1. 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

2. 4-2 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

3. 4-3 Figure 4-1: A Short History of Ethernet Standards Ethernet –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)

4. 4-4 Figure 4-1: A Short History of Ethernet Standards The 802 Committee creates working groups for specific types of standards –802.1 for general standards –802.3 for Ethernet standards The terms 802.3 and Ethernet are interchangeable –802.11 for wireless LAN standards –802.16 for WiMax wireless metropolitan area network standards

5. 4-5 Figure 4-1: A Short History of Ethernet 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 802.3 standards; ISO ratifies them –In practice, when 802.3 finishes standards, vendors begin building compliant products

6. Ethernet Physical Layer Standards

7. 4-7 Figure 4-3: Baseband Versus Broadband Transmission Baseband Transmission Source Signal Transmitted Signal (Same) 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

8. 4-8 Figure 4-3: Baseband Versus Broadband Transmission, Continued Broadband Transmission Source Radio Tuner Modulated Signal Radio Channel 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).

9. 4-9 Figure 4-2: Ethernet Physical Layer Standards UTP Physical Layer Standards Medium Required Maximum Run Length Speed 100BASE-TX4-pair Category 5 or higher100 meters100 Mbps 1000BASE-T (Gigabit Ethernet) 4-pair Category 5 or higher100 meters1,000 Mbps 10BASE-T4-pair Category 3 or higher100 meters10 Mbps 100BASE-TX dominates access links today, Although 1000BASE-T is growing in access links today

10. 4-10 Fiber Physical Layer Standards Medium 850 nm light (inexpensive) Multimode fiber Maximum Run Length Speed 1000BASE-SX275 m1 Gbps 1000BASE-SX500 m1 Gbps 1000BASE-SX220 m1 Gbps 1000BASE-SX550 m1 Gbps Figure 4-2: Ethernet Physical Layer Standards, Continued 62.5 microns 160 MHz-km 62.5200 50400 50500 The 1000BASE-SX standard dominates trunk links today. Carriers use 1310 and 1550 nm light and single-mode fiber.

11. 4-11 10 Gbps Ethernet 10 Gbps Ethernet usage is small but growing Several 10 Gbps fiber standards are defined, but none is dominant Revised

12. 4-12 10 Gbps Ethernet 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) Revised

13. 4-13 100 Gbps Ethernet 100 Gbps has been selected as the next Ethernet speed –Chosen over 40 Gbps 100 Gbps Ethernet standards development is just getting underway New Information

14. 4-14 Figure 4-4: Link Aggregation (Trunking or Bonding) 1 Gbps Cord 1 Gbps Cord 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 1000BASE-SX Switch

15. 4-15 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.

16. 4-16 Figure 4-5: Data Link Using Multiple Switches, Continued Original Signal Received Signal Received Signal Received Signal Regenerated Signal Regenerated Signal Thanks to regeneration, signals can travel far across a series of switches

17. 4-17 Figure 4-5: Data Link Using Multiple Switches, Continued Original Signal Received Signal Received Signal Received Signal Regenerated Signal Regenerated Signal UTP 62.5/125 Multimode Fiber 100BASE-TX (100 m maximum) Physical Link 100BASE-TX (100 m maximum) Physical Link 1000BASE-SX (220 m maximum) Physical Link Each trunk line along the way has a distance limit

18. 4-18 Figure 4-5: Data Link Using Multiple Switches, Continued Station-to-station data link does not have a maximum distance (420 m maximum distance in this example) Original Signal Received Signal Received Signal Received Signal Regenerated Signal Regenerated Signal UTP 62.5/125 Multimode Fiber 100BASE-TX (100 m maximum) Physical Link 100BASE-TX (100 m maximum) Physical Link 1000BASE-SX (220 m maximum) Physical Link

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

20. 4-20 Figure 4-6: Layering in 802 Networks, Continued TCP/IP Internet Layer Standards (IP, ARP, etc.) Other Internet Layer Standards (IPX, etc.) 802.2 Ethernet 802.3 MAC Layer Standard Physical Layer Media Access Control Layer Non-Ethernet MAC Standards (802.5, 802.11, etc.) 100BASE- TX 1000 Base- SX … Logical Link Control Layer Non-Ethernet Physical Layer Standards (802.11, etc.) Data Link Layer Internet Layer 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, 802.11 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

