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A local area network (LAN) is a group of computers that are connected together in a localized area to communicate with one another and share resources such as printers. Data is sent in the form of packets and to regulate the transmission of the packets, different technologies can be used. The most widely used LAN technology is the Ethernet and it is specified in a standard called IEEE 802.3. (Other types of LAN networking technologies include token ring and FDDI.)
Ethernet uses a star topology in which the individual nodes (devices) are networked with one another via active networking equipment such as switches. The number of networked devices in a LAN can range from two to several thousand.
The physical transmission medium for a wired LAN involves cables, mainly twisted pair or fiber optics. A twisted pair cable consists of eight wires, forming four pairs of twisted copper wires and is used with RJ-45 plugs and sockets. The maximum cable length of a twisted pair is 100 m (328 ft.) while for fiber, the maximum length ranges from 10 km to 70 km, depending on the type of fiber. Depending on the type of twisted pair or fiber optic cables used, data rates today can range from 100 Mbit/s to 10,000 Mbit/s.
A rule of thumb is to always build a network with greater capacity than is currently required. To future-proof a network, it is a good idea to design a network such that only 30% of its capacity is used. Since more and more applications are running over networks today, higher and higher network performance is required. While network switches (discussed below) are easy to upgrade after a few years, cabling is normally much more difficult to replace.
Types of Ethernet networks
Fast Ethernet refers to an Ethernet network that can transfer data at a rate of 100 Mbit/s. It can be based on a twisted pair or fiber optic cable. (The older 10 Mbit/s Ethernet is still installed and used, but such networks do not provide the necessary bandwidth for some network video applications.)
Most devices that are connected to a network, such as a laptop or a network camera, are equipped with a 100BASE-TX/10BASE-T Ethernet interface, most commonly called a 10/100 interface, which supports both 10 Mbit/s and Fast Ethernet. The type of twisted pair cable that supports Fast Ethernet is called a Cat-5 cable.
Gigabit Ethernet, which can also be based on a twisted pair or fiber optic cable, delivers a data rate of 1,000 Mbit/s (1 Gbit/s) and is becoming very popular. It is expected to soon replace Fast Ethernet as the de facto standard.
The type of twisted pair cable that supports Gigabit Ethernet is a Cat-5e cable, where all four pairs of twisted wires in the cable are used to achieve the high data rates. Cat-5e or higher cable categories are recommended for network video systems. Most interfaces are backwards compatible with 10 and 100 Mbit/s Ethernet and are commonly called 10/100/1000 interfaces.
For transmission over longer distances, fiber cables such as 1000BASE-SX (up to 550 m/1,639 ft.) and 1000BASE-LX (up to 550 m with multimode optical fibers and 5,000 m with single-mode fibers) can be used.
10 Gigabit Ethernet
10 Gigabit Ethernet is the latest generation and delivers a data rate of 10 Gbit/s (10,000 Mbit/s), and a fiber optic or twisted pair cable can be used. 10GBASE-LX4, 10GBASE-ER and 10GBASE-SR based on an optical fiber cable can be used to bridge distances of up to 10,000 m (6.2 miles). With a twisted pair solution, a very high quality cable (Cat-6a or Cat-7) is required. 10 Gbit/s Ethernet is mainly used for backbones in high-end applications that require high data rates.
When only two devices need to communicate directly with one another via a twisted pair cable, a so-called crossover cable can be used. The crossover cable simply crosses the transmission pair on one end of the cable with the receiving pair on the other end and vice versa.
To network multiple devices in a LAN, however, network equipment such as a network switch is required. When using a network switch, a regular network cable is used instead of a crossover cable.
The main function of a network switch is to forward data from one device to another on the same network. It does it in an efficient manner since data can be directed from one device to another without affecting other devices on the same network.
How it works is that a switch registers the MAC (Media Access Control) addresses of all devices that are connected to it. (Each networking device has a unique MAC address, which is made up of a series of numbers and letters that is set by the manufacturer and the address is often found on the product label.) When a switch receives data, it forwards it only to the port that is connected to a device with the appropriate destination MAC address.
Switches typically indicate their performance in per port rates and in backplane or internal rates (both in bit rates and in packets per second). The port rates indicate the maximum rates on specific ports. This means that the speed of a switch, for example 100 Mbit/s, is often the performance of each port.
A network switch normally supports different data rates simultaneously. The most common rates used to be 10/100, supporting 10 Mbit/s as well as Fast Ethernet. However, 10/100/1000 are quickly taking over as the standard switch, thus supporting 10 Mbit/s, Fast Ethernet and Gigabit Ethernet simultaneously. The transfer rate and mode between a port on a switch and a connected device are normally determined through auto-negotiation, whereby the highest common data rate and best transfer mode are used. A switch also allows a connected device to function in full-duplex mode, i.e. send and receive data at the same time, resulting in increased performance.
