Ethernet LANs Chapter 4 Updated January 2007

Ethernet LANs Chapter 4 Updated January 2007
Panko’s Business Data Networks and Telecommunications, 6th edition Copyright 2007 Prentice-Hall May only be used by adopters of the book

Orientation Chapters 2 and 3 Looked at Standards
Chapter 2: Layered standards (data link to application) Chapter 3: Physical layer standards Chapters 4-7 Deal With Single Networks Chapter 4: Ethernet LANs Chapter 4a deals with obsolete Token-Ring Networks Chapter 5: Wireless LANs Chapters 6 and 7: WANs Flow is from LANs to WANs

Figure 4-1: A Short History of Ethernet Standards
The dominant wired LAN technology today Only “competitor” is wireless LANs (which actually are supplementary) The IEEE 802 Committee LAN standards development is done primarily by the Institute for Electrical and Electronics Engineers (IEEE) IEEE created the 802 LAN/MAN Standards Committee for LAN standards (the 802 Committee)

The 802 Committee creates working groups for specific types of standards 802.1 for general standards 802.3 for Ethernet standards The terms and Ethernet are interchangeable for wireless LAN standards for WiMax wireless metropolitan area network standards

Ethernet Standards are OSI Standards Single networks, including LANs, are governed by physical and data link layer standards Layer 1 and Layer 2 standards are almost universally OSI standards Ethernet is no exception The IEEE makes standards; ISO ratifies them In practice, when finishes standards, vendors begin building compliant products

Ethernet Physical Layer Standards

Figure 4-3: Baseband Versus Broadband Transmission
Baseband Transmission Signal Transmitted Signal (Same) Source Transmission Medium Signal is injected directly into the transmission medium (wire, optical fiber) Inexpensive, so dominates wired LAN transmission technology BASE in standard names means baseband

Figure 4-3: Baseband Versus Broadband Transmission, Continued
Modulated Signal Radio Channel Source Radio Tuner The radio tuner modulates the signal to a higher frequency. The transceiver then sends the signal in a radio channel. Expensive but needed for radio-based networks. Not used in Ethernet, but is used in wireless LANs (discussed in Chapter 5).

Figure 4-2: Ethernet Physical Layer Standards
UTP Physical Layer Standards Speed Maximum Run Length Medium Required 10BASE-T 10 Mbps 100 meters 4-pair Category 3 or higher 100BASE-TX 100 Mbps 100 meters 4-pair Category 5 or higher 1000BASE-T (Gigabit Ethernet) 1,000 Mbps 100 meters 4-pair Category 5 or higher 100BASE-TX dominates access links today, Although 1000BASE-T is growing in access links today

Figure 4-2: Ethernet Physical Layer Standards, Continued
Fiber Physical Layer Standards Speed Maximum Run Length Medium 850 nm light (inexpensive) Multimode fiber 1000BASE-SX 1 Gbps 220 m 62.5 microns 160 MHz-km 1000BASE-SX 1 Gbps 275 m 62.5 200 1000BASE-SX 1 Gbps 500 m 50 400 1000BASE-SX 1 Gbps 550 m 50 500 The 1000BASE-SX standard dominates trunk links today. Carriers use 1310 and 1550 nm light and single-mode fiber.

10 Gbps Ethernet 10 Gbps Ethernet usage is small but growing
Revised 10 Gbps Ethernet usage is small but growing Several 10 Gbps fiber standards are defined, but none is dominant

10 Gbps Ethernet 10 Gbps Ethernet usage is small but growing
Revised 10 Gbps Ethernet usage is small but growing Several 10 Gbps 10GBASE-x fiber standards are defined, but none is dominant Copper is cheaper than fiber but cannot go as far 10GBASE-CX4 (shielded Infiniband cable) up to 15 m UTP Category 6: 55 meters maximum (UTP) Category 6A: 100 meters (UTP) Category 7: 100 meters (shielded twisted pair, STP, which has metal shielding around each pair and around the cord)

100 Gbps Ethernet New Information 100 Gbps has been selected as the next Ethernet speed Chosen over 40 Gbps 100 Gbps Ethernet standards development is just getting underway

Figure 4-4: Link Aggregation (Trunking or Bonding)
1000BASE-SX Switch We have been looking at single cords Link aggregation or bonding allows you to bond two or more cords between two switches In this example, if you need 1.6 Gbps, two bonded 1 Gbps links will meet your need at lower cost than moving to a 10 Gbps switch. Link aggregation allows incremental growth in speed and cost 1 Gbps Cord 1 Gbps Cord 1000BASE-SX Switch

Figure 4-5: Data Link Using Multiple Switches
Original Signal Received Signal Regenerated Signal Switches regenerate signals before sending them out; this removes propagation effects. It therefore allows signals to travel farther.