21. 4-21 Figure 4-6: Layering in 802 Networks, Continued TCP/IP Internet Layer Standards (IP, ARP, etc.) Other Internet Layer Standards (IPX, etc.) 802.2 Ethernet 802.3 MAC Layer Standard Physical Layer Media Access Control Layer Non-Ethernet MAC Standards (802.5, 802.11, etc.) 100BASE- TX 1000 BASE- SX … Logical Link Control Layer Non-Ethernet Physical Layer Standards (802.11, etc.) Data Link Layer Internet Layer Ethernet only has a single MAC standard (The 802.3 MAC Layer Standard) Ethernet has many physical layer standards (Fig. 4-2)

22. 4-22 Figure 4-7: The Ethernet MAC Layer Frame Preamble (7 Octets) 10101010 … Start of Frame Delimiter (1 Octet) 10101011 Destination MAC Address (48 bits) Source MAC Address (48 bits) 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

23. 4-23 Figure 4-7: The Ethernet MAC-Layer Frame, Continued Preamble (7 Octets) 10101010 … Start of Frame Delimiter (1 Octet) 10101011 Destination MAC Address (48 bits) Source MAC Address (48 bits) Field Computers use raw 48-bit MAC addresses; Humans use Hexadecimal notation (A1-23-9C-AB-33-53), which is discussed next.

24. 4-24 Figure 4-8: Hexadecimal Notation 4 Bits (Base 2)* Decimal (Base 10) Hexadecimal (Base 16) Symbol 000000 hex 000111 hex 001022 hex With 4 bits, there are 2 4 =16 possible symbols. For example, 01-34-CD-7B-DF hex begins with 00000001 for 01. 001133 hex 010044 hex 010155 hex 011066 hex 011177 hex Begin Counting at Zero

25. 4-25 Figure 4-8: Hexadecimal Notation, Continued 4 Bits (Base 2) Decimal (Base 10) Hexadecimal (Base 16) Symbol 100088 hex 100199 hex 101010A hex 101111B hex 110012C hex 110113D hex 111014E hex 111115F hex After 9, Count A Through F

26. 4-26 Figure 4-8: Hexadecimal Notation, Continued Converting 48-Bit MAC Addresses to Hex –Start with the 48-bit MAC Address 1010000110111011 … –Break the MAC address into twelve 4-bit “nibbles” 1010 0001 1101 1101 … –Convert each nibble to a hex symbol A 1 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

27. 4-27 Figure 4-7: The Ethernet MAC Layer Frame, Continued Length (2 Octets) PAD Field Packet (Variable Length) LLC Subheader (Usually 8 Octets) Data Field (Variable Length) Frame Check Sequence (4 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.) Length field gives the length of the data field in octets

28. 4-28 Figure 4-7: The Ethernet MAC Layer Frame, Continued Length (2 Octets) PAD Field Packet (Variable Length) LLC Subheader (Usually 8 Octets) Data Field (Variable Length) Frame Check Sequence (4 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.

29. 4-29 Figure 4-7: The Ethernet MAC Layer Frame, Continued Length (2 Octets) PAD Field Packet (Variable Length) LLC Subheader (Usually 8 Octets) Data Field (Variable Length) Frame Check Sequence (4 Octets) 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.

30. 4-30 Figure 4-9: Multiswitch Ethernet LAN Switch 2 Switch 1 Switch 3 Port 5 on Switch 1 to Port 3 on Switch 2 Port 7 on Switch 2 to Port 4 on Switch 3 A1-44-D5-1F-AA-4C Switch 1, Port 2 E5-BB-47-21-D3-56 Switch 3, Port 6 D5-47-55-C4-B6-9F Switch 3, Port 2 B2-CD-13-5B-E4-65 Switch 1, Port 7 The Situation: A1… Sends to E5… Frame must go through 3 switches along the way (1, 2, and then 3)

31. 4-31 Figure 4-9: Multiswitch Ethernet LAN, Continued Switching Table Switch 1 PortStation 2A1-45-D5-1F-AA-4C 7B2-CD-13-5B-E4-65 5D5-47-55-C4-B6-9F 5E5-BB-47-21-D3-56 Switch 2 Switch 1 Port 5 on Switch 1 to Port 3 on Switch 2 A1-44-D5-1F-AA-4C Switch 1, Port 2 B2-CD-13-5B-E4-65 Switch 1, Port 7 E5-BB-47-21-D3-56 Switch 3, Port 6 On Switch 1

32. 4-32 Figure 4-9: Multiswitch Ethernet LAN, Continued Switch 2 Switch 1 Switch 3 Port 5 on Switch 1 to Port 3 on Switch 2 Port 7 on Switch 2 to Port 4 on Switch 3 Switching Table Switch 2 PortStation 3A1-44-D5-1F-AA-4C 3B2-CD-13-5B-E4-65 7D5-47-55-C4-B6-9F 7E5-BB-47-21-D3-56 E5-BB-47-21-D3-56 Switch 3, Port 6 On Switch 2

