Hub – A physical layer network device used to connect multiple Ethernet devices together. Active hubs act as a repeater and boost the signal in order to allow for it to travel farther, while passive hubs simply pass the signal through. Most hubs have an uplink port that allows them to connect to other hubs, a router, or other network devices.

Repeater – A physical layer device that boosts signals in order to allow a signal to travel farther and prevent attenuation. Attentuation is the degradation of a signal as it travels farther from its origination. Repeaters do not filter packets and will forward broadcasts. Both segments must use the same access method, which means that you can’t connect a token ring segment to an Ethernet segment. Repeaters can connect different cable types as shown in the image.

Modem – The modem is a device that converts digital information to analog by MODulating it on the sending end and DEModulating the analog information into digital information at the receiving end. Most modern modems are internal, however, they can be internal or external. External modems are connected to the back of the system board via a RS-232 serial connection. Internal modems are installed in one of the motherboard’s PCI or ISA expansion slots depending on the modem. The modem contains an RJ-11 connection that is used to plug in the telephone line. Modems have different transmission modes as follows:

• Simplex – Signals can be passed in one direction only.
• Half Duplex – Half duplex means that signals can be passed in either direction, but not in both simultaneously. Half-duplex modems can work in full-duplex mode.
• Full Duplex – Full duplex means that signals can be passed in either direction simultaneously.

Modems can also be classified by their speed which is measured by the BAUD rate. One baud is one electronic state change per second. Since a single state change can involve more than a single bit of data, the Bits Per Second(BPS) unit of measurement has replaced it as a better expression of data transmission speed. Common modem speeds are V.34 at 28.8 kbps, V.34+ at 33.6 kbps and V.90 at 56 Kbps.

Network Interface Card – A Network Interface Card, often abbreviated as NIC, is an expansion board you insert into a computer so the computer can be connected to a network. Most NICs are designed for a particular type of network, protocol and media, although some can serve multiple networks.

Media Converters – simple networking devices that make it possible to connect two dissimilar media types such as twisted pair with fiber optic cabling. They were introduced to the industry nearly two decades ago, and are important in interconnecting fiber optic cabling-based systems with existing copper-based, structured cabling systems. They are also used in MAN access and data transport services to enterprise customers. Fiber media converters support many different data communication protocols including Ethernet, Fast Ethernet, Gigabit Ethernet, T1/E1/J1, DS3/E3, as well as multiple cabling types such as coax, twisted pair, multi-mode and single-mode fiber optics. Media converter types range from small standalone devices and PC card converters to high port-density chassis systems that offer many advanced features for network management.

Switch – A switch is a network device that filters and forwards packets between LAN segments and ensures that data goes straight from its origin to its proper destination. Switches remember the address of every node on the network, and anticipate where data needs to go. A switch only operates with the computers on the same LAN. This reduces competition for bandwidth between devices on the network. It isn’t smart enough to send data out to the internet, or across a WAN. These functions require a router.

Bridge – Functions the same as a repeater, but can also divide a network in order to reduce traffic problems. A bridge can also connect unlike network segments (ie. token ring and ethernet). Bridges create routing tables based on the source address. If the bridge can’t find the source address it will forward the packets to all segments. Bridging methods:

• Transparent – Only one bridge is used.
• Source-Route – Bridging address tables are stored on each PC on the network
• Spanning Tree – Prevents looping where there exists more than one path between segments

Wireless Access Point – A Wireless Access Point is a radio frequency transceiver which allows your wireless devices to connect to a network. The WAP usually connects to a wired network, and can relay data between the wireless devices (such as computers or printers) and wired devices on the network. A wireless access point will support up to 32 wireless devices. The range of the wireless signal depends greatly on obstructions such as walls. For more information about wireless standards, see domain 1.7.

Router – Functioning at the network later of the OSI model, a router is similar to a switch, but it can also connect different logical networks or subnets and enable traffic that is destined for the networks on the other side of the router to pass through. Routers create or maintain a table of the available routes and can be configured to use various routing protocols to determine the best route for a given data packet. Routers can connect networks that use disimilar protocols. Routers also typically provide improved security functions over a switch.

Firewall – Either a hardware or software entity (or a combination of both) that protects a network by stopping network traffic from passing through it. In most cases, a firewall is placed on the network to allow all internal traffic to leave the network (email to the outside world, web access, etc.), but stop unwanted traffic from the outside world from entering the internal network. This is achieved by granting and denying access to resources based on a set of configurable rules.

DHCP Server – A server that is responsible for assiging unique IP address to the computers on a network. A DHCP server prevents the assignment of duplicate IP addresses to clients and reduces administrative effort in network configuration. A DHCP server is actually more of a service that is found on network operating systems such as Windows 2002/2008 server, or on network devices such as routers.

**Source by wikipedia**

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