Solutions to Delay and Delay Variability

We can reduce delay and delay variability by overprovisioning, but this reduces bandwidth
efficiency and increases cost. We can control delay and delay variability by introducing
traffic shaping protocols, which means we can determine that some users will
have low delay and delay variability at the expense of others. This is the philosophical
basis of an IP network. The challenge is to make a wireless IP network perform as well
as a wireline IP network—to deliver wireless/wireline transparency. This means we
have to avoid introducing additional delays on the radio layer. We cannot do this
because there are a number of irreducible delay mechanisms—source and channel coding/
interleaving, for example. We therefore must consider reducing, or at least managing
and controlling, network delay/delay variability to make the wireless IP user
experience transparent—that is, equal to the wireline IP user experience.
Implicitly this means that we need to provide more control of the end-to-end channel
in a wireless IP network. We can achieve this by using IP traffic shaping protocols, effectively
in-band packet-level signaling, or by using existing signaling plane control mechanisms
(SS7). The SS7 signaling plane sits on top of the network traffic and is designed to
manage the process of call setup, call maintenance, and call clear-down—that is, the
management and billing involved in a circuit-switched voice call. This is inefficient if
users are exchanging short bursts of data. It is therefore more efficient to use IP protocols.
However, we have said that it is the job of the application layer to increase session persistency
(and session complexity, because session persistency and session complexity
increase session value). Asession may start as a nonpersistent session but is manipulated
to become more persistent, more complex, and ideally more interactive as the session
progresses. In IP terms, this implies that an initial best-effort service may need to be
upgraded to an interactive or conversational session as the session progresses. As session
persistency increases, we are effectively setting up the session, managing the session,
and clearing down the session. Session management becomes directly analogous to call
management.
A conversational complex rich media exchange represents the highest added-value
service available within our network. We have to be very careful how we preserve the
value of this exchange. Continuing to use SS7 for call setup, call maintenance, and call
clear-down, and extending SS7 to manage session setup, session management, and session
clear-down is one option (and the preferred option presently for most European
and Asian, and many U.S. network operators).
If we use IP for session management, using protocols, like Session Initiation Protocol
(SIP), then we need to be sure that these protocols perform at least as well and preferably
better than the protocols they are replacing.