33. 4-33 Figure 4-9: Multiswitch Ethernet LAN, Continued Switch 2 Switch 3 Port 7 on Switch 2 to Port 4 on Switch 3 A1-44-D5-1F-AA-4C Switch 1, Port 2 D5-47-55-C4-B6-9F Switch 3, Port 2 Switching Table Switch 3 PortStation 4A1-44-D5-1F-AA-4C 4B2-CD-13-5B-E4-65 2D5-47-55-C4-B6-9F 6E5-BB-47-21-D3-56 E5-BB-47-21-D3-56 Switch 3, Port 6 On Switch 3

34. 4-34 Figure 4-10: Hierarchical Ethernet LAN Client PC 1 Ethernet Switch F Server Y Server X Single Possible Path Between Client PC 1 and Server Y Ethernet Switch E Ethernet Switch D Ethernet Switch B Ethernet Switch A Ethernet Switch C

35. 4-35 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 PortStation 2A1-44-D5-1F-AA-4C 7B2-CD-13-5B-E4-65 5E5-BB-47-21-D3-56

36. 4-36 Figure 4-10: Hierarchical Ethernet LAN, Continued Workgroup Ethernet Switch F Core Switches Workgroup Ethernet Switch E Workgroup Ethernet Switch D Core Ethernet Switch B Core Ethernet Switch A Core Ethernet Switch C Core Workgroup Switch As noted in Chapter 3, there are workgroup and core switches. Core switches need more capacity.

37. 4-37 Figure 4-11: Single Point of Failure in a Switch Hierarchy No Communication Switch 1 Switch 2 Switch 3 Switch Fails A1-44-D5-1F-AA-4C B2-CD-13-5B-E4-65 D4-47-55-C4-B6-9F E5-BB-47-21-D3-56

38. 4-38 Figure 4-12: 802.1D Spanning Tree Protocol (STP) Switch 1 Switch 2 Switch 3 A1-44-D5-1F-AA-4C B2-CD-13-5B-E4-65 D4-47-55-C4-B6-9F E5-BB-47-21-D3-56 Activated Deactivated Normal Operation Loop, but Spanning Tree Protocol Deactivates One Link

39. 4-39 Figure 4-12: 802.1D Spanning Tree Protocol (STP), Continued Switch 1 Switch 2 Switch 3 A1-44-D5-1F-AA-4C B2-CD-13-5B-E4-65 C3-2D-55-3B-A9-4F D4-47-55-C4-B6-9F E5-BB-47-21-D3-56 Deactivated Reactivated Switch 2 Fails

40. 4-40 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

41. Virtual LANs (VLANs)

42. 4-42 Figure 4-13: Virtual LAN (VLAN) with Ethernet Switches Client A Client B Client C Server DServer E Server Broadcast Server Broadcasting without VLANS Servers Sometimes Broadcast; Goes To All Stations; Latency Results

43. 4-43 Figure 4-13: Virtual LAN (VLAN) with Ethernet Switches, Continued Server Broadcasting with VLANS Client A on VLAN1 Client B on VLAN2 Client C on VLAN1 Server D on VLAN2 Server E on VLAN1 Server Broadcast No With VLANs, Broadcasts Only Go To a Server’s VLAN Clients; Less Latency

44. 4-44 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

45. 4-45 Figure 4-14: Tagged Ethernet Frame (Governed By 802.1Q) Destination Address (6 Octets) Destination 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) 1000000100000000 81-00 hex; 33,024 decimal. Larger than 1,500, So not a Length Field 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 802.3 MAC FrameTagged 802.3 MAC Frame Start-of-Frame Delimiter (1 Octet) Preamble (7 octets) Start-of-Frame Delimiter (1 Octet) Preamble (7 octets) Source Address (6 Octets)

46. 4-46 Figure 4-14: Tagged Ethernet Frame (Governed By 802.1Q), Continued Tag Control Information (2 Octets) Priority Level (0-7) (3 bits); VLAN ID (12 bits) 1 other bit Basic 802.3 MAC FrameTagged 802.3 MAC Frame Length (2 Octets) Data Field (variable) PAD (If Needed) Frame Check Sequence (4 Octets) PAD (If Needed) Frame Check Sequence (4 Octets)

47. 4-47 Figure 4-15: Handling Momentary Traffic Peaks with Overprovisioning and Priority Traffic Network Capacity Momentary Traffic Peak: Congestion and Latency Time Momentary Traffic Peak: Congestion and Latency 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.

