Adtran is a company that rpovides all kinds of network devices. CSUs DSUs, although they provide a lot more than they use to. The Lab manual is probably referring to devices used in the Cisco Network Academy Lab, and I think its the following http://www.adtran.com/web/page/portal/Adtran/group/42
Yep, I guess at one time they were trying to incorporate those into the academy CCNA/CCNP class curriculum...we had several on the racks and I never bothered trying to hook one up, and recently the dept. head just unscrewed them all and tossed them in a dark closet somewhere.
They simulate wan connection to a central office.
Just for interest, heres a little blurb about it:http://www.adtran.com/pub/Library/Case_Studies/Cisco_Academy_Benefits_From_WAN_Emulation.pdf