Chapter 8: Glossary
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encapsulation used to carry the TCP/IP traffic over the ATM connection and pass
the data into the ISPs normal network. In addition, if PPP is one of the
encapsulating protocols, the BAS performs the user authentication (perhaps using
an authentication server such as RADIUS to verify the username/password
combination). Splitting this functionality from the DSLAM increases the
implementation flexibility by allowing the connection to the ISP’s IP network (and
the PPP authentication, if necessary) to take place at the ISP’s premises, across
an ATM network from the DSLAM. In some configurations, the BAS will allow
switching between ISPs based on the user id/password that the user supplies in
the PPP authentication. This architecture allows the ISP to replace the Access
Server that terminates and authenticates the PPP connection from customers with
analogue modems and ISDN lines with the BAS for SHDSL customers. If PPP is
used on the SHDSL connection, then the SHDSL user connected through the BAS
appears in the ISP’s network in exactly the same way as an analogue modem or
ISDN user; the same authentication server could be handling both conventional
and SHDSL users. This explains the popularity of PPP as an encapsulating
protocol.
Bridged/Routed connections
Bridge-based solutions are perceived by some telcos and ISPs as the simpler,
cheaper, entry level SHDSL offering. These telcos/ISPs tend to regard routed
connections as ‘complicated, expensive and advanced’. It is difficult to see how
this can be justified, since the thinnest of passive devices can use a routed
connection using Microsoft’s PPP over ATM stack in Windows 98SE, Me and 2000.
Note that some telcos launching services more recently have chosen to provide
only routed solutions even for their low-end services, skipping the bridge.Bridged
solutions are normally either ethernet with LLC/SNAP (this is one of the options of
RFC 1483) or PPP over Ethernet (PPPoE - there is no ‘real’ RFC for this, only an
‘informational’).
Routed solutions are normally either PPP over ATM (PPPoATM - RFC 2364) or IP
with LLC/SNAP (this is another option of RFC 1483). 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 SHDSL modems and
consolidates all the data connections onto a single ATM fibre connection, typically
at 155Mbps currently.
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
