Wireless data: Systems, standards, services

Antonio DeSimone and Sanjiv Nanda

One-line summary: 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.

Overview/Main Points

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.

Relevance

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.

Flaws

Low-level network layers are explored, but higher-level issues for wireless data systems such as application level issues are glossed over.
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