Cisco CCNA Study Guide - Router Alley

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CCNA Study Guide v2.62 – Aaron Balchunas 34 Power over Ethernet (PoE) Power over Ethernet (PoE) allows both data and power to be sent across the same twisted-pair cable, eliminating the need to provide separate power connections. This is especially useful in areas where installing separate power might be expensive or difficult. PoE can be used to power many devices, including: • Voice over IP (VoIP) phones • Security cameras • Wireless access points • Thin clients PoE was originally formalized as 802.3af, which can provide roughly 13W of power to a device. 802.3at further enhanced PoE, supporting 25W or more power to a device. Ethernet, Fast Ethernet, and Gigabit Ethernet all support PoE. Power can be sent across either the unused pairs in a cable, or the data transmission pairs, which is referred to as phantom power. Gigabit Ethernet requires the phantom power method, as it uses all eight wires in a twisted-pair cable. The device that provides power is referred to as the Power Source Equipment (PSE). PoE can be supplied using an external power injector, though each powered device requires a separate power injector. More commonly, an 802.3af-compliant network switch is used to provide power to many devices simultaneously. The power supplies in the switch must be large enough to support both the switch itself, and the devices it is powering. (Reference: http://www.belden.com/docs/upload/PoE_Basics_WP.pdf) * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
CCNA Study Guide v2.62 – Aaron Balchunas 35 Layered Communication Section 4 - Hubs vs. Switches vs. Routers - Network communication models are generally organized into layers. The OSI model specifically consists of seven layers, with each layer representing a specific networking function. These functions are controlled by protocols, which govern end-to-end communication between devices. As data is passed from the user application down the virtual layers of the OSI model, each of the lower layers adds a header (and sometimes a trailer) containing protocol information specific to that layer. These headers are called Protocol Data Units (PDUs), and the process of adding these headers is referred to as encapsulation. The PDU of each lower layer is identified with a unique term: # Layer PDU Name 7 Application - 6 Presentation - 5 Session - 4 Transport Segments 3 Network Packets 2 Data-link Frames 1 Physical Bits Commonly, network devices are identified by the OSI layer they operate at (or, more specifically, what header or PDU the device processes). For example, switches are generally identified as Layer-2 devices, as switches process information stored in the Data-Link header of a frame (such as MAC addresses in Ethernet). Similarly, routers are identified as Layer-3 devices, as routers process logical addressing information in the Network header of a packet (such as IP addresses). However, the strict definitions of the terms switch and router have blurred over time, which can result in confusion. For example, the term switch can now refer to devices that operate at layers higher than Layer-2. This will be explained in greater detail in this guide. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
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