Databases Reference
In-Depth Information
Another issue affecting network performance is compatibility among dif-
ferent vendors' networks. If network protocols are different, gateways must
be used to translate messages across networks. Gateways handle any con-
versions necessary to go from one set of protocols to another, including
message format conversion, address translation, and protocol conversion.
Protocol conversion increases processing overhead, further reducing
throughput and performance.
In a distributed environment, high availability requires that all necessary
components be operational when they are needed. Availability is measured
in Mean Time Between Failures and mean time between repair. The level of
availability is guided by the level of replication and the number of nonlocal
queries made. If data is manipulated on a local level and updates are done
in batches, availability may not be a critical requirement. Data availability
becomes an issue when data is not replicated between sites, and there are
frequent queries or updates of nonlocal data. Maximum availability requires
that data be fully replicated at all sites. In such a case, if a network node
failed, processing could continue until the failed node is recovered.
SECURITY CONSIDERATIONS
In a distributed database environment, data becomes more vulnerable
because it exists in more than one location and is transferred by a commu-
nications network. Data traveling over communications circuits is espe-
cially vulnerable to tampering. Areas in which data may be tampered with
include the media itself and the hardware involved in transmitting the data.
At a local level, twisted-pair and coaxial cable are very easy to tap.
Fiber, on the other hand, is very difficult to tap. When communicating over
longer distances, microwave and satellite communications are very inse-
cure. Because they employ broadcast media, anyone with the proper
receiver can intercept the signals. Encryption can be used to ensure that
if the data is intercepted, it will not be in a readable form. Although such
precautions virtually guarantee security, there is a performance trade-off
in the overhead required to first encrypt, then decrypt the data. In highly
fragmented or fully replicated databases, the additional overhead may
affect system performance.
The network manager must also take actions to protect the data commu-
nications equipment used to transfer the data. This includes actions to
ensure the physical security of communications equipment (e.g., modems,
encryption devices, earth stations for satellites or microwave, and any
other key equipment).
NETWORK DESIGN CONSIDERATIONS
Distance is the major determinant of the network configuration chosen.
Several configurations are available. If the distance between sites is short,
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