Ethernet LANs Chapter 4 Updated January 2009 XU Zhengchuan Fudan University

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

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 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 and Ethernet are interchangeable – for wireless LAN standards – for WiMax wireless metropolitan area network standards

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 standards; ISO ratifies them –In practice, when finishes standards, vendors begin building compliant products

4-6 Test Your Understanding P 188 2

Ethernet Physical Layer Standards

4-8 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

4-9 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).

4-10 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

4-11 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 The 1000BASE-SX standard dominates trunk links today. Carriers use 1310 and 1550 nm light and single-mode fiber.

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

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

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

4-15 Test Your Understanding P 193 3

4-16 Figure 4-4: Link Aggregation (Trunking or Bonding) 链路聚合（中继 或 捆绑） 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

4-17 链路聚合 IEEE 802.3ad 标准定义了如何将两个以上的千兆位以太网连 接组合起来，为高带宽网络连接实现负载共享、负载平衡， 以及提供更好的可伸缩性服务。由于在链路聚合技术的支持 下，网络传输的数据流被动态地分布到加入链路的各个端口， 因此在聚合链路中自动地完成了对实际流经某个端口的数据 管理。 链路聚合的另一个主要优点是可靠性。链路聚合技术在点到 点链路上提供了固有的、自动的冗余性。如果链路使用的多 个端口中的一个出现故障，网络传输的数据流可以动态地快 速转向链路中其他工作正常的端口进行传输。

4-18

4-19 Test Your Understanding P 194 4

4-20 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.

4-21 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

4-22 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

4-23 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

4-24 Answer Question (Page 196) 5 c 5 f

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

4-26 Figure 4-6: Layering in 802 Networks, Continued TCP/IP Internet Layer Standards (IP, ARP, etc.) Other Internet Layer Standards (IPX, etc.) Ethernet MAC Layer Standard Physical Layer Media Access Control Layer Non-Ethernet MAC Standards (802.5, , 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, 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

4-27 Figure 4-6: Layering in 802 Networks, Continued TCP/IP Internet Layer Standards (IP, ARP, etc.) Other Internet Layer Standards (IPX, etc.) Ethernet MAC Layer Standard Physical Layer Media Access Control Layer Non-Ethernet MAC Standards (802.5, , 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 MAC Layer Standard) Ethernet has many physical layer standards (Fig. 4-2)

4-28 Figure 4-7: The Ethernet MAC Layer Frame Preamble (7 Octets) … Start of Frame Delimiter (1 Octet) 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

4-29 Figure 4-7: The Ethernet MAC-Layer Frame, Continued Preamble (7 Octets) … Start of Frame Delimiter (1 Octet) 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.

4-30 Figure 4-8: Hexadecimal Notation 4 Bits (Base 2)* Decimal (Base 10) Hexadecimal (Base 16) Symbol hex hex hex With 4 bits, there are 2 4 =16 possible symbols. For example, CD-7B-DF hex begins with for hex hex hex hex hex Begin Counting at Zero

4-31 Figure 4-8: Hexadecimal Notation, Continued 4 Bits (Base 2) Decimal (Base 10) Hexadecimal (Base 16) Symbol hex hex A hex B hex C hex D hex E hex F hex After 9, Count A Through F

4-32 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 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

4-33 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

4-34 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.

4-35 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.

4-36 Answer Question (Page 201) 7 a 7 h

4-37 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 D3-56 Switch 3, Port 6 D 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)

4-38 Figure 4-9: Multiswitch Ethernet LAN, Continued Switching Table Switch 1 PortStation 2A1-45-D5-1F-AA-4C 7B2-CD-13-5B-E4-65 5D C4-B6-9F 5E5-BB 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 D3-56 Switch 3, Port 6 On Switch 1

4-39 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 7D C4-B6-9F 7E5-BB D3-56 E5-BB D3-56 Switch 3, Port 6 On Switch 2

4-40 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 D C4-B6-9F Switch 3, Port 2 Switching Table Switch 3 PortStation 4A1-44-D5-1F-AA-4C 4B2-CD-13-5B-E4-65 2D C4-B6-9F 6E5-BB D3-56 E5-BB D3-56 Switch 3, Port 6 On Switch 3

4-41 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

4-42 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 D3-56

4-43 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.

