Sunday, July 20, 2008

Universal Serial Bus

USBUniversal Serial Bus
A USB Series “A” plug, the most common USB plug

Universal Serial Bus (USB) is a serial bus standard to interface devices. USB was designed to allow many peripherals to be connected using a single standardized interface socket and to improve the plug-and-play capabilities by allowing devices to be connected and disconnected without rebooting the computer (hot swapping). Other convenient features include providing power to low-consumption devices without the need for an external power supply and allowing many devices to be used without requiring manufacturer specific, individual device drivers to be installed.

USB is intended to help retire all legacy varieties of serial and parallel ports. USB can connect computer peripherals such as computer mouse, keyboards, PDAs, gamepads and joysticks, scanners, digital cameras, printers, personal media players, and flash drives. For many of those devices USB has become the standard connection method.
USB was originally designed for personal computers, but it has become commonplace on other devices such as PDAs and video game consoles. As of 2008, there are about 2 billion USB devices in the world.[1]

The design of USB is standardized by the USB Implementers Forum (USB-IF), an industry standards body incorporating leading companies from the computer and electronics industries. Notable members have included Agere (now merged with LSI Corporation), Apple Inc., Hewlett-Packard, Intel, NEC, and Microsoft.

Command line interface

The cmd.exe command line interface in Windows Vista
Command Line Interface (CLI) is a mechanism for interacting with a computer operating system or software by typing commands to perform specific tasks. This contrasts with the use of a mouse pointer with a graphical user interface (GUI) to click on options, or menus on a Text user interface (TUI) to select options.

This method of instructing a computer to perform a given task is referred to as "entering" a command: the system waits for the user to conclude the submitting of the text command by pressing the "Enter" key (a descendant of the "carriage return" key of a typewriter keyboard). A command line interpreter then receives, analyses, and launches the requested command. The command line interpreter may be a text terminal or a remote shell client such as PuTTY. Upon completion, the command usually returns output to the user in the form of text lines on the CLI. This output may be an answer if the command was a question, or otherwise a summary of the operation.

The concept of the CLI originated when teletype machines (TTY) were connected to computers in the 1950s, and offered results on demand, compared to 'batch' oriented mechanical punch card input technology. Dedicated text-based CRT terminals followed, with faster interaction and more information visible at one time, then graphical terminals enriched the visual display of information. Currently personal computers encapsulate both functions in software.
The CLI continues to coevolve with GUIs like those provided by Microsoft Windows, Mac OS and the X Window System. In some applications, such as MATLAB, a CLI is integrated with the GUI, with the benefits of both.

1 Usage
2 Anatomy of a Shell CLI
3 Programming languages in Interactive mode
3.1 CLI and Resource Protection
3.2 Command prompt
4 References
5 See also
6 External links


Diff btw Hub and switch

What is the difference between an Ethernet hub and switch?
Although hubs and switches both glue the PCs in a network together, a switch is more expensive and a network built with switches is generally considered faster than one built with hubs. Why?
When a hub receives a packet (chunk) of data (a frame in Ethernet lingo) at one of its ports from a PC on the network, it transmits (repeats) the packet to all of its ports and, thus, to all of the other PCs on the network. If two or more PCs on the network try to send packets at the same time a collision is said to occur. When that happens all of the PCs have to go though a routine to resolve the conflict. The process is prescribed in the Ethernet Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol. Each Ethernet Adapter has both a receiver and a transmitter. If the adapters didn't have to listen with their receivers for collisions they would be able to send data at the same time they are receiving it (full duplex). Because they have to operate at half duplex (data flows one way at a time) and a hub retransmits data from one PC to all of the PCs, the maximum bandwidth is 100 Mhz and that bandwidth is shared by all of the PC's connected to the hub. The result is when a person using a computer on a hub downloads a large file or group of files from another computer the network becomes congested. In a 10 Mhz 10Base-T network the affect is to slow the network to nearly a crawl. The affect on a small, 100 Mbps (million bits per scond), 5-port network is not as significant.
Two computers can be connected directly together in an Ethernet with a crossover cable. A crossover cable doesn't have a collision problem. It hardwires the Ethernet transmitter on one computer to the receiver on the other. Most 100BASE-TX Ethernet Adapters can detect when listening for collisions is not required with a process known as auto-negotiation and will operate in a full duplex mode when it is permitted. The result is a crossover cable doesn't have delays caused by collisions, data can be sent in both directions simultaneously, the maximum available bandwidth is 200 Mbps, 100 Mbps each way, and there are no other PC's with which the bandwidth must be shared.
An Ethernet switch automatically divides the network into multiple segments, acts as a high-speed, selective bridge between the segments, and supports simultaneous connections of multiple pairs of computers which don't compete with other pairs of computers for network bandwidth. It accomplishes this by maintaining a table of each destination address and its port. When the switch receives a packet, it reads the destination address from the header information in the packet, establishes a temporary connection between the source and destination ports, sends the packet on its way, and then terminates the connection.
Picture a switch as making multiple temporary crossover cable connections between pairs of computers (the cables are actually straight-thru cables; the crossover function is done inside the switch). High-speed electronics in the switch automatically connect the end of one cable (source port) from a sending computer to the end of another cable (destination port) going to the receiving computer on a per packet basis. Multiple connections like this can occur simultaneously. It's as simple as that. And like a crossover cable between two PCs, PC's on an Ethernet switch do not share the transmission media, do not experience collisions or have to listen for them, can operate in a full-duplex mode, have bandwidth as high as 200 Mbps, 100 Mbps each way, and do not share this bandwidth with other PCs on the switch. In short, a switch is "more better."



Network switch

Typical SOHO network switch.

