12 ADSL NETWORK ARCHITECTURE,PROTOCOLS,AND EQUIPMENT communicate.For example,one PVC could be assigned for an ISP and another to communicate with corporate headquarters.However,higher layer protocols used by NSPs could not effectively be overlaid on top of the RFC 1483-based protocol stack. For Internet access and remote access to corporate servers it was highly desirable to use the same "legacy"programs for authentication,billing,and encryption/security that are used by NSPs today.These are all operational over the Internet engineering task force (IETF)'s point-to-point protocol(PPP).To facilitate the PPP over ATM over ADSL capability,the ADSL Forum has completed TR-0012:an end-to-end architecture for the transport of PPP over ATM over ADSL.This is likely to be implemented by the majority of ADSL equipment vendors.In this scheme,the entire ATM network is reduced to a set of virtual point-to-point leased lines,and all traffic is sent on a "best effort basis" using the ATM unspecified bit rate (UBR)traffic class. Whereas today,only PVCs are used with ADSL,a catalyst for SVCs will be widespread use of Microsoft's ATM protocol stack,including SVC signaling,in Windows 98 and Windows 2000(formerly known as Windows NT).This will encourage use of SVCs end to end,which are much easier than PVCs to maintain in a large network.A potential problem for SVCs is that the ATM address plans of telcos differ,creating nonunique ATM addresses,which may be either E.164 public network addresses or private network service access points (NSAPs). Another issue is mapping SVC UNI signaling messages to/from the ADSL facilities and the ATM network behind the access node (see the discussion of DSLAM in Section 2.3.1). Since there is no guality of service (QoS)capability or multicasting with the PPP over ATM architecture,vendors desiring to provide video/high-quality audio on demand,VToA/VolP,or real-time video conferencing over ADSL, must chose a classical ATM protocol stack.These stacks have been well defined by the ATM Forum and ITU-T and include: .MPEG2 over ATM(using AAL 5) Structured circuit emulation service'(for n x 64 kbit/s circuits) VToA desktop(using AAL 5 or AAL2) Video conferencing over AAL 5 or AAL 1 Thus there are three ATM protocol stacks for ATM over ADSL: .RFC 1483 encapsulation/bridging .PPP over ATM as in ADSLF TR-0012 Classical ATM(ATMF and ITU-T for real-time,interactive applications such as VoD,video streaming,VToA,conferencing,etc.) 3These applications may use the network timing reference (see Section 8.2.1).￾￾    ￾    ￾￾ ￾        ￾   !￾  ￾     "#$%&'  ￾￾(   ￾￾   ￾￾ ￾         )) ￾ ** ￾ ￾ + ￾    ! ,           (￾ -.,/*0123454150123406141718-/ , ￾   9,:  9;<￾  9;<  ￾  ,"=>>#?@ == ￾ ￾    9,:  9;< ,(       A 9;<    ￾   9,: ( ￾   ==   B￾ ))  **  9,: C3D0E72FEGH246I4E-JK"/B￾￾ L      9;<￾     :￾*9,:￾￾(￾ LM%L?>>>-  (L!,/ , ￾    ￾ ￾     ( 9   9,:   ￾ ￾  9,:  ￾   . #N$ ￾ (   3E4O16PDE6Q27EI77EDD01234D-!9/ 9  J!  +  9;< ￾  9,:  (  ￾￾  -  ￾ ;<9:  ￾? & #/ ￾   RCI824S1TDE6Q27E-U/￾ ￾  9,: ￾ ￾     +=  ,9+ =  ￾  ￾ 9;< ￾ 78IDD27I8VWX06141718D4I7P ,  ￾(    B    9,: ,J=,￾ @ Y :.Z? 9,: -99<[/ Y ￾ ￾￾  ￾ &-3\N$(+￾￾/ Y ,9 (&-99<[99<?/ Y  ￾  ￾ 99<[99<# ,    9,: ￾￾(9,:  9;<@ Y "#$%& ￾+ Y  9,: 9;<,"=>>#? Y ￾9,: -9,:,J=, =  ￾ ￾ ￾;  ,9￾  ￾ ￾ / &,  ￾    (   ￾ -  ￾% ? #/ ]^ _`abcdefghi_hjkledjemhdnohgegjgban_c`dpmloqdce