Figure 4-5: Data Link Using Multiple Switches, Continued
Received Signal Original Signal Received Signal Regenerated Signal Received Signal Regenerated Signal Thanks to regeneration, signals can travel far across a series of switches

Received Signal Received Signal Original Signal Received Signal Regenerated Signal Regenerated Signal 62.5/125 Multimode Fiber UTP UTP 100BASE-TX (100 m maximum) Physical Link 1000BASE-SX (220 m maximum) Physical Link 100BASE-TX (100 m maximum) Physical Link Each trunk line along the way has a distance limit

Received Signal Original Signal Received Signal Received Signal Regenerated Signal Regenerated Signal 62.5/125 Multimode Fiber UTP UTP 100BASE-TX (100 m maximum) Physical Link 1000BASE-SX (220 m maximum) Physical Link 100BASE-TX (100 m maximum) Physical Link Station-to-station data link does not have a maximum distance (420 m maximum distance in this example)

Ethernet Data Link (MAC) Layer Standards
802 Layering Frame Syntax Switch Operation

Figure 4-6: Layering in 802 Networks, Continued
Internet Layer TCP/IP Internet Layer Standards (IP, ARP, etc.) Other Internet Layer Standards (IPX, etc.) The 802 LAN/MAN Standards Committee subdivided the data link layer The media access control (MAC) layer handles details specific to a particular technology (Ethernet 802.3, for wireless LANs, etc.) The logical link control layer handles some general functions: Connection to the internet layer, etc.; Not important to corporate networking professionals Data Link Layer Logical Link Control Layer 802.2 Media Access Control Layer Ethernet MAC Layer Standard Non-Ethernet MAC Standards (802.5, 802.11, etc.) Physical Layer 100BASE- TX 1000 Base- SX … Non-Ethernet Physical Layer Standards (802.11, etc.)

Figure 4-6: Layering in 802 Networks, Continued
Internet Layer TCP/IP Internet Layer Standards (IP, ARP, etc.) Other Internet Layer Standards (IPX, etc.) Ethernet only has a single MAC standard (The MAC Layer Standard) Ethernet has many physical layer standards (Fig. 4-2) Data Link Layer Logical Link Control Layer 802.2 Media Access Control Layer Ethernet MAC Layer Standard Non-Ethernet MAC Standards (802.5, 802.11, etc.) Physical Layer 100BASE- TX 1000 BASE- SX … Non-Ethernet Physical Layer Standards (802.11, etc.)

Figure 4-7: The Ethernet MAC Layer Frame
Field Preamble and Start of Frame Delimiter Strong repeating 10… pattern. Synchronizes receiver’s clock with sender’s clock Like quarterback calling out “Hut 1, Hut 2, Hut 3 …” to synchronize the team Preamble (7 Octets) … Start of Frame Delimiter (1 Octet) Destination MAC Address (48 bits) Source MAC Address (48 bits)

Figure 4-7: The Ethernet MAC-Layer Frame, Continued
Field Preamble (7 Octets) … Start of Frame Delimiter (1 Octet) Computers use raw 48-bit MAC addresses; Humans use Hexadecimal notation (A1-23-9C-AB-33-53), which is discussed next. Destination MAC Address (48 bits) Source MAC Address (48 bits)

Figure 4-8: Hexadecimal Notation
4 Bits (Base 2)* Decimal (Base 10) Hexadecimal (Base 16) Symbol Begin Counting at Zero 0000 0 hex 0001 1 1 hex 0010 2 2 hex 0011 3 3 hex 0100 4 4 hex 0101 5 5 hex 0110 6 6 hex 0111 7 7 hex With 4 bits, there are 24=16 possible symbols. For example, CD-7B-DF hex begins with for 01.

