Installation manual

So, you place each document into its own ‘internal’ envelope before placing all the ‘internal’
envelopes inside the big one.
Another, more appropriate analogy: An ISP may specify that all TCP/IP messages must be
placed inside a brown envelope marked ‘PPP’ with your name and encrypted password written
on it before being placed inside the ‘real’ ATM envelope and sent to the ISP. The reason: ‘we
like to be sure who all messages come from and we’ve always done it this way’.
This is how PPP encapsulation is used in ADSL.
Ethernet over LLC/SNAP
This is a bridged connection method and is one of the options 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.
Filter
In this context, a device that separates the low frequency (voice) from the high frequencies
(data). Without a filter, picking up a phone that is connected to an ADSL line can cause
sufficient disturbance to the line to cause a retrain to occur; this may stop data transmission for
up to 15 seconds.
G.hs = G.994.1
G.994.1 defines the “handshaking” protocol that defines how the ADSL modems each whistle to
allow their detection by the other and agree how the ADSL line is going to work.
This is just like two V.34/V.90 modems whistling, burbling and bonging to each other to decide
how slow a connection to give you.
The synchronisation and training phase terminates in ‘showtime’. The G.994.1 recommendation
defines the use of multiple tones in parallel to give resilience to interference; earlier
handshaking techniques used a single tone and were susceptible to external interference
‘knocking out’ this tone and preventing handshaking from proceeding.
G.Lite = G.992.2, derived from T1.413 Issue 2
In 1999, it seemed that this was going to be very important, but now ‘Full rate’ has fought back.
In any case, the distinction is largely academic because all current and planned ADSL chipsets
and DSLAMs that do G.992.2 also do G.992.1 (Full rate).
G.992.2 specifies ATM as the low level protocol, maximum up/down speeds of 512kbps/1.5Mbps,
the fast retrain option and power saving.
Fast retrain is intended to reduce the impact of picking up a phone on the ADSL enabled line.
Without a filter to prevent the phone interfering with the data, taking a phone off-hook leads to a
retrain sequence that could last 15 seconds.
Fast retrain uses stored information in the user’s modem as a start-point for the training process
rather than starting again from the beginning. If the stored configuration still works, the fast
retrain can be completed in less than a second.
Allied Data Technologies 49
CopperJet 81x
CJT 81x 02-10-2001 11:45 Pagina 49