RFC 2757 (rfc2757) - Page 3 of 46

Long Thin Networks

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