Figure 4-8: Hexadecimal Notation, Continued
4 Bits (Base 2) Decimal (Base 10) Hexadecimal (Base 16) Symbol 1000 8 8 hex 1001 9 9 hex 1010 10 A hex 1011 11 B hex After 9, Count A Through F 1100 12 C hex 1101 13 D hex 1110 14 E hex 1111 15 F hex

Figure 4-8: Hexadecimal Notation, Continued
Converting 48-Bit MAC Addresses to Hex Start with the 48-bit MAC Address … Break the MAC address into twelve 4-bit “nibbles” … Convert each nibble to a hex symbol A D D Write the hex symbols in pairs (each pair is an octet) and put a dash between each pair A1-DD-3C-D7-23-FF

Figure 4-7: The Ethernet MAC Layer Frame, Continued
Field Length field gives the length of the data field in octets Length (2 Octets) Data Field (Variable Length) LLC Subheader (Usually 8 Octets) Data field contains A packet of variable length Packet is preceded in the data field by an LLC subheader that describes the type of packet (IP, IPX, etc.) Packet (Variable Length) PAD Frame Check Sequence (4 Octets)

Field Length (2 Octets) A PAD is added if the data field is less than 46 octets; length is set to make the data field plus PAD field 46 octets; A PAD field is not added if data field is greater than 46 octets long. Data Field (Variable Length) LLC Subheader (Usually 8 Octets) Packet (Variable Length) PAD Frame Check Sequence (4 Octets)

Field Sender computes the frame check sequence field value based on the bits in the other fields. The receiver redoes the computation. If it gets a different results, the frame must have a transmission error. The receiver discards the frame. There is no error correction. Ethernet is not reliable. Length (2 Octets) Data Field (Variable Length) LLC Subheader (Usually 8 Octets) Packet (Variable Length) PAD Frame Check Sequence (4 Octets)

Figure 4-9: Multiswitch Ethernet LAN
Port 7 on Switch 2 to Port 4 on Switch 3 Port 5 on Switch 1 to Port 3 on Switch 2 The Situation: A1… Sends to E5… Frame must go through 3 switches along the way (1, 2, and then 3) Switch 1 Switch 3 B2-CD-13-5B-E4-65 Switch 1, Port 7 A1-44-D5-1F-AA-4C Switch 1, Port 2 E5-BB D3-56 Switch 3, Port 6 D C4-B6-9F Switch 3, Port 2

Figure 4-9: Multiswitch Ethernet LAN, Continued
On Switch 1 Switch 2 Switching Table Switch 1 Port Station 2 A1-45-D5-1F-AA-4C 7 B2-CD-13-5B-E4-65 5 D C4-B6-9F 5 E5-BB D3-56 Port 5 on Switch 1 to Port 3 on Switch 2 Switch 1 B2-CD-13-5B-E4-65 Switch 1, Port 7 A1-44-D5-1F-AA-4C Switch 1, Port 2 E5-BB D3-56 Switch 3, Port 6

On Switch 2 Switch 2 Port 5 on Switch 1 to Port 3 on Switch 2 Port 7 on Switch 2 to Port 4 on Switch 3 Switch 1 Switch 3 Switching Table Switch 2 Port Station A1-44-D5-1F-AA-4C 3 B2-CD-13-5B-E4-65 D C4-B6-9F 7 E5-BB D3-56 E5-BB D3-56 Switch 3, Port 6

Switching Table Switch 3 Port Station 4 A1-44-D5-1F-AA-4C B2-CD-13-5B-E4-65 2 D C4-B6-9F 6 E5-BB D3-56 Port 7 on Switch 2 to Port 4 on Switch 3 Switch 3 On Switch 3 A1-44-D5-1F-AA-4C Switch 1, Port 2 D C4-B6-9F Switch 3, Port 2 E5-BB D3-56 Switch 3, Port 6

Figure 4-10: Hierarchical Ethernet LAN
Single Possible Path Between Client PC 1 and Server Y Ethernet Switch A Ethernet Switch C Ethernet Switch B Ethernet Switch F Ethernet Switch D Ethernet Switch E Server X Server Y Client PC 1

Figure 4-10: Hierarchical Ethernet LAN, Continued
With only one possible path between stations… Therefore there is only one possible port on a switch to send the frame back out Therefore only one row per MAC address in switching table Switch can find the one row quickly This makes Ethernet switches inexpensive per frame Low cost has led to Ethernet’s LAN dominance Port Station 2 A1-44-D5-1F-AA-4C 7 B2-CD-13-5B-E4-65 5 E5-BB D3-56

