Shared Network
Platform Performance
Most cable modem systems rely on
a shared access platform, much like an office LAN. Because cable modem
subscribers share available bandwidth during their sessions, there are
concerns that cable modem users will see poor performance as the number of
subscribers increases on the network. Common sense dictates that 200 cable
data subscribers sharing a 27-Mbps connection would each get only about 135
Kbps of throughput -- virtually the same speed as a 128-Kbps ISDN connection
-- right? Not necessarily.
Unlike circuit-switched
telephone networks where a caller is allocated a dedicated connection, cable
modem users do not occupy a fixed amount of bandwidth during their online
session. Instead, they share the network with other active users and use the
network's resources only when they actually send or receive data in quick
bursts. So instead of 200 cable online users each being allocated 135 Kbps,
they are able to grab all the bandwidth available during the millisecond
they need to download their data packets -- up to many megabits per second.
If congestion does begin to
occur due to high usage, cable operators have the flexibility to add more
bandwidth for data services. A cable operator can simply allocate an
additional 6 MHz video channel for high-speed data, doubling the downstream
bandwidth available to users. Another option for adding bandwidth is to
subdivide the physical cable network by running fibre-optic lines deeper
into neighbourhoods. This reduces the number of homes served by each network
segment, and thus, increases the amount of bandwidth available to end-users.
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