Game Development Reference
In-Depth Information
Network speeds
It may be disappointing that the Raspberry Pi foundation decided to use a 100
MBps LAN chip instead of a gigabit one. But, we need to crunch some numbers to
justify this decision. Let's convert megabits to more familiar megabytes. To get to
"megabytes per second" from "megabits per second", we divide 100 MBps by 8
(there are 8 bits in a byte). This equates to 12.5 megabytes per second at 100 percent
LAN capacity. For a single user, this is only roughly 20 percent of what the USB
hub can handle. That means that, by design, this is an unchangeable bandwidth
limitation for networking.
If you plan to share files with several users at the same time, each new user
will bump down the other user's bandwidth to accommodate their own. As a
work-around, you could add a gigabit USB LAN peripheral to increase the
bandwidth. But, due to the speed constraints of the USB hub, you will only use
approximately 48 percent of the gigabit LAN. To make matters worse, the hard
drives running on the USB port will start to fight for bandwidth. The USB Controller
has to share 480 MBps across all the four ports! One port is used by the 100 MBit
network card, and the other connects the hub to the GPU. For one user, this
means a maximum bandwidth of 240 MBps. Why 240 MBps? This is because
240 MBps goes to LAN and 240 MBps goes to the hard drive; and theoretically,
there is no USB bandwidth left for anything else.
This can be a problem for a multiuser environment, but for home use, you would not
run into any major problems as the bandwidth can accommodate HD video streams
while serving other clients. This is why the cheaper 100 MBit version was used.
USB bottlenecks
As it was made clear by the bottlenecks found with LAN, the worst thing about
USB bottlenecks is that there is no way to work around this problem! This is because
the USB Controller connects to the Broadcom chip and the LAN chip on the PCB,
without any possibility of expanding or replacing this chip.
Time
The Pi does not come with a real-time clock, so timekeeping is left to Internet-based
time servers. For most people, this might not cause a problem. But if you want to
create a remote, disconnected device that depends on events that are recorded at
various times of the day, you might be left a little bewildered.