4-44 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 D C4-B6-9F E5-BB D3-56

4-45 Figure 4-12: 802.1D Spanning Tree Protocol (STP) 生成树协议 Switch 1 Switch 2 Switch 3 A1-44-D5-1F-AA-4C B2-CD-13-5B-E4-65 D C4-B6-9F E5-BB D3-56 Activated Deactivated Normal Operation Loop, but Spanning Tree Protocol Deactivates One Link

4-46 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 D C4-B6-9F E5-BB D3-56 Deactivated Reactivated Switch 2 Fails

4-47 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)

4-49 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

4-50 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

4-51 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

4-52 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) 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 MAC FrameTagged MAC Frame Start-of-Frame Delimiter (1 Octet) Preamble (7 octets) Start-of-Frame Delimiter (1 Octet) Preamble (7 octets) Source Address (6 Octets)

4-53 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 MAC FrameTagged MAC Frame Length (2 Octets) Data Field (variable) PAD (If Needed) Frame Check Sequence (4 Octets) PAD (If Needed) Frame Check Sequence (4 Octets)

4-54 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.

4-55 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)

4-56 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.

4-57 Answer Question 13 c, page c d e f, page 213

Box: Hubs and Switches

4-59 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

4-60 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

4-61 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

4-62 Figure 4.11: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) 载波侦听多路访问 / 冲突检测 1. Carrier Sense Multiple Access (CSMA) –If a NIC wishes to transmit, it must listen for traffic If there is no traffic, the NIC may transmit If there is traffic, the NIC must wait to transmit until no traffic is being transmitted; then it may send

4-63 Figure 4.11: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) 2. Collision Detection (CD) –If there is a collision (by two or more stations transmitting at the same time), All NICs stop transmitting and wait for a random amount of time The first NIC that finishes its wait my transmit –but only if there is no traffic! –If there is traffic, the NIC must wait until there is no traffic

4-64 Figure 4.11: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) 3. Collision Detection (CD) –If there are multiple collisions, The random wait is increased each time After 16 collisions, the sending NIC discards the frame

4-65 Figure 4.11: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) CSMA/CD Recap –Hubs do not implement it when talking to stations –Switches do not implement it when talking to stations –NICs implement it CSMA/CD

4-66 Figure 4.11: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Recap: Three basic elements for describing CSMA/CD –CSMA—transmit only if line is clear; waiting otherwise –CD—handling a collision –CD—Handling of multiple collisions test

4-67 Network Interface Cards (NICs) PC Card NICInternal NIC

4-68 Network Interface Card (NIC) RJ-45 Jack PCI Connector Pins

4-69 Motherboard Intel D850MV Desktop Board PCI Slots For Expansion Boards (NICs, etc.) Slots for RAM Slot for Microprocessor (Pentium 4)

4-70 Internal NICs Fit into Motherboard Slots

4-71 Figure 4.12: Logical Link Control (LLC) Layer 逻辑链路控制层 LLC Standard is Used in All 802 LANs –All MAC layer standards interact only with at the layer above them –All internet layer standards interact only with at the layer below them LLC Ethernet MACWireless MAC IPIPX

4-72 Figure 4.12: Logical Link Control (LLC) Layer Optional Error Correction –Error detection, and retransmission –Almost never used –NICs rarely allow user to control LLC functionality

4-73 Ethernet Frame Organization Including LLC Here is how Ethernet frames are organized, including the LLC header Ethernet Header LLC Header IP or Other Packet Ethernet Trailer Ethernet Data Field: LLC Frame test

Purchasing Switches

4-75 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

4-76 Figure 4-18: Switching Matrix Mbps 1234 Port 1 to Port 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

4-77 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

4-78 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)

4-79 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

4-80 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

4-81 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

Box: Advanced Switch Purchase Considerations Physical and Electrical Features Box

4-83 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

4-84 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

4-85 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

4-86 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

4-87 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

4-88 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

4-89 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

4-90 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

4-91 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

4-92 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 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

Topics Covered

4-94 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

4-95 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

4-96 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

4-97 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