are defined in Recommendation Q.7. 3.2.3ÁHÁÃÃType II exchange connectionÄÄ ÁHÁThe exchange should carry out the functions defined in the following signalling systems. 3.2.3.1Á  ÁOn the subscriber side ÁHÁThe defined digital subscriber access signalling system(s) (see Recommendations I.430 and Q.920, Q.930©Series of Recommendations). 3.2.3.2Á  ÁOn the network side ÁHÁOne or more of the signalling systems defined in Recommendation Q.7. Ð à ÐÃÃNoteÄÄ © Further study is required for information transfer protocol or access to PSPDN. 3.2.4ÁHÁÃÃType III exchange connectionÄÄ 3.2.4.1Á  ÁOn the subscriber side ÁHÁFor further study 3.2.4.2Á  ÁOn the network side ÁHÁThe signalling associated with the messages/packets may be: ÁHÁa)Á   Ácontained in the individual message/packets; or Ð ° ÐÂHHÂb)Âh   Âtransported separately, in accordance with one or more of the signalling systems defined in Recommendation Q.7.ÆÆ Ð ` Ð Ð H ÐÁHÁA local exchange which supports such services must contain a function that is capable of either interpreting them and routing them appropriately, or of sending them directly to an appropriate interworking function. 3.2.5ÁHÁÃÃType IV exchange connectionÄÄ ÁHÁOn the subscriber side: ÁHÁThe signalling associated with the messages/packets may be: ÁHÁa)Âh   Âcontained in the individual message/packet, orÆÆ ÁHÁb)Âh   Âtransported separately as s©information (see Recommendations I.430 and Q.920 and Q.930©Series of Recommendations).ÆÆ ÁHÁOn the trunk side: ÁHÁa)Âh   Âcontained in the individual message/packet (pÃÃ1ÄÄ); orÆÆ ÁHÁb)Âh   Âtransported separately (sÃÃ1ÄÄ information), in accordance with one or more of the signalling systems defined in Recommendation Q.7.ÆÆ ÁHÁA local exchange which supports such services must contain a function that is capable of either interpreting them and routing appropriately, or of sending them directly to an appropriate interworking function. 3.3ÂàHÂÃÃDigital subscriber access © D Channel and protocol handling layers 1, 2ÄÄ ÃÃand 3ÄÄÆÆ ÁHÁThe following text refers to handling the D Channel protocol on the exchange side of the interfaces U and V1. ÁHÁThe functions associated with handling the D©channel protocol are defined in the parts of Recommendations I.430, Q.920 and Q.930©Series of Recommendations relating to call establishment for subscribers connected to the U or V1 interfaces. Exchange functions for D©channel signalling procedures for users connected via a primary rate multiple access are also given in I.431, Q.920 and Q.930©Series of Recommendations. 3.4ÁHÁÃÃUser©to©user signallingÄÄ ÁHÁThe exchange may receive signals from the user (e.g. from a PABX) for transport across the network. It must be capable of receiving this information, verifying its acceptability, and if the service is permitted to the requesting user, send it via the inter©exchange signalling or other network to the distant exchange. Similarly the exchange may receive information from the signalling network for transmission to the subscriber. This capability may not be provided on all types of connection. ÁHÁWhere user©to©user signalling involves network inter©exchange facilities, it may be necessary for the originating local exchange to process this signalling information before sending it to the network, to ensure that it is compatible with signalling, charging and flow control requirements of the originating exchange and network. 4.ÁHÁÃÃAncillary functionsÄÄ 4.1ÁHÁÃÃConnection of ancillary equipmentÄÄ ÁHÁAncillary equipment may be connected in the following way: ÁHÁi)Âh   ÂSerially. This may require more than one connection through the exchange. Examples of serially connected equipment include:ÆÆ ÁHÁÁ  Á©Á øÁecho control devices, ÁHÁÁ  Á©Á øÁencoding law converters, ÁHÁÁ  Á©Âà øÂmanual board access equipment (for operator controlled traffic).ÆÆ ÁHÁii)Âð   ÂAs terminal connected equipment usually requiring one connection through the exchange. Examples of such equipment include:ÆÆ ÁHÁÁ  Á©Á øÁrecorded announcements, Ô w.