Re: [PWE3] TDM Channels and TDMPW Payload

[Javi <[e-mail]>]

Thanks, some comments inline

a) [YJS] AAL2 is Loop Emulation (LES) and not Circuit Emulation (CES). 
It has no mechanism for indicating circuit level defects, as it does not 
emulate the circuit.

[Javi] Does it mean AAL2 may not emulate a TDM trunk?

[YJS] No, absolutely not. You can emulate a full trunk by emulating all 
of its channels. However, you can also use AAL2 to glue together 
channels that do not come from a single trunk.


[Javi-New] Then I'm confused. You say "AAL2 does not emulate the 
circuit. but "AAL2 can emulate a full trunk". For me, a TDM circuit and 
a TDM trunk are the same. Which is the difference?

[Javi] I thought circuit level defects are indicated by control flags 
(L, R, M) from Control Word, so that, each TDMPW:

                                                          May emulate
     0x0011   Structure-agnostic E1 over Packet           One Full TDM 
Trunk per TDMPW (but no two trunks per TDMPW)
     0x0012   Structure-agnostic T1 (DS1) over Packet     Idem
     0x0013   Structure-agnostic E3 over Packet           Idem
     0x0014   Structure-agnostic T3 (DS3) over Packet     Idem
     0x0015   CESoPSN basic mode                          One Full 
E1/DS1 TDM Trunk per TDMPW, or one Nx64 service*
     0x0017   CESoPSN TDM with CAS                        Idem with CAS 
signalling
     0x0016   TDMoIP AAL1 Mode                            One Full 
E1/DS1 TDM Trunk per TDMPW, or one Nx64 service*
     0x0018   TDMoIP AAL2 Mode                            One Full 
E1/DS1 TDM Trunk per TDMPW, or one Nx64 service*

  * Nx64 services: "N" timeslots, from one or several TDM trunks, per 
TDMPW. Does it not include "circuit level defects"?

[YJS] Yes, trunk defects can be so indicated. If the AAL2 channels come 
from a single E1 or T1 you can use the L, R and M.

[Javi-New] ITU-T Recommendation Y.1452 (clause 8.3.1),  describing 
TDMoIP AAL.2, indicates bits R, M of the CW are reserved (value always 
0). If indeed a TDMPW AAL.2 can emulate a full trunk, where does it 
carry circuit level defects?


b) [Javi] Emulation service of fractional TDM trunk: What exactly?. I 
thought that indicates TDMPW emulates TDM circuit end to end, carrying 
only the channels in use in order to conserve bandwidth, but then it 
would not CBR, but VBR.

[YJS] I am not sure what you are asking. You can use CESoPSN and 
TDMoIP-AAL1 to emulate a fraction of a trunk -- but the fraction

and which channels are transported, can not change. Using AAL2 allows 
you to dynamically add and remove channels as needed.

[Javi-New] I detail my question. RFC 5087, clause 5.2: "Although AAL1 
may be configured to transport fractional E1 or T1 circuits...". Also, 
ITU-T Recommendation Y.1453 (clause 7.1) indicates a "TDMPW" (TDM stream 
exactly) must be able to carry "N × 64 k" and "Fractional DS1/E1". Which 
is the difference between a fractional TDM circuit and a Nx64 
(application/service)?


e) [Javi] In L2TPv3 TDMoIP AAL.2, how do you know the length of the 
payload TDMPW? (since it may contain a number of minicells dynamically 
variable).

[YJS] L2TPv3 TDMoIP has not been defined in the IETF.

[Javi-New] RFC 5087 (clause 4.3) defines the data plane for a TDMPW 
based on L2TPv3. Under the protocol stack that there is proposed, how do 
you know the payload length (variable)  of each TDMPW?

[Javi] In TDMoIP AAL.2, ITU-T I.366.1 (SSCS for CCS) and I.366.2 (SSCS 
for bearer channel) are used?

[YJS] I.366.2

[Javi-new] When CCS signalling is carried by TDMPW in transparent mode 
(not in HDLC mode), If AAL2 carries CCS signaling  using I.366.1 
(SSCSAR), it would use I.366.1. Is not that correct?


f) [JYS] CES services do NOT try to identify the channels -- the 
channels (if there are any) are transparently transported.

So all you have to do is to match up the trunks. The TDM end equipment 
knows how to pick out the channels.

If you need visibility of the channels, you need a loop (channel) 
emulation service.

[Javi-New] Then, how does output PE identify TDM trunk of  input PE to 
which TDMPW is associated?


Regards