Figure 4-10: Hierarchical Ethernet LAN, Continued
Core Core Switches Core Ethernet Switch A Workgroup Ethernet Switch D Core Ethernet Switch C Core Ethernet Switch B Workgroup Ethernet Switch F Workgroup Ethernet Switch E As noted in Chapter 3, there are workgroup and core switches. Core switches need more capacity. Workgroup Switch

Figure 4-11: Single Point of Failure in a Switch Hierarchy
Switch Fails Switch 2 No Communication No Communication Switch 1 Switch 3 B2-CD-13-5B-E4-65 D C4-B6-9F E5-BB D3-56 A1-44-D5-1F-AA-4C

Figure 4-12: 802.1D Spanning Tree Protocol (STP)
Loop, but Spanning Tree Protocol Deactivates One Link Normal Operation Activated Switch 2 Activated Deactivated Switch 1 Switch 3 B2-CD-13-5B-E4-65 D C4-B6-9F E5-BB D3-56 A1-44-D5-1F-AA-4C

Figure 4-12: 802.1D Spanning Tree Protocol (STP), Continued
Switch 2 Fails Deactivated Deactivated Switch 2 Reactivated Switch 1 Switch 3 C3-2D-55-3B-A9-4F B2-CD-13-5B-E4-65 D C4-B6-9F A1-44-D5-1F-AA-4C E5-BB D3-56

Figure 4-12: 802.1D (STP), Continued
Spanning Tree Protocol (STP) Works but when there is a break in the hierarchy, the network converges to a new hierarchy too slowly Rapid Spanning Tree Protocol (RSTP) Newer algorithm that converges very quickly

Virtual LANs (VLANs)

Figure 4-13: Virtual LAN (VLAN) with Ethernet Switches
Server Broadcasting without VLANS Servers Sometimes Broadcast; Goes To All Stations; Latency Results Server Broadcast Client C Client B Client A Server D Server E

Figure 4-13: Virtual LAN (VLAN) with Ethernet Switches, Continued
Server Broadcasting with VLANS With VLANs, Broadcasts Only Go To a Server’s VLAN Clients; Less Latency Server Broadcast No No Client C on VLAN1 Client B on VLAN2 Client A on VLAN1 Server D on VLAN2 Server E on VLAN1

Figure 4-13: Virtual LAN (VLAN) with Ethernet Switches, Continued
VLANs primarily reduce congestion due to latency They can also be used for security Only people on a server’s VLAN can reach it This provides some degree of security Not sufficient by itself, but it can help Wireless LANs In wireless LANs, wireless clients may be initially placed in a VLAN that only has a single server—a server that authenticates the clients After authentication, clients are allowed beyond the initial VLAN

Figure 4-14: Tagged Ethernet Frame (Governed By 802.1Q)
By looking at the value in the 2 octets after the addresses, the switch can tell if this frame is a basic frame (value less than 1,500) or a tagged (value is 33,024). Basic MAC Frame Tagged MAC Frame Preamble (7 octets) Preamble (7 octets) Start-of-Frame Delimiter (1 Octet) Start-of-Frame Delimiter (1 Octet) Destination Address (6 Octets) Destination Address (6 Octets) Source Address (6 Octets) Source Address (6 Octets) Length (2 Octets) Length of Data Field in Octets 1,500 (Decimal) Maximum Tag Protocol ID (2 Octets) 81-00 hex; 33,024 decimal. Larger than 1,500, So not a Length Field

Figure 4-14: Tagged Ethernet Frame (Governed By 802.1Q), Continued
Basic MAC Frame Tagged MAC Frame Data Field (variable) Tag Control Information (2 Octets) Priority Level (0-7) (3 bits); VLAN ID (12 bits) 1 other bit PAD (If Needed) Length (2 Octets) Frame Check Sequence (4 Octets) Data Field (variable) PAD (If Needed) Frame Check Sequence (4 Octets)

Momentary Traffic Peak: Congestion and Latency
Figure 4-15: Handling Momentary Traffic Peaks with Overprovisioning and Priority Momentary Traffic Peak: Congestion and Latency Traffic Momentary Traffic Peak: Congestion and Latency Network Capacity Momentary traffic peaks usually last only a fraction of a second; They occasionally exceed the network’s capacity. When they do, frames will be delayed, even dropped. Time

