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Black Box Explains...Power over Ethernet (PoE).

What is PoE?
The seemingly universal network connection, twisted-pair Ethernet cable, has another role to play, providing electrical power to low-wattage electrical devices. Power over Ethernet (PoE) was ratified by the... more/see it nowInstitute of Electrical and Electronic Engineers (IEEE) in June 2000 as the 802.3af-2003 standard. It defines the specifications for low-level power delivery—roughly 13 watts at 48 VDC—over twisted-pair Ethernet cable to PoE-enabled devices such as IP telephones, wireless access points, Web cameras, and audio speakers.

Recently, the basic 802.3af standard was joined by the IEEE 802.3at PoE standard (also called PoE+ or PoE plus), ratified on September 11, 2009, which supplies up to 25 watts to larger, more power-hungry devices. 802.3at is backwards compatible with 802.3af.

How does PoE work?
The way it works is simple. Ethernet cable that meets CAT5 (or better) standards consists of four twisted pairs of cable, and PoE sends power over these pairs to PoE-enabled devices. In one method, two wire pairs are used to transmit data, and the remaining two pairs are used for power. In the other method, power and data are sent over the same pair.

When the same pair is used for both power and data, the power and data transmissions don’t interfere with each other. Because electricity and data function at opposite ends of the frequency spectrum, they can travel over the same cable. Electricity has a low frequency of 60 Hz or less, and data transmissions have frequencies that can range from 10 million to 100 million Hz.

Basic structure.
There are two types of devices involved in PoE configurations: Power Sourcing Equipment (PSE) and Powered Devices (PD).

PSEs, which include end-span and mid-span devices, provide power to PDs over the Ethernet cable. An end-span device is often a PoE-enabled network switch that’s designed to supply power directly to the cable from each port. The setup would look something like this:

End-span device → Ethernet with power

A mid-span device is inserted between a non-PoE device and the network, and it supplies power from that juncture. Here is a rough schematic of that setup:

Non-PoE switch → Ethernet without PoE → Mid-span device → Ethernet with power

Power injectors, a third type of PSE, supply power to a specific point on the network while the other network segments remain without power.

PDs are pieces of equipment like surveillance cameras, sensors, wireless access points, and any other devices that operate on PoE.

PoE applications and benefits.

  • Use one set of twisted-pair wires for both data and low-wattage appliances.
  • In addition to the applications noted above, PoE also works well for video surveillance, building management, retail video kiosks, smart signs, vending machines, and retail point-of-information systems.
  • Save money by eliminating the need to run electrical wiring.
  • Easily move an appliance with minimal disruption.
  • If your LAN is protected from power failure by a UPS, the PoE devices connected to your LAN are also protected from power failure.

  • Converters and Scalers Selector
    PoE Standards PoE
    IEEE 802.3 af
    PoE IEEE 802.3 at
    Power available at powered device 12.95 W 25.5
    Maximum power delivered 15.40 W 34.20
    Voltage range at powred source 44.0-57.0 V 50.0-57.0 V
    Voltage range at powred device 37.0-57.0 42.5-57.0 V
    Maximum current 350 mA 600 mA
    Maximum cable resistance 20 ohms 12.5 ohms
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    • Manual... 
    • PoE+ Gigabit Managed Switch Eco CLI Guide
      CLI Guide for the LPB2810A, LPB2826A, and LPB2848A (Version 1)
     

    Black Box Explains...Layer 3 switching.

    In the last decade, network topologies have typically featured routers along with hubs or switches. The hub or switch acts as a central wiring point for LAN segments while the... more/see it nowrouter takes care of higher-level functions such as protocol translation, traffic between LAN segments, and wide-area access.

    Layer 3 switching, which combines Layer 2 switching and Layer 3 IP routing, provides a more cost-effective way of setting up LANs by incorporating switching and routing into one device. While a traditional Layer 2 switch simply sends data along without examining it, a Layer 3 switch incorporates some features of a router in that it examines data packets before sending them on their way. The integration of switching and routing in a Layer 3 switch takes advantage of the speed of a switch and the intelligence of a router in one economical package.

    There are two basic types of Layer 3 switching: packet-by-packet Layer 3 (PPL3) and cut-through Layer 3.

    PPL3 switches are technically routers in that they examine all packets before forwarding them to their destinations. They achieve top speed by running protocols such as OSPF (Open Shortest Path First) and by using cache routing tables. Because these switches understand and take advantage of network topology, they can blow the doors off traditional routers with speeds of more than 7,000,000 (that’s seven million!) packets per second.

