Some time ago I had to setup a router for a customer who required a 4 Mbps link. The chosen solution was based on a 4 Mbps SHDSL with four-wire bonding, which has a better throutput than a 4 Mbps with ATM IMA. These days a friend needed a help on setting this solution up again, so I would like to share it on my blog.

Technology

The DSLAM obtains the 4 Mbps link by multiplexing 2 links at 2 Mbps each, with no overhead (unlike IMA) and a gross max data rate of 4608 Kbps; this is what we can read in the Telecom Italia Bitstream Reference Offer:

accesso simmetrico “4 Mbit/s bonding” senza modem, con 2 coppie in rame, ognuna delle quali è terminata su una borchia RJ11, e protocollo ATM in tecnologia SHDSL ITU-T G.991.2 Annex B in modalità four-wire (bonding fisico)

“4 Mbps bonding” symmetric access without modem, with 2 copper pairs, each on a RJ11 wall jack, ATM protocol on SHDSL ITU-T G.991.2 Annex B technology, four-wire (physical bonding) mode

(my English is quite poor, maybe you can have more luck with the Google Translator)

Hardware Platform and IOS version

The hardware platform I used for the solution is based on a 2651XM router with WIC-1SHDSL-V2 card; V2 is the first release of the card supporting the 4-wire mode. For what concerns the IOS, be sure to use a release with support for the “standard” option on the “line-mode 4-wire standard” command; it has been introduced in the 12.4(2)XA release and it is needed to enable handshaking only on the master wire pair.

Cabling

A Y-cable is needed to connect the two RJ11 wall jacks to the RJ11 port on the controller; to build the cable you can use the following diagrams I found on the G.SHDSL Cisco High-Speed WAN Interface Cards Q&A page:

Basic configuration

This is the basic configuration; in the example the point-to-point interface is on X.Y.Z.0/30 subnet, with .2 assigned to our router:
 

controller DSL 0/0
 mode atm
 line-term cpe
 line-mode 4-wire standard
 dsl-mode shdsl symmetric annex B
 line-rate 4608
!
interface ATM0/0
 no ip address
 no atm ilmi-keepalive
!
interface ATM0/0.1 point-to-point
 ip address X.Y.Z.2 255.255.255.252
 pvc 12/35
  protocol ip X.Y.Z.1
  oam-pvc manage
  oam retry 3 5 1
  encapsulation aal5snap

References

Cisco.com: Symmetrical High-Bit Rate DSL Interface Card for Cisco Routers

Cisco.com: ATM Mode for Two-Wire or Four-Wire SHDSL

Cisco.com: G.SHDSL Cisco High-Speed WAN Interface Cards Q&A

Telecom Italia: Telecom Italia WholeSale