Installation manual

Note that ‘RFC 1483’ by itself doesn’t tell you what the protocol is - you need to know if the
connection is bridged or routed to know what ‘RFC 1483’ means.
DSLAM
The unit in the telco’s local exchange that houses the ADSL modems and consolidates all the
data connections onto a single ATM fibre connection, typically at 155Mbps currently.
The key configuration parameters of the DSLAM are for each line (user):
* ADSL standards supported (for instance, is G.lite supported?)
* Minimum and maximum allowed downstream speeds (minimum is normally 32kbps)
* Minimum and maximum allowed upstream speeds (minimum is normally 32kbps)
* User-side ATM VPI/VCI - can be the same for all users for support simplicity
* The ATM VPI/VCI that the user’s ATM connection is mapped to on the fibre connection.
This must be different for each user; it determines the BAS that the user is connected to.
If there is a choice of ISPs, then this VPI/VCI (and the routing in any further ATM switches) may
determine which ISP the user is connected to.
In some configurations, the BAS will allow switching between ISPs based on the
userid/password that the user supplies.
A typical DSLAM might serve several hundred users, so it is clear that all these users cannot
expect 8Mbps bandwidth at the same time if the upstream ATM fibre link is running at 155Mbps.
Encapsulation
The process of placing data inside a wrapper (or envelope) in order to make it compatible with
a protocol. The term is typically used where one protocol is placed inside another in a non-
standard way (often where a low-level protocol is placed within a high-level protocol or one
network protocol is placed inside another).
Analogy: A letter has to be placed inside a correctly addressed and stamped envelope for it to
be compatible with the postal service. A postcard is already compatible with the postal service
(as long as it is correctly stamped and addressed) so does not need encapsulation but the
postcard has severe limitations, not least, the short message length. A letter does not have an
inherent address like a postcard but, inside an envelope, the letter gives much greater flexibility
to the user (you can write much more using several pieces of paper and include a photograph
or even include a lock of hair). The postal service sees only the envelope and doesn’t care
what’s inside it. There’s nothing to stop you putting a postcard inside an envelope, so is this
encapsulation?
You may need to encapsulate data in a protocol just to get it across a network, or it may be
necessary to put an ‘extra’ envelope around the data before it is passed to the network so that
the recipient will know what to do with the data when it arrives.
Analogy: Inter-office mail may be consolidated into a single envelope to save postage charges.
But if individual documents are placed directly into one big envelope then the receiving post
room will not be able to deliver them to the correct place.
Allied Data Technologies48
CopperJet 81x
CJT 81x 02-10-2001 11:45 Pagina 48