Back view of Atlantis network switch with Ethernet ports.
A network switch is a computer networking device that connects network segments. In the past, it was faster to use Layer 2 techniques to switch, when only MAC addresses could be looked up in content addressable memory (CAM). With the advent of ternary CAM (TCAM), it was equally fast to look up an IP address or a MAC address. TCAM is expensive, but very appropriate for enterprise switches that use default routes plus a moderate number of other routes. For routers that need a full Internet routing table, TCAM may not be cost-effective.
The first Ethernet switch was introduced by Kalpana in 1989. [1]
1 Function
2 Role of switches in networks
3 Layer-specific functionality
3.1 Layer-1 hubs versus higher-layer switches
3.2 Layer 2
3.3 Layer 3
3.4 Layer 4
3.5 Layer 7
4 Types of switches
4.1 Form factor
4.2 Configuration options
4.2.1 Traffic monitoring on a switched network
4.2.2 Typical switch management features
5 See also
6 References
7 External links

From Wikipedia, the free encyclopedia
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A standard Ethernet cord.
Ethernet is a family of frame-based computer networking technologies for local area networks (LANs). The name comes from the physical concept of the ether. It defines a number of wiring and signaling standards for the physical layer, through means of network access at the Media Access Control (MAC)/Data Link Layer, and a common addressing format.
Ethernet is standardized as IEEE 802.3. The combination of the twisted pair versions of Ethernet for connecting end systems to the network, along with the fiber optic versions for site backbones, is the most widespread wired LAN technology. It has been in use from around 1980[1] to the present, largely replacing competing LAN standards such as token ring, FDDI, and ARCNET.
1 History
2 General description
3 Dealing with multiple clients
3.1 CSMA/CD shared medium Ethernet
3.1.1 Main procedure
3.1.2 Collision detected procedure
3.2 Ethernet repeaters and hubs
3.3 Bridging and switching
3.4 Dual speed hubs
3.5 More advanced networks
4 Autonegotiation and duplex mismatch
5 Physical layer
6 Ethernet frame types and the EtherType field
6.1 Runt frames
7 Varieties of Ethernet
7.1 Some early varieties
7.2 10Mbit/s Ethernet
7.3 Fast Ethernet
7.4 Gigabit Ethernet
7.5 10 gigabit Ethernet
8 Related standards
9 See also
10 References
11 External links

Sunday, February 10, 2008

Network Devices and Components Overview

Network components and devices are the physical entities connected to a network. There are many types of network devices and increasing daily. The basic network devices are: Computers either a PC or a Server, Hubs, Switches, Bridges, Routers, Gateways, Network interface cards (NICs), Wireless access points (WAPs), Printers and Modems. The following is a overview of the main network components and devices:
Individual Computers: The personal computer is typically a desktop computer, a workstation or a notebook for individual users. The individual computers are the most common type of microcomputer and is found in the majority of organizations.
Server: A computer on a network or other network device that stores all necessary information and is dedicated to provide a particular service. For example, a database server would store all data and software related to a certain database and allows other network devices to access and process database queries. A file server is a computer and storage device dedicated to storing files for any user on the network to store files on the server. A print server is a device that manages one or more printers, and a network server is a computer that manages network traffic.
Network Interface Card: Network Interface Cards (NIC) are adaptors attached with a computer or other network device to provide the connection between the computer with the network. Each NIC is design for a specific type of network such as Ethernet, Token Ring, FDDI or wireless LAN. The NIC operates using the physical layer (layer 1) and data link layer (layer 2) specifications. NIC basically defines the physical connection methods with the cable and the framing methods used to transmit bit streams over the network. It also defines the control signals that provide the timing of data transfers across network.
Hubs: Hubs are the simplest network devices. Computers connect to a hub via a length of twisted-pair cabling. On a hub, data is forwarded to all ports, regardless of whether the data is intended for the system connected to the port. In addition to ports for connecting computers, even a very inexpensive hub generally has a port designated as an uplink port that enables the hub to be connected to another hub to create larger networks.
Switches:Switch is a layer 2 and multi-port device. Switch provides similar functions as a hub or a bridge but has more advanced features that can temporarily connect any two ports together. It contains a switch matrix or switch fabric that can rapidly connect and disconnect ports. Unlike Hub, a switch only forward frame from one port to the other port where the destination node is connected without broadcast to all other ports.
Routers: Routers route data around the network from data senders to receivers. A router is able to determine the destination address for the data and determines the best way for the data to continue its journey. Unlike bridges and switches, which use the hardware-configured MAC address to determine the destination of the data, routers use the logic network address such as IP address to make decisions.
Gateway: The term gateway is applied to any device, system, or software application that can perform the function of translating data from one format to another. Gateway will not change the data itself. For example, a router that can route data from an IPX network to an IP network is, technically, a gateway. The same can be said of a translational switch that converts from an Ethernet network to a Token Ring network and back again.
Modems: Modems are access devices that translate digital signals from a computer into analog signals that can travel across conventional phone lines. The modem modulates the signal at the sending end and demodulates at the receiving end. Modems are required for many access methods such as 56k data modern, ISDN, DSL etc. They can be as internal devices that plug into expansion slots in a system; external devices that plug into serial or USB ports; PCMCIA cards designed for use in laptops; and specialized devices designed for use in systems such as handheld computers. In addition, many laptops now come with integrated modems. For large-scale modem implementations, such as at an ISP, rack-mounted modems are also available.
Network Devices and Components Overview
Related Terms:Server, NIC, Gateway, Router, Hub, Switch, Modem
‹ Managed Switches Support Port Mirroring, Port Spanning or Port Monitoring FunctionsupNetworking TCP/IP ›
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