Ethernet LANs Chapter 4

4-2 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)

4-3 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

4-4 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

Ethernet Physical Layer Standards

4-6 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

4-7 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).

4-8 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

4-9 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 –Chosen over 40 Gbps 100 Gbps Ethernet standards development is just getting underway New Information

4-13 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

4-14 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.

4-15 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

4-16 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

4-17 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

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

4-19 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

4-20 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)

4-21 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

4-22 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.

4-23 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

4-24 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

4-25 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

4-26 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

4-27 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) 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.

4-28 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) 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.

4-29 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)

4-30 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

4-31 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

4-32 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

4-33 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

4-34 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

4-35 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.

4-36 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

4-37 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 D C4-B6-9F E5-BB D3-56 Activated Deactivated Normal Operation Loop, but Spanning Tree Protocol Deactivates One Link

4-38 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

4-39 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)

4-41 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

4-42 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

4-43 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

4-44 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)

4-45 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)

4-46 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.

4-47 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)

4-48 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.