    Cut-through Layer 3 switching relies on a shortcut for top speed. Cut-through Layer 3 switches, rather than examining every packet, examine only the first in a series to determine its destination. Once the destination is known, the data flow is switched at Layer 2 to achieve high speeds. collapse

    • Manual... 
    • Gigabit Managed Switch User Manual
      User Manual for the LGB1108A, LGB1126A, and LGB1148A (Version 1)
     

    Black Box Explains...Layer 2, 3, and 4 switches.



    ...more/see it now
    OSI Layer Physical
    Component
    7-Application Applicaton Software

    LAN-Compatible Software
    E-Mail, Diagnostics, Word Processing, Database


    Network Applications
    6-Presentation Data-
    Conversion Utilities
    Vendor-Specific Network Shells and Gateway™ Workstation Software
    5-Session Network Operating System SPX NetBIOS DECnet™ TCP/IP AppleTalk®
    4-Transport Novell® NetWare® IPX™ PC LAN LAN Mgr DECnet PC/TCP® VINES™ NFS TOPS® Apple
    Share®
    3-Network Control
    2-Data Link Network E A TR P TR E TR E E E P E P
    1-Physical E=Ethernet; TR=Token Ring; A=ARCNET®; P=PhoneNET®

    With the rapid development of computer networks over the last decade, high-end switching has become one of the most important functions on a network for moving data efficiently and quickly from one place to another.


    Here’s how a switch works: As data passes through the switch, it examines addressing information attached to each data packet. From this information, the switch determines the packet’s destination on the network. It then creates a virtual link to the destination and sends the packet there.


    The efficiency and speed of a switch depends on its algorithms, its switching fabric, and its processor. Its complexity is determined by the layer at which the switch operates in the OSI (Open Systems Interconnection) Reference Model (see above).


    OSI is a layered network design framework that establishes a standard so that devices from different vendors work together. Network addresses are based on this OSI Model and are hierarchical. The more details that are included, the more specific the address becomes and the easier it is to find.


    The Layer at which the switch operates is determined by how much addressing detail the switch reads as data passes through.


    Switches can also be considered low end or high end. A low-end switch operates in Layer 2 of the OSI Model and can also operate in a combination of Layers 2 and 3. High-end switches operate in Layer 3, Layer 4, or a combination of the two.


    Layer 2 Switches (The Data-Link Layer)

    Layer 2 switches operate using physical network addresses. Physical addresses, also known as link-layer, hardware, or MAC-layer addresses, identify individual devices. Most hardware devices are permanently assigned this number during the manufacturing process.


    Switches operating at Layer 2 are very fast because they’re just sorting physical addresses, but they usually aren’t very smart—that is, they don’t look at the data packet very closely to learn anything more about where it’s headed.


    Layer 3 Switches (The Network Layer)

    Layer 3 switches use network or IP addresses that identify locations on the network. They read network addresses more closely than Layer 2 switches—they identify network locations as well as the physical device. A location can be a LAN workstation, a location in a computer’s memory, or even a different packet of data traveling through a network.


    Switches operating at Layer 3 are smarter than Layer 2 devices and incorporate routing functions to actively calculate the best way to send a packet to its destination. But although they’re smarter, they may not be as fast if their algorithms, fabric, and processor don’t support high speeds.


    Layer 4 Switches (The Transport Layer)

    Layer 4 of the OSI Model coordinates communications between systems. Layer 4 switches are capable of identifying which application protocols (HTTP, SNTP, FTP, and so forth) are included with each packet, and they use this information to hand off the packet to the appropriate higher-layer software. Layer 4 switches make packet-forwarding decisions based not only on the MAC address and IP address, but also on the application to which a packet belongs.


    Because Layer 4 devices enable you to establish priorities for network traffic based on application, you can assign a high priority to packets belonging to vital in-house applications such as Peoplesoft, with different forwarding rules for low-priority packets such as generic HTTP-based Internet traffic.


    Layer 4 switches also provide an effective wire-speed security shield for your network because any company- or industry-specific protocols can be confined to only authorized switched ports or users. This security feature is often reinforced with traffic filtering and forwarding features.

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    • Manual... 
    • Gigabit Managed Switch CLI Guide
      CLI Guide for the LGB1126A and LGB1148A (Version 1)
     
    • Firmware... 
    • 52-Port Gigabit Ethernet Managed Switch
      Firmware for the LGB5052A (Version v2.00)
     
    • Firmware... 
    • 48-Port PoE+ Gigabit Managed Switch Eco
      Firmware for the LPB2848A (Version v1.58)
     

    Product Data Sheets (pdf)...Gigabit L3 Managed Switches with 10G Uplinks

    • Manual... 
    • LPB5000 Series Gigabit PoE+ Ethernet Managed Switch Eco User Manual
      Installation & Getting Started Guide for the LPB5028A and LPB5052A (Version 2)
     
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