Chapter 8: Glossary
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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.
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 SHDSL.
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
SHDSL 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 SHDSL modems
each whistle to allow their detection by the other and agree how the SHDSL 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 SHDSL 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 SHDSL 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.
G.dmt = G.992.1
Derived from T1.413; also known as ’Full rate’. This is the type of SHDSL most
commonly implemented now.
IP over LLC/SNAP
This is a routed connection method and is one of the options of RFC 1483.
