Back to index
Wireless data: Systems, standards, services
Antonio DeSimone and Sanjiv Nanda
MAC, link-layer, and network layer issues associated with wireless
data systems are presented through the discussion of existing
services, with the emphasis placed on the CDPD system.
- Wireless systems: A large number of wireless data systems
and standards currently exist - wireless LAN technology, while
relatively mature, has not been successful in the market. Wireless
data WAN services promise to have more demand. CDPD is the main focal
point of this paper.
- CDPD MAC: The 802.11 DFWMAC protocol uses CSMA/CA - the
protocol is known as the distributed coordination function (DCF). All
stations are quiet for some minimum time (the time may differ
depending on traffic priority). Then, each station randomly selects a
back-off time CW x n, 0<=n<1, for all frames they want to
transmit. CW is the contention window (# slots), initialized to CWmin
and exponentionally increased on retransmissions to CWmax.
- MAC performance: CSMA/CA parameters (CW, slot time) must be
tuned to expected load and physical link characteristics. Large slot
times imply high collision vulnerability but greater efficiency, while
low load implies collisions are unimportant. FDD from base-station
with busy/idle feedback further improves collision avoidance performance.
- CDPD channel availability: CDPD sniffs voice channels to find
data channels. If the voice channels saturate, high latencies for data
(many seconds) are possibile.
- Link layer retransmission: Providing link-layer reliability
may increase throughput, especially for high-error rate physical
links. Link-layer retransmissions increase end-to-end variability
in latency and error rate, which can negatively affect end-to-end transport
mechanisms like TCP slow start and congestion control.
- RLP: A radio-link protocol performs partial recovery on a
link; receivers transmit NACKs for lost frames, up to the level of
an abort timer. If the abort timer goes off, the link-layer recovery is
aborted and end-to-end mechanisms (TCP) are left to perform recovery.
- ARQ schemes: Receivers maintain state - a bitmap containing
received frames in the window, and next and largest so
far sequence numbers. This state is periodically sent to the
transmitter so "smart" retransmission can occur.
- Buffering at cellular base-station: When using cell-phones
to connect to data services, cell-phones will shut down after inactivity.
If data is then sent to the mobile, the ciruit must be reestablished;
the reestablishment time is high, causing many packets to be retransmitted.
Buffering at the BS may help alleviate this.
- Mobility management: Mobile IP and the CDPD equavalent
solve this problem.
This paper provides a good overview of some issues involved in deploying
wireless data systems. It focusses on link-layer issues such as MAC and
link-layer retranmission schemes, and their impact on higher layer
protocols. The moral of the paper is that protocols designed for
wireline networks are not directly applicable to wireless networks.
- Low-level network layers are explored, but higher-level issues
for wireless data systems such as application level issues are glossed
- CDPD is an interesting example of a wireless data service, but it
is hardly representative.
- A number of alternative link-layer or MAC schemes are presented,
but no real insight into relative advantages are given.
Back to index