Back to index
An Architecture and Communication Protocol for Picocellular Networks
Rohit Ghai and Suresh Singh
As cell size decreases, the number of handoffs and therefore handoff
overhead within a wireless picocellular networks increases; a method
for reducing handoff overhead and buffer space required using
multicast groups and mobile trajectory prediction is described, as well as
a network architecture that supports this technique.
- Picocells: Picocells (indoor, O(10m)) offer greater
frequency re-use, lower power transmitters, and higher
throughput. There will also be many more handoffs between cells;
overcoming the overhead associated with handoffs therefore is the
critical design element in a picocellular network.
- "Dumb" Handoff: The wrong way to do handoff
is to require old basestations to forward data received
during handoffs for open connections to the new basestation.
The number of forwarded messages can be shown to be NxMxR,
where N is the number of packets that arrive at the old BS
during and after a handoff, M is the number of mobile users per
cell, and R is the number of cells in the network.
- "Smart" Handoff: BS's (called mobility support
stations or MSSs in this story) no longer retain intelligence,
but rely on a centralized authority called a supervisory
host (SH). The supervisory host calculates the mobile's
likely trajectory, and forms a multicast group for MSSs
that the mobile is likely to handoff to in the near future. All
packets are multicast to this group; MSSs that do not
currently host the mobile buffer packets in anticipation
of a handoff; such buffered packets are tossed when the MSSs
receive an update of the mobile's ackowledged sequence numbers.
- Delayed Buffering: By keeping track of how long a mobile
has spent in previous cells, the SH can guess how long the
mobile is likely to remain in the current cell. The SH therefore
can let MSSs in the mobiles multicast group know when they
should start buffering in anticipation of the handoff. This
reduces the buffer space required at the MSSs, but increases
handoff delay when the SH's guess is wrong.
- Network Architecture: A connection-oriented network
architecture is also described in this paper. Virtual circuits
are set up between end-points. The communication is optimized
for the case of two mobiles in the same network region managed
by a one SH (the ACTIVE_LOCAL connection state).
Communication with a mobile under a foreign SH is in the
ACTIVE_REMOTE state. POINT means that a SH is
forwarding packets for a mobile that moved out of its network
region. ACTIVE_LOCAL destinations are easy to find
since the common SH is omniscient. ACTIVE_REMOTE
destinations are found by broadcasting LOCATE packets.
Finally, a NASCENT connection is one in the process of
Handoff latency and overhead is a serious issue in picocellular (and even
microcellular) wireless networks. The multicast group approach is
well-known and has been demonstrated to work.
- The network architecture described is centralized, connection
oriented, and has broadcast route-finding as a common case.
These are all bad properties (IMHO) for a wireless network -
the handoff work in the paper is much more interesting than the
network architecture work.
- The buffering approach was justified with some rather dubious
analytical calculations. The queuing model used for buffering
packets is known to be weak. The numbers chosen were totally
unbased in reality. The probability distributions used were
ad-hoc. No measurements of any real-life working implementation
were presented. (Possibly because there is not an implementation
- From the paper: "During our experiments we noted that, on
occasion, packets do not necessarily get delivered to an MH in
either the current cell or even the next one..." They
don't talk about fading or packet loss at all in this model, nor
any other known bad property of wireless links.
Back to index