Covad is the dominant SDSL provider (CLEC) in the USA-occupied territories. Nowadays Covad can act as an ISP itself (that's what happens when an end user orders a line directly from Covad), but they also serve as a CLEC partner for other ISPs, which is the traditional business model. In the latter case Covad enables the ISP to provide DSL in virtually any physical location by virtue of having their DSLAMs installed in almost every telco CO in the country and provides OSI Layer 2 transport for the ISP. The ISP then only needs to have one backhaul router or at most a few of them connected to Covad's backhaul network. In this traditional model Covad or other CLEC sticks to OSI layers 1 and 2, whereas the ISP issues IP addresses and deals with layers 3 through 7. ISPs which go through Covad per this model include some big guys like AT&T, MegaPath and Verizon Business as well a myriad of small shops.
Covad operates a pure ATM backhaul network and uses Nokia DSLAMs to serve SDSL and IDSL. These DSLAMs function primarily as dumb ATM switches: for SDSL and ADSL they send ATM cells to the CPE, whereas for IDSL and T1 they perform FRF.8 Layer 2 conversion and send FR to the CPE.
The encapsulation served on the line depends on whether one uses Covad's own IP service or a 3rd party ISP. The former uses PPPoA with CHAP authentication (unknown whether PAP would work as well or not), the latter provides a transparent Layer 2 cross-connect to the ISP who can serve any encapsulation in theory. In practice 3rd party ISPs going through Covad serve either routed or bridged circuits with RFC 1483 encapsulation.
The above is how the encapsulation looks to the ISP or to an SDSL user. For IDSL and T1 the FRF.8 IWF in the DSLAM turns RFC 1483 into RFC 1490; PPPoA gets turned into PPPoFR by the same IWF. It still looks the same to the ISP though, i.e., the ISP always sees ATM whereas the user sees ATM or FR for SDSL and IDSL/T1, respectively.
The Open SDSL Connectivity Project has achieved successful connectivity to both SDSL and IDSL circuits served through Covad which cross-connect to a 3rd party ISP who provides a simple routed IP encapsulation (no PPP). This has been done with our Hack-o-Rocket platform.
Covad's SDSL lines use the proprietary Nokia flavor which we have now fully reverse-engineered and documented here.
The higher level encapsulation running over ATM on Covad SDSL lines is PPPoA for Covad IP (i.e., lines ordered by end users from Covad acting as an ISP) and RFC 1483 on lines which cross-connect to other ISPs. VPI is 0 and VCI is 38.
Covad IDSL lines run at full 144 kbps and the IDSL bit stream
occupies channel positions B1+B2+D in this order.
Devices which only support IDSL-128 are thus of no help on Covad lines.
Our testing with the Hack-o-Rocket has revealed however that Covad's
flavor of IDSL is otherwise completely vanilla at the physical layer.
No EOC messages were ever received and no unusual patterns have ever
been observed on the other ISDN frame overhead bits in our testing.
The HDLC encapsulation on the line is Frame Relay with DLCI 16. When the line goes to Covad's own IP network there is a further encapsulation of PPP over FR, corresponding to their use of PPPoA on the ATM-flavored lines. The CPE has to transmit the first PPP LCP packet over DLCI 16, otherwise the line from the DSLAM is completely silent until then.
When the line goes to another ISP, the encapsulation is standard RFC 1490. No LMI is necessary as attested by the fact that our Hack-o-Rocket works despite not implementing any.
When Covad had switched from Inefficient Networks 5851 and 5871 to the newer Siemens 5890 as their standard CPE, I thought there was a possibility of Covad SDSL becoming G.shdsl, but their subsequent switch to Netopia 4652 has disproven that hypothesis.
There does exist a G.shdsl line card for the Nokia D50 DSLAMs used by
Covad, but given the status of SDSL in general,
it seems very unlikely that Covad would have any motivation to do anything
at all with its Nokia SDSL/IDSL infrastructure.
They are getting new DSLAMs from someone other than Nokia, but they
are ADSL only.
I am afraid that if Covad makes any changes at all to their SDSL/IDSL
infrastructure, it'll be the total discontinuation of these slow and
old-fashioned
symmetric business services.
Let's just hope they keep their old Nokias as they are rather than kill the
service altogether.