RFC 2757 (rfc2757) - Page 3 of 46
Long Thin Networks
Alternative Format: Original Text Document
RFC 2757 Long Thin Networks January 2000
9 References ................................................... 36
Authors' Addresses ............................................. 44
Full Copyright Statement ....................................... 46
1 Introduction
Optimized wireless networking is one of the major hurdles that Mobile
Computing must solve if it is to enable ubiquitous access to
networking resources. However, current data networking protocols have
been optimized primarily for wired networks. Wireless environments
have very different characteristics in terms of latency, jitter, and
error rate as compared to wired networks. Accordingly, traditional
protocols are ill-suited to this medium.
Mobile Wireless networks can be grouped in W-LANs (for example,
802.11 compliant networks) and W-WANs (for example, CDPD [CDPD],
Ricochet, CDMA [CDMA], PHS, DoCoMo, GSM [GSM] to name a few). W-WANs
present the most serious challenge, given that the length of the
wireless link (expressed as the delay*bandwidth product) is typically
4 to 5 times as long as that of its W-LAN counterparts. For example,
for an 802.11 network, assuming the delay (round-trip time) is about
3 ms. and the bandwidth is 1.5 Mbps, the delay*bandwidth product is
4500 bits. For a W-WAN such as Ricochet, a typical round-trip time
may be around 500 ms. (the best is about 230 ms.), and the sustained
bandwidth is about 24 Kbps. This yields a delay*bandwidth product
roughly equal to 1.5 KB. In the near future, 3rd Generation wireless
services will offer 384Kbps and more. Assuming a 200 ms round-trip,
the delay*bandwidth product in this case is 76.8 Kbits (9.6 KB). This
value is larger than the default 8KB buffer space used by many TCP
implementations. This means that, whereas for W-LANs the default
buffer space is enough, future W-WANs will operate inefficiently
(that is, they will not be able to fill the pipe) unless they
override the default value. A 3rd Generation wireless service
offering 2 Mbps with 200-millisecond latency requires a 50 KB buffer.
Most importantly, latency across a link adversely affects
throughput. For example, [MSMO97] derives an upper bound on TCP
throughput. Indeed, the resultant expression is inversely related to
the round-trip time.
The long latencies also push the limits (and commonly transgress
them) for what is acceptable to users of interactive applications.
As a quick glance to our list of references will reveal, there is a
wealth of proposals that attempt to solve the wireless networking
problem. In this document, we survey the different solutions
available or under investigation, and issue the corresponding
recommendations.
Montenegro, et al. Informational