Switches may come with different features or functions. Some switches include the function of a router. A switch may also support Power over Ethernet or Quality of Service, which controls how much bandwidth is used by different applications.
Power over Ethernet
Power over Ethernet (PoE) provides the option of supplying devices connected to an Ethernet network with power using the same cable as for data communication. Power over Ethernet is widely used to power IP phones, wireless access points and network cameras in a LAN.
The main benefit of PoE is the inherent cost savings. Hiring a certified electrician and installing a separate power line are not needed. This is advantageous, particularly in difficult-to-reach areas. The fact that no power cable has to be installed can save, depending on the camera location, up to a few hundred dollars per camera. Having PoE also makes it easier to move a camera to a new location, or add cameras to a video surveillance system.
Additionally, PoE can make a video system more secure. A video surveillance system with PoE can be powered from the server room, which is often backed up with a UPS (Uninterruptible Power Supply). This means that the video surveillance system can be operational even during a power outage.
Due to the benefits of PoE, it is recommended for use with as many devices as possible. The power available from the PoE-enabled switch or midspan should be sufficient for the connected devices and the devices should support power classification. These are explained in more detail in the sections below.
802.3af standard and High PoE
Most PoE devices today conform to the IEEE 802.3af standard, which was published in 2003. The IEEE 802.3af standard uses standard Cat-5 or higher cables, and ensures that data transfer is not affected. In the standard, the device that supplies the power is referred to as the power sourcing equipment (PSE). This can be a PoE-enabled switch or midspan. The device that receives the power is referred to as a powered device (PD). The functionality is normally built into a network device like a network camera, or provided in a standalone splitter (see section below).
Backward compatibility to non PoE-compatible network devices is guaranteed. The standard includes a method for automatically identifying if a device supports PoE, and only when that is confirmed will power be supplied to the device. This also means that the Ethernet cable that is connected to a PoE switch will not supply any power if it is not connected to a PoE-enabled device. This eliminates the risk of getting an electrical shock when installing or rewiring a network.
In a twisted pair cable, there are four pairs of twisted wires. PoE can use either the two ‘spare’ wire pairs, or overlay the current on the wire pairs used for data transmission. Switches with built-in PoE often supply electricity through the two pairs of wires used for transferring data, while midspans normally use the two spare pairs. A PD supports both options.
According to IEEE 802.3af, a PSE provides a voltage of 48 V DC with a maximum power of 15.4 W per port. Considering that power loss takes place on a twisted pair cable, only 12.95 W is guaranteed for a PD. The IEEE 802.3af standard specifies various performance categories for PDs.
PSE such as switches and midspans normally supply a certain amount of power, typically 300 W to 500 W. On a 48-port switch, that would mean 6 W to 10 W per port if all ports are connected to devices that use PoE. Unless the PDs support power classification, the full 15.4 W must be reserved for each port that uses PoE, which means a switch with 300 W can only supply power on 20 of the 48 ports. However, if all devices let the switch know that they are Class 1 devices, the 300 W will be enough to supply power to all 48 ports.
Most fixed network cameras can receive power via PoE using the IEEE 802.3af standard and are normally identified as Class 1 or 2 devices.
With IEEE 802.3at pre-standard or PoE+, the power limit will be raised to at least 30 W via two pairs of wires from a PSE. The final specifications are still to be determined and the standard is expected to be ratified in mid-2009.
In the meantime, IEEE 802.3at pre-standard (High PoE) midspans and splitters can be used for devices such as PTZ cameras and PTZ dome cameras with motor control, as well as cameras with heaters and fans, which require more power than can be delivered by the IEEE 802.3af standard.
Midspans and splitters
Midspans and splitters (also known as active splitters) are equipment that enable an existing network to support Power over Ethernet.
The midspan, which adds power to an Ethernet cable, is placed between the network switch and the powered devices. To ensure that data transfer is not affected, it is important to keep in mind that the maximum distance between the source of the data (e.g., switch) and the network video products is not more than 100 m (328 ft.). This means that the midspan and active splitter(s) must be placed within the distance of 100 m.
A splitter is used to split the power and data in an Ethernet cable into two separate cables, which can then be connected to a device that has no built-in support for PoE. Since PoE or High PoE only supplies 48 V DC, another function of the splitter is to step down the voltage to the appropriate level for the device; for example, 12 V or 5 V.
PoE and High PoE midspans and splitters are available from Axis. See Power over Ethernet products.