Overprovisioned Traffic Capacity in Ethernet
Figure 4-15: Handling Momentary Traffic Peaks with Overprovisioning and Priority, Continued Overprovisioned Traffic Capacity in Ethernet Traffic Overprovisioned Network Capacity Momentary Peak: No Congestion Overprovisioning: Build high capacity than will rarely if ever be exceeded. This wastes capacity. But cheaper than using priority (next) Time

Figure 4-15: Handling Momentary Traffic Peaks with Overprovisioning and Priority, Continued
Priority in Ethernet Traffic Momentary Peak High-Priority Traffic Goes Low-Priority Waits Network Capacity Priority: During momentary peaks, give priority to traffic that is intolerant of latency (delay), such as voice. No need to overprovision, but expensive to implement. Ongoing management is very expensive. Time

Box: Hubs and Switches

Figure 4-16: Hub Versus Switch Operation
Today, All Corporations Use Ethernet Switches An Ethernet Switch Sends Frame Out One Port If A Is Transmitting to C, B Can Transmit to D Simultaneously Ethernet Switch Box C D A B

Figure 4-16: Hub Versus Switch Operation, Continued
Years Ago, Corporations Used Ethernet Hubs A Hub Broadcasts Each Bit Out All Other Ports. Simple and Cheap --- But If A Is Transmitting, B Must Wait to Transmit In Large Hub Networks, Delays Are Intolerable Ethernet Hub Box X A B C D

Figure 4-16: Hub Versus Switch Operation, Continued
Box Hubs Need Media Access Control This limits when a station may transmit Ethernet NICs must use CSMA/CD with hubs Carrier Sense Multiple Access (CSMA) Only transmit if no other station is transmitting Otherwise, wait With Collision Detection (CD) If two NICs transmit at the same time, this is a collision Both will stop, wait a random amount of time, and the go back to CSMA to send again

Purchasing Switches

Figure 4-17: Switch Purchasing Considerations
Number and Speeds of Ports Buyers must decide on the number of ports needed and the speed of each Buyers often can buy a prebuilt switch with this configuration

Figure 4-18: Switching Matrix
100 Mbps 1 Port 1 to Port 3 100 Mbps 400 Mbps Aggregate Capacity to Be Nonblocking 2 Any-to-Any Switching Matrix 100 Mbps 3 100 Mbps 4 100BASE-TX Input Ports Input Queue(s) 100BASE-TX Output Ports 1 2 3 4 Note: Input Port 1 and Output Port 1 are the same port. Aggregate switching matrix capacity is its total switching speed. Maximum input for this switch is 400 Mbps (4 x 100 Mbps). 400 Mbps aggregate capacity is needed for switch to be nonblocking

Figure 4-17: Switch Purchasing Considerations, Continued
Store-and-Forward Versus Cut-Through Switching (see Figure 4-19) Store-and-forward Ethernet switches read whole frame before passing the frame on Cut-through Ethernet switches read only some fields before starting to pass the frame back out Cut-through switches have less latency, but this is rarely important

Figure 4-19: Store-and-Forward Versus Cut-Through Switching
2. Cut-Through Based On MAC Destination Address (14 Octets) Preamble Start-of-Frame Delimiter Destination Address 3.. Cut-Through for Priority or VLANs (24 Octets) Source Address 1. Store-and- Forward Processing Ends Here (Often Hundreds Of Bytes) Tag Fields if Present Length Data (and Perhaps PAD) 4. Cut-Through at 64 Bytes (Not a Runt) Cyclical Redundancy Check

Figure 4-20: Managed Switches
cost much more than unmanaged switcheds Manager can manage all switches remotely Get Data Data Requested Managed Switch Manager Command to Change Configuration (can fix many problems remotely) Managed Switch

Ethernet Security Port-Based Access Control (802.1X)
Attackers on site can walk up to any Ethernet port and plug in a computer, bypassing the firewall 802.1X standard Computer attaching to a port must first authenticate itself. (More details in Chapter 5) or be rejected. No Access Without Authentication

Ethernet Security MAC Security (MACsec) 802.1AE
Switches must talk to one another for STP, VLANs, and other supervisory protocols An attacker on a PC can pretend to be a switch and send false supervisory messages 802.1AE MACsec protects supervisory communication, preventing many types of attacks False Supervisory Message PC impersonating a switch Stops Fake Message

