Overview of Distributed File Systems (DFS)

A distributed file system or network file system is any file system that allows access to files from multiple hosts sharing via a computer network. This makes it possible for multiple users on multiple machines to share files and storage resources.
In order to understand the structure of a distributed file system, the terms service, server and client should be defined. A service is a software entity running on one or more machines and providing a particular type of function. A server is the service software running on a single machine. A client is a process that can invoke a service using a set of operations that forms its client interface.
A distributed file system (DFS) is a file system whose clients, servers, and storage devices are dispersed among the machines of a distributed system. A service activity has to be carried out across the network, and instead of a single centralized data repository, there are multiple and independent storage devices. the distinctive features of a DFS are the multiplicity and autonomy of clients and servers in the system.
A DFS should look to its clients like a conventional, centralized file system. The client interface of a DFS should not distinguish between local and remote files. The most important performance measurement of a DFS is the amount of time needed to satisfy various service requests. In a DFS, a remote access has the additional overhead attributed to the distributed structure. This overhead includes the time needed to deliver the request to the server, as well as the time for getting the response across the network back to the client. DFS manages a set of dispersed storage devices which is the DFS's key distinguishing feature.

Quick Tech Tip: CDMA - Code Division Multiple Access

CDMA (Code-Division Multiple Access) refers to any of several protocols used in so-called second-generation (2G) and third-generation (3G) wireless communications. As the term implies, CDMA is a form of multiplexing, which allows numerous signals to occupy a single transmission channel, optimizing the use of available bandwidth. CDMA, a proprietary standard designed by Qualcomm in the United States, has been the dominant network standard for North America and parts of Asia.
Coverage: The most important factor is getting service in the areas you will be using your phone. Upon viewing competitors' coverage maps you may discover that CDMA carriers offer cellular service in your area. If so, there is no decision to be made, but most people will find that they do have a choice.
For radio systems there are two resources, frequency and time. Division by frequency, so that each pair of communicators is allocated part of the spectrum for all of the time, results in Frequency Division Multiple Access (FDMA). Division by time, so that each pair of communicators is allocated all (or at least a large part) of the spectrum for part of the time results in Time Division Multiple Access (TDMA). In Code Division Multiple Access (CDMA), every communicator will be allocated the entire spectrum all of the time. CDMA uses codes to identify connections.


CDMA is a form of spread-spectrum, a family of digital communication techniques that
have been used in military applications for many years. The core principle of spread
spectrum is the use of noise-like carrier waves, and, as the name implies, bandwidths
much wider than that required for simple point-to-point communication at the same data rate. CDMA is a Direct Sequence Spread Spectrum system. The CDMA system works directly on 64 kbit/sec digital signals. These signals can be digitized voice, ISDN channels, modem data, etc.
CDMA is altering the face of cellular and PCS communication by:
· Dramatically improving the telephone traffic capacity.
· Dramatically improving the voice quality and eliminating the audible effects of
multipath fading.
· Reducing the incidence of dropped calls due to handoff failures.
· Providing reliable transport mechanism for data communications, such as
facsimile and internet traffic.
· Reducing the number of sites needed to support any given amount of traffic.
· Simplifying site selection.
· Reducing deployment and operating costs because fewer cell sites are needed.
· Reducing average transmitted power.
· Reducing interference to other electronic devices.
· Reducing potential health risks.