ÔŒÁHÁÁ  Á©Á øÁmanual board terminations, ÁHÁÁ  Á©Á øÁspeech codecs, ÁHÁÁ  Á©Á øÁdata terminal facilities, ÁHÁÁ  Á©Á øÁtest equipment (such as a test call sender), ÁHÁÁ  Á©Á øÁtone generators, ÁHÁÁ  Á©Á øÁsignalling receivers. ÁHÁThe interface between the exchange and the items of equipment listed above may be left to the national designers. However, the use of internationally standardized interfaces is preferred. ÃÃNoteÄÄ © In some cases it may be necessary to establish more than one connection to one timeslot at the same time. 4.2ÁHÁÃÃDigitally generated tones and frequenciesÄÄ ÁHÁWhen tones and frequencies are digitally generated the following minimum requirements apply on a provisional basis. 4.2.1ÁHÁÃÃService tonesÄÄ ÁHÁDigitally generated tones should meet the recommended limits specified in Recommendation Q.35 when decoded. 4.2.2ÁHÁÃÃSignalling frequenciesÄÄ ÁHÁDigitally generated signalling frequencies should be such that they can be detected after decoding by any analogue receivers designed to CCITT Recommendations. 4.3ÁHÁÃÃEcho control devicesÄÄ ÁHÁThe exchange should be able to be equipped with echo control devices (echo suppressors/echo cancellers conforming to Recommendations G.164 and G.165 respectively). When required the exchange should be able to control such devices to meet the requirements of Recommendation Q.115. The means of control by the exchange is for further study. (ÃÃNoteÄÄ © It is recognized that there is a need for an internationally agreed method of disabling and enabling echo control devices for the purposes of making end©to©end circuit transmission maintenance measurements, e.g. as recommended in Recommendation V.25.) 5.ÁHÁÃÃControl functions associated with call handlingÄÄ 5.1ÁHÁÃÃBasic control functionsÄÄ ÁHÁThe requirements for the basic control functions are implicit in the requirements recommended for the other functions of the exchange. However recommendation of a number of new requirements for the control functions associated with the handling of digital subscriber lines and the use of a digital local exchange within an ISDN, may be necessary. ÁHÁThe exchange should provide the capability to avoid fraudulent use of the connection. Such capabilities may be based on the use of digital pads or an asymmetrical through©connect procedure. ÁHÁAn originating local exchange should be capable of supporting either symmetric or asymmetric through©connect procedures. The choice of procedure may be determined on the basis of service. ÁHÁTerminating and transit exchange need only support symmetric through© connect procedure. ÁHÁThis subject is for further study. 5.2ÁHÁÃÃExchange connections Types I©IV, general control aspectsÄÄ 5.2.1ÁHÁÃÃType IÄÄ ÁHÁThese connections will be set up between the accesses associated with network addresses specified in response to the signalling information received. Voice©associated facilities, e.g. tones, should be provided where appropriate and telephony supplementary services may be invoked if provided. 5.2.2ÁHÁÃÃType IIÄÄ ÁHÁSuch connections will be set up between the accesses associated with network addresses specified in response to the signalling messages received. Compatibility checking may be provided before the connection is completely established (see ÀÀ 2.3.1 a)). Voice associated facilities (e.g. tones, pads) will be disabled in order to provide a transparent digital path, (the means of doing this is for further study). Data supplementary services may be invoked if provided. 5.2.3ÁHÁÃÃType IIIÄÄ ÁHÁSuch connections will be set up between the accesses associated with network addresses specified in response to the signalling messages received. Compatibility checking may be provided before the connection is completely established (see ÀÀ 2.3.1 a)). Voice associated facilities (e.g. tones, pads) will be disabled in order to provide a transparent digital path, (the means of doing this is for further study). Data supplementary services may be invoked if provided. ÁHÁFor option a) in the switched access case, originating calls will be set up over the B Channel towards the 64 kbit/s digital circuit using the ISDN signalling procedures prior to starting X.25 layer 2 and layer 3 functions. The corresponding service requested in the Q.931 SET©UP message is ISDN packet mode bearer service. For calls originated by the network, the same consideration applies (see X.31/I.462). ÁHÁThe distant packet handling must be selected by the called address in the D Channel protocol when the terminal sets up the circuit switched connection. ÁHÁPacket data communications, when using a switched B Channel, will be established by separating the establishment phase of the B Channel (carried out by the exchange) and the control phase of the virtual circuit using the X.25 link layer and packet layer protocol (carried out in the distant packet handling function). ÁHÁFor options b) and c) the same considerations as above apply except that the control phase of the virtual circuit is performed within the exchange. 