XS Server Hardware
This is a description of the first implementation of the School server. It describes the hardware and software implementation details, including the management of services. The actual services provided by the School server are described in the Server Services document, with an accompanying discussion of desired services.
Hardware Specifications
The School server hardware platform has not been selected at this time.
The current plan is to provide two hardware platforms. One, XSX, available by mid-March, will allow for School server software testing and early deployments in areas where power mains are at least periodic if not continuous. The second platform, XS, would be reduced in cost and power, and should be available for deployment beginning in October.
XS Specifications
This is the hardware platform for actual deployment.
The school server must meet all of the environmental constraints of the XO laptop with the exception of daily rough handling!
Processor Architecture
This hardware platform may or may not be based on an x86 platform. Perhaps multiple hardware platforms will be supported.
Reasons to use x86
- Support required for only a single toolchain
- Able to use an XO as an emergency school server replacement ?
Reasons to use non-x86
- Lower power
- Lower cost
- Usually integrated with more onchip peripherals.
Network Interfaces
What network interfaces should be provided on the School server ?
802.11b/g
How many WiFi interfaces should be built into the server ? The current proposal is to include two distinct channels (MACs).
If we really have 100 students in close proximity, can we make do with only two channels ?
The number of WiFi interfaces will be expanded using external USB connected WiFi nodes. A reasonable number of these (two to four) can be added to a server to increase the WiFi performance.
100baseT
A wired ethernet interface provides for reliable, high-bandwidth connection between a school server and its internet connection (if through a DSL or satellite modem) or other school servers.
The current plan is to provide at least two 100baseT ports on the server (unless Powerline is substituted for one port).
Powerline
This technology has the potential to provide 100baseT performance over the power lines used to power the school servers. While known as power line communications, the high speed short distance version being referred to here is best represented by the HomePlug Powerline Alliance.
The advantage is that even if power lines aren't currently deployed in the school, the cable required for powerline networking is more likely to be available/cheaper than Cat-3 or Cat-5 cable.
A drawback is the lack of standardization and regulatory concerns in many nations. Correcting the regulatory situation isn't necessarily just a matter of time. Powerline networking emits strongly in the lower HF band, increasing the noise floor for existing communication systems (not WiFi, WiMax, or cellular).
Other Interfaces
What other interfaces should the School server have ?
USB 2.0
A number of Universal Serial Bus (USB) 2.0 interfaces will be provided for extending the storage and communication capabilities of a School server.
Perhaps four ports, on two separate buses ? This assumes that up to two external ports will regularly be used to add additional WiFi channels (or the WAN connection), and two others will be available for external disk drives.
If no internal WiFi modules are included in the School server, then the number of USB ports should be increased by two.
Non-Volatile Storage
Internal Disk Drive
An internal disk drive will be provided. This may be either PATA or SATA. The size of this disk drive WILL vary, but a minimum size of 300 GB seems reasonable given a target of 100 students per server.
If the humidity environmental constraints require excessive encapsulation of internal drives, perhaps a single modular and "stackable" approach is optimum.
Otherwise, including the "first" drive internally reduces cost (less plastic enclosure).
External Disk Drives
Additional disks may be added using external USB 2.0 ports.
Flash
Enough flash (solid state non-volatile) memory should be provided on the server to allow the operating system and minimal services to continue operation even though the primary disk drive has failed. Current estimates are that 256 MB of memory will be sufficient for this task.
Power
The power specifications of the School server are important. Many schools do not have adequate, or regular, power. While the power consumption should be minimized (8W is a good target), consideration should be given to an integral (or optional modular) uninterruptible power supply (UPS). This is nothing more than a larger version of the laptop power supply!
In some test schools with minimal power, we are already deploying multiple (gang) battery chargers with integral UPS.
Possible methods of obtaining power are summarized in Battery_and_power.
Environmental
The environmental constraints on the school server are similar but slightly less constrained than those of the laptop, at least in terms of water and dirt penetration and drop resistance.
Temperature
The school server should meet the same environmental specifications for temperature as the laptop. This is 50 C ambient.
Water and Dust
This is one area where the school server doesn't have to meet the standards of the laptop. The server should be resistant to water exposure from a single direction (above), but does not have to survive immersion. It should be capable of operation in a constantly humid (100%) environment.
- Can any hard drive operate at 100% humidity ? If the storage device needs to be encapsulated to meet humidity specs., it might as well be designed as separate, easy to replace modules (USB disk units.)
Dust intrusion should be considered. While the server will not contain a fan, it will be air cooled and dust collection on the cooling surfaces will be a problem. User cleanable filtration should be provided on the air intakes and vents.
Connectors and buttons should be resistant to dust intrusion. Buttons should be sealed against water, and connectors located and cables dressed to prevent water intrusion.
Mounting
While the school server should be designed to sit on a flat surface, it should probably also be mountable (hangable) from a wall or post.
This shouldn't cause a problem unless the server includes batteries for a optional/modular UPS...
Drop and Shake
The school server should meet the drop and shake specifications of standard consumer desktop PCs.
- Perhaps the shake specification should be higher to account for rougher transport during deployment ?
XSX Specifications
This is a version with a target date of mid-March. The primary goal is to support development of the School server/laptop software, including early trials. As this is a very limited production model (12-40 ?) and the design criteria are still undefined, flexibility wins out over cost. Off-the-shelf consumer PC hardware is being used for this platform. See the XSX Server Implementation for details.
Selection criteria are:
- 1GHz+ x86 processor
- 1 GB main memory
- four to six USB interfaces, with power for three Marvell Wifi nodes and an external disk drive.
- one 300GB+ 3.5in SATA drive
- power and space for a second disk drive
- two 100baseT network interfaces
- minimal fans
There are no power consumption targets for this device. A separate UPS is suggested for trial deployments in areas of uncertain power.
Software Specifications
The School server should run roughly the same Linux OS as that in the XO laptop. The release process will probably be different, but the kernel and most libraries should be identical.
A description of the software used for the school server in early trials is available.
Development Toolchain
What is the cost of this toolchain being different from that used for the XO laptop?
Upgrades
How is the School server upgraded ?
Service Management
How are services on the School server installed, configured, managed, and updated ?
A web based interface is a natural candidate for performing these actions on the School server.
Scalability
A School server should serve up to a hundred students. We could choose any number as this target, but have selected one hundred for a number of reasons:
- Higher student/server ratios push the processor requirements into the desktop PC range, with their higher prices/power consumption, and require multiple disks to provide the needed storage.
- Higher student/server ratios will probably require dedicated access points in addition to those provided locally by the server.
- Smaller student/server ratios spend too much money on the case and power supply for each unit.
- Ideally, the number of students supported by a single server results in at least two servers in almost all schools, providing a degree of redundancy.
In schools with multiple school servers (the normal situation), the servers are interconnected (preferrably using a wired network connection). They will work together to maximize the performance and reliability of the system.
The management interface for a school will be independent of the number of servers needed to serve the school.
The monitoring and management interface for school server software in a country/region should be independent of the number of server managed.