Box: Advanced Switch Purchase Considerations
Physical and Electrical Features

Figure 4-21: Physical and Electrical Features
Box Physical Size Switches fit into standard 19-in (48-cm) wide equipment racks Switch heights usually are multiples of 1U (1.75 in or 4.4 cm) 19 inches (48 cm)

Figure 4-21: Physical and Electrical Features, Continued
Box Port Flexibility Fixed-port switches No flexibility: the number of ports is fixed 1 or 2 U tall Most workgroup switches are fixed-port switches

Box Port Flexibility Stackable Switches Fixed number of ports 1 or 2 U tall High-speed interconnect bus connects stacked switches When demand increases, firm can simply add a new stackable switch

Box Port Flexibility Modular Switches 1 or 2 U tall Contain one or a few slots for modules Each module usually contains 1 to 4 ports Module

Box Port Flexibility Chassis switches Several U tall Contain several expansion slots Each expansion board contains several slots Most core switches are chassis switches

Box Switch and NIC Ports Normal Ethernet RJ-45 switch ports transmit on Pins 3 and 6 and listen on Pins 1 and 2 NICs transmit on Pins 1 and 2 and listen on Ports 3 and 6 Pins 1 & 2 Pins 3 & 6 Normal PC NIC Port Normal Switch Port

Box Switch and NIC Ports If you connect two normal ports on different switches via UTP cords, BOTH will send on Pins 3 & 6 and neither will listen on Pins 3 & 6 Communication will be impossible Normal Switch Port On Parent Switch Normal Switch Port Pins 3 & 6 Pins 3 & 6

Box Switch Uplink Ports On a growing number of switches, normal ports change automatically to uplink ports if used that way 2. Changes automatically to Pins 1 & 2 Normal Switch Port On Parent Switch Pins 3 & 6 Normal Switch Port 1. Normally Transmits on Pins 3 & 6

Box / New Crossover Cables Designed to connect ordinary ports on two switches Internally, connect Pins 1 & 2 on one machine to Pins 3 & 6 on the other switch Do NOT use to connect NICs to switches or a switch uplink port to another switch! Pins 1 & 2 Normal Switch Port On Parent Switch Normal Switch Port Crossover Cable Pins 3 & 6

Box Electrical Power Under the 802.3af standard, switches can provide electrical power to devices over the UTP cord Currently limited to watts; sufficient for most wireless access points (Chapter 5) and voice over IP telephones (Chapter 6) but not sufficient for computers New slightly higher-power version of the standard is being developed to be able to serve sophisticated access points; still not good enough for computers.

Topics Covered

Topics Covered Ethernet Standards Setting Physical Layer Standards
802.3 Working Group Physical and data link layer standards OSI standards Physical Layer Standards BASE means baseband 100BASE-TX dominates for access lines 10GBASE-SX dominates for trunk lines Link aggregation for small capacity increases Regeneration to carry signals across multiple switches

Topics Covered Ethernet MAC Layer Standards
Data link layer subdivided into the LLC and MAC layers The Ethernet MAC Layer Frame Preamble and Start of Frame Delimiter fields Destination and Source MAC addresses fields Hexadecimal notation Length field Data field LLC subheader Packet PAD if needed Frame Check Sequence field

Topics Covered Ethernet MAC Layer Standards Switch operation
Operation of a hierarchy of switches Single possible path between any two computers Hierarchy gives low price per frame transmitted Single points of failure and the Spanning Tree Protocol VLANs and frame tagging to reduce broadcasting Momentary traffic peaks: addressed by overprovisioning and priority Hubs and CSMA/CD

Topics Covered Switch Purchasing Considerations
Number and speed of ports Switching matrix (nonblocking) Store-and-forward versus cut-through switches Managed switches Ethernet security 802.1X Port-Based Access Control 802.1AE MACsec

Topics Covered Advanced Switch Purchasing Considerations Physical size
Box Advanced Switch Purchasing Considerations Physical size Fixed-Port-Speeches Stackable Switches Modular Switches Chassis Switches Pins in Switch Ports and Uplink Ports Electrical Power (802.3af)

Ethernet LANs Chapter 4 Updated January 2007

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