5.2.4ÁHÁÃÃType IVÄÄ ÁHÁThese connections will be of the message/packet type (e.g. virtual circuit). The "p©information handling function" and "packet switching interworking function" shown in Figure 4/Q.521 will implement procedures for control of the logical links on the D Channel (e.g. flow control, error control) (see also ÀÀ 3.2.5). Voice associated facilities (e.g. tones, pads) will be disabled inÔ w. Ô order to provide a transparent digital path. ÁHÁThe D Channel provides a semi©permanent physical connection which enables the user terminal to access a packet handling function by establishing a link layer connection (with a specific SAPI) to that function which can then be used to support packet communications according to X.25 layer 3 procedures. The X.25 packet layer will use the acknowledged information service provided by LAP D (see Q.920) X.25 layer 3 procedures are transferred transparently over the D Channel link. ÁHÁA single or multiple LAP D link must support the multiplexing of logical channels at layer 3. 5.3ÂàHÂÃÃControl functions associated with calls over a digital subscriberÄÄ ÃÃaccess via Interfaces U and V1ÄÄÆÆ 5.3.1ÁHÁÃÃControl of circuit switched calls Types I, II and IIIÄÄ ÁHÁIn response to s©information carried on the D Channel and network signalling messages, the exchange must have the following capabilities. ÁHÁa)Âh   ÂSetting up a callÆÆ Ð Ð ÐÂHHÂÁ  ÁThe exchange must receive address information (overlap sending or en bloc), establish the desired path (digital only or mixed) and send further (e.g. Signalling System No. 7) signalling, if necessary (e.g. address, calling line identity, service indicator) into the network.ÆÆ Ð ` Ð ÂHHÂÁ  ÁThe call set up procedure may include steps to verify compatibility based on the record in the exchange of the services permitted for the subscriber. The degree of compatibility checking provided by the exchange requires further study.ÆÆ ÁHÁb)Á   ÁDuring a call Ð H ÐÂHHÂÁ  ÁIn addition to the basic functions of maintaining a call record, supervising the call, charging for the call, etc., the exchange must be able to handle in©call service/facility requests. These include for example transfer of a call to another terminal or conferencing.ÆÆ Ð ` Ð Ð À ÐÂHHÂÁ  ÁIf it is required that a terminal be moved from one location to another on the same access during a call, the exchange must be able to hold the call while the transfer is made and to re©establish communication on request by the user (including carrying out any compatibility checks). The exchange may limit the time allowed for moving a terminal. In addition, the user must send a signal to the exchange indicating that terminal movement is about to take place. Signalling procedures for terminal movement are given in Recommendation Q.930.ÆÆ Ð ` Ð ÁHÁc)Á   ÁClearing a call Ð H ÐÂHHÂÁ  ÁThe exchange will need to initiate call clearing on receipt of a clear request signal from the terminal or network.ÆÆ Ð ` Ð ÁHÁd)Á   ÁWithout a call path Ð ° ÐÂHHÂÁ  ÁThe exchange may be required to handle signalling information without establishment of a call path (subscriber © network transactions).ÆÆ Ð ` Ð Ð p Ð5.3.2ÁHÁÃÃControl of message/packet calls over the D Channel, Type IVÄÄ Ð X ÐÁHÁAny messages carrying p©information on the D Channel must be handled by the exchange in accordance with the applicable Recommendation for services (e.g. Recommendation X.25) requested by the user. It is not necessary that every digital local or combined exchange in an ISDN should be able to carry out all the possible functions associated with handling this information. It is possible for example that the exchange may route such traffic to another node which has the appropriate handling facilities. 6.ÂXHÂÃÃControl functions associated with maintenance and automaticÄÄ ÃÃsupervisionÄÄÆÆ ÁHÁSee Recommendation Q.542. ÁàÀPÁÃà          Äă