48. 4-48 Figure 4-15: Handling Momentary Traffic Peaks with Overprovisioning and Priority, Continued Traffic Overprovisioned Network Capacity Momentary Peak: No Congestion Time Overprovisioned Traffic Capacity in Ethernet Overprovisioning: Build high capacity than will rarely if ever be exceeded. This wastes capacity. But cheaper than using priority (next)

49. 4-49 Figure 4-15: Handling Momentary Traffic Peaks with Overprovisioning and Priority, Continued Traffic Network Capacity Momentary Peak Time Priority in Ethernet High-Priority Traffic Goes Low-Priority Waits 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.

50. Box: Hubs and Switches

51. 4-51 Figure 4-16: Hub Versus Switch Operation AB CD Ethernet Switch An Ethernet Switch Sends Frame Out One Port If A Is Transmitting to C, B Can Transmit to D Simultaneously Box Today, All Corporations Use Ethernet Switches

52. 4-52 Figure 4-16: Hub Versus Switch Operation, Continued ABCD Ethernet Hub 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 X Box Years Ago, Corporations Used Ethernet Hubs

53. 4-53 Figure 4-16: Hub Versus Switch Operation, Continued 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 Box

54. Purchasing Switches

55. 4-55 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

56. 4-56 Figure 4-18: Switching Matrix 1 2 3 4 100 Mbps 1234 Port 1 to Port 3 400 Mbps Aggregate Capacity to Be Nonblocking Input Queue(s) 100BASE-TX Input Ports 100BASE-TX Output Ports Any-to-Any Switching Matrix 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

57. 4-57 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

58. 4-58 Figure 4-19: Store-and-Forward Versus Cut- Through Switching Preamble Start-of-Frame Delimiter Destination Address Source Address Tag Fields if Present Length Cyclical Redundancy Check Data (and Perhaps PAD) 2. Cut-Through Based On MAC Destination Address (14 Octets) 3.. Cut-Through for Priority or VLANs (24 Octets) 4. Cut-Through at 64 Bytes (Not a Runt) 1. Store-and- Forward Processing Ends Here (Often Hundreds Of Bytes)

59. 4-59 Figure 4-20: Managed Switches Manager Command to Change Configuration (can fix many problems remotely) Get Data Data Requested Managed Switch Manager can manage all switches remotely Managed switches cost much more than unmanaged switcheds

60. 4-60 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

61. 4-61 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 Stops Fake Message PC impersonating a switch False Supervisory Message

62. Box: Advanced Switch Purchase Considerations Physical and Electrical Features Box

63. 4-63 Figure 4-21: Physical and Electrical Features 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) Box

64. 4-64 Figure 4-21: Physical and Electrical Features, Continued 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

65. 4-65 Figure 4-21: Physical and Electrical Features, Continued 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

66. 4-66 Figure 4-21: Physical and Electrical Features, Continued 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 Box Module

67. 4-67 Figure 4-21: Physical and Electrical Features, Continued Port Flexibility –Chassis switches Several U tall Contain several expansion slots Each expansion board contains several slots Most core switches are chassis switches Box

68. 4-68 Figure 4-21: Physical and Electrical Features, Continued 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 Box Normal PC NIC Port Normal Switch Port Pins 1 & 2 Pins 3 & 6

69. 4-69 Figure 4-21: Physical and Electrical Features, Continued 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 Box Normal Switch Port Normal Switch Port On Parent Switch Pins 3 & 6 Pins 3 & 6

70. 4-70 Figure 4-21: Physical and Electrical Features, Continued Switch Uplink Ports –On a growing number of switches, normal ports change automatically to uplink ports if used that way Box Normal Switch Port Normal Switch Port On Parent Switch Pins 3 & 6 1. Normally Transmits on Pins 3 & 6 2. Changes automatically to Pins 1 & 2

71. 4-71 Figure 4-21: Physical and Electrical Features, Continued 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! Box / New Normal Switch Port Normal Switch Port On Parent Switch Pins 3 & 6 Pins 1 & 2 Crossover Cable

72. 4-72 Figure 4-21: Physical and Electrical Features, Continued Electrical Power –Under the 802.3af standard, switches can provide electrical power to devices over the UTP cord –Currently limited to 12.95 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. Box

73. Topics Covered

74. 4-74 Topics Covered Ethernet Standards Setting –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

75. 4-75 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

76. 4-76 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

77. 4-77 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

78. 4-78 Topics Covered 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) Box