spending $23 billion to wire 18 million homes with FiOS (Fiber-optic Service) by 2010. Comcast also recently an-
6, 7
nounced it plans to offer speeds of up to 100Mbps within a year. 3
Demand drives this last-mile boom: Pew Internet’s 2008 report shows that one-third of U.S. broadband users have chosen to pay more for faster connections. 4 Akamai Technologies’ data, shown in Figure 1, reveals that 59% of its global users have broadband connections (with speeds greater than 2 Mbps), and 19% of global users have “high broadband” connections greater than 5Mbps—fast enough to support DVD-quality content. 2 The high-
figure 1: Broadband penetration by country.
Broadband
Ranking Country
— Global
1 South Korea
22 Belgium
33 Japan
44 Hong Kong
5 Switzerland
6 Slovakia
7 Norway
8 Denmark
9 Netherlands
10 Sweden
…
20.
united States
fast Broadband
Ranking Country
— Global
1 South Korea
22 Japan
33 Hong Kong
44 Sweden
5 Belgium
6 united States
7 Romania
8 Netherlands
9 Canada
10 Denmark
Source: akamai’s State of the Internet Report, 02 2008
broadband numbers represent a 19% increase in just three months.
Along with the greater demand and availability of broadband comes a rise in user expectations for faster sites, richer media, and highly interactive applications. The increased traffic loads and performance requirements in turn put greater pressure on the Internet’s internal infrastructure—the middle mile. In fact, the fast-rising popularity of video has sparked debate about whether the Internet can scale to meet the demand.
Consider, for example, delivering a TV-quality stream (2Mbps) to an au-
> 2mbps
59%
90%
90%
87%
87%
85%
83%
82%
79%
77%
75%
71%
> 5mbps
19%
64%
52%
37%
32%
26%
26%
22%
22%
18%
18%
dience of 50 million viewers, approximately the audience size of a popular TV show. The scenario produces aggregate bandwidth requirements of 100Tbps. This is a reasonable vision for the near term—the next two to five years—but it is orders of magnitude larger than the biggest online events today, leading to skepticism about the Internet’s ability to handle such demand. Moreover, these numbers are just for a single TV-quality show. If hundreds of millions of end users were to download Blu-ray-quality movies regularly over the Internet, the resulting traffic load would go up by an additional one or two orders of magnitude.
Another interesting side effect of the growth in video and rich media file sizes is that the distance between server and end user becomes critical to end-user performance. This is the result of a somewhat counterintuitive phenomenon that we call the Fat File Paradox: given that data packets can traverse net works at close to the speed of light, why does it takes so long for a “fat file” to cross the country, even if the network is not congested?
It turns out that because of the way the underlying network protocols work, latency and throughput are directly coupled. TCP, for example, allows only small amounts of data to be sent at a time (that is, the TCP window) before having to pause and wait for acknowledgments from the receiving end. This means that throughput is effectively throttled by network round-trip time (latency), which can become the bottleneck for file download speeds and video viewing quality.
Packet loss further complicates the problem, since these protocols back off and send even less data before waiting for acknowledgment if packet loss is detected. Longer distances increase the chance of congestion and packet loss to the further detriment of throughput.
Figure 2 illustrates the effect of distance (between server and end user) on throughput and download times. Five or 10 years ago, dial-up modem speeds would have been the bottleneck on these files, but as we look at the Internet today and into the future, middle-mile distance becomes the bottleneck.
46 CommunICatIons of the aCm | feBRuaRY2009 | vol. 52 | No. 2
References:
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