Ethernet LANs Chapter 4 Panko s Business Data Networks

Perspective n Ethernet is the dominant LAN technology n n n You need to know it well Basic Ethernet switching is very simple However, large Ethernet networks require more advanced knowledge 2

Ethernet History n Developed at Xerox Palo Alto Research Center in the 1970 s n n After a trip to the University of Hawai`i’s Alohanet project Taken over by the IEEE 802 LAN/MAN Standards Committee is in charge of LAN Standards n 802. 3 Working Group develops Ethernet standards n Other working groups create other standards n 3

Ethernet Physical Layer Standards

Figure 4 -1: Ethernet Physical Layer Standards Physical Layer Standard Speed Maximum Medium Run Length UTP 10 Base-T 100 Base-TX 1000 Base-T 10 Mbps* 100 meters 4 -pair Category 3 or better 100 Mbps 100 meters 4 -pair Category 5 or better 1, 000 Mbps 100 meters 4 -pair Category 5 or better *With autosensing, 100 Base-TX NICs and switches will slow to 10 Mbps for 10 Base-T devices. Often called 10/100 Ethernet 6

Figure 4 -1: Ethernet Physical Layer Standards, Continued Physical Layer Standard Speed Maximum Medium Run Length Optical Fiber 100 Base-FX 100 Mbps 2 km 62. 5/125 multimode, 1300 nm, switch 7

Perspective n n Access links to client stations today are dominated by 100 Base-TX Trunk links today are dominated by 1000 Base. SX Short trunk links, however, use UTP n Longer and faster trunk links use other fiber standards n 10

Figure 4 -1: Ethernet Physical Layer Standards, Continued n Notes: n For 10 GBase-x, LAN versions (R) transmit at 10 Gbps. WAN versions (W) transmit at 9. 95328 Gbps for carriage over SONET/SDH links (see Chapter 6) n The 40 Gbps Ethernet standards are still under preliminary development 14

Figure 4 -2: Baseband Versus Broadband Transmission Baseband Transmission Signal Source Transmitted Signal (Same) Transmission Medium Signal is injected directly into the transmission medium (wire, optical fiber) Inexpensive, so dominates wired LAN transmission technology 15

Figure 4 -2: Baseband Versus Broadband Transmission, Continued Broadband Transmission Modulated Signal Source Radio Tuner Radio Channel Signal is first modulated to a higher frequency, then sent in a radio channel Expensive but needed for radio-based networks 16

Figure 4 -3: Link Aggregation (Trunking) 100 Base-TX Switch Two links provide 200 Mbps of trunk capacity between the switches UTP Cord No need to buy a more expensive Gigabit Ethernet port Switch must support link aggregation (trunking) 100 Base-TX Switch 17

Figure 4 -4: Data Link Using Multiple Switches Original Signal Received Regenerated Signal Switches regenerate signals before sending them out; this removes errors 18

Figure 4 -4: Data Link Using Multiple Switches, Continued Received Original Received Regenerated Signal Received Signal Regenerated Signal Thanks to regeneration, signals can travel far across a series of switches 19

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

Figure 4 -5: Layering in 802 Networks Internet Layer Data Link Layer Logical Link Control Layer Governs aspects of the communication needed by all LANs, e. g. , error correction. These functions not used in practice. Media Access Control Layer Governs aspects of the communication specific to a particular LAN technology, e. g. , Ethernet, 802. 11 wireless LANs, etc. Physical Layer 23

Figure 4 -5: Layering in 802 Networks, Continued Internet Layer Data Link Layer TCP/IP Internet Layer Standards (IP, ARP, etc. ) Logical Link Control Layer Media Access Control Layer Physical Layer Other Internet Layer Standards (IPX, etc. ) 802. 2 Ethernet 802. 3 MAC Layer Standard Other MAC Standards (802. 5, 802. 11, etc. ) 1000 Base. SX Other Physical Layer Standards (802. 11, etc. ) 10 Base-T … 24

Figure 4 -6: The Ethernet Frame Field Preamble (7 Octets) 1010 … Start of Frame Delimiter (1 Octet) 10101011 Destination MAC Address (48 bits) Source MAC Address (48 bits) Computers use raw 48 -bit MAC addresses; Humans use Hexadecimal notation (A 1 -23 -9 C-AB-33 -53), 25

Figure 4 -6: The Ethernet Frame, Continued Field Length (2 Octets) Data Field (Variable Length) LLC Subheader (Usually 8 Octets) Packet (Variable Length) PAD Field Frame Check Sequence (4 Octets) Added if data field is less than 46 octets; length set to make data field plus PAD field 46 octets; Not added if data field is greater than 46 octets long. If an error is found, the frame is discarded. 26

Figure 4 -8: Multiswitch Ethernet LAN The Situation: Switch 2 Port 5 on Switch 1 to Port 3 on Switch 2 A 1… Sends to E 5… Port 7 on Switch 2 to Port 4 on Switch 3 Switch 1 Switch 3 C 3 -2 D-55 -3 B-A 9 -4 F Switch 2, Port 5 B 2 -CD-13 -5 B-E 4 -65 Switch 1, Port 7 A 1 -44 -D 5 -1 F-AA-4 C Switch 1, Port 2 D 4 -47 -55 -C 4 -B 6 -9 F Switch 3, Port 2 E 5 -BB-47 -21 -D 3 -56 Switch 3, Port 6 30

Figure 4 -8: Multi-Switch Ethernet LAN, Continued On Switch 1 Switch 2 Port 5 on Switch 1 to Port 3 on Switch 2 Switching Table Switch 1 Port Station 2 A 1 -44 -D 5 -1 F-AA-4 C 7 B 2 -CD-13 -5 B-E 4 -65 5 C 3 -2 D-55 -3 B-A 9 -4 F 5 D 4 -47 -55 -C 4 -B 6 -9 F 5 E 5 -BB-47 -21 -D 3 -56 B 2 -CD-13 -5 B-E 4 -65 Switch 1, Port 7 A 1 -44 -D 5 -1 F-AA-4 C Switch 1, Port 2 E 5 -BB-47 -21 -D 3 -56 Switch 3, Port 6 31

Figure 4 -8: Multi-Switch Ethernet LAN, Continued Switch 2 Port 5 on Switch 1 to Port 3 on Switch 2 Switch 1 C 3 -2 D-55 -3 B-A 9 -4 F Switch 2, Port 5 Switching Table Switch 2 Port Station 3 A 1 -44 -D 5 -1 F-AA-4 C 4 3 B 2 -CD-13 -5 B-E 4 -65 5 C 3 -2 D-55 -3 B-A 9 -4 F 7 D 4 -47 -55 -C 4 -B 6 -9 F 8 7 E 5 -BB-47 -21 -D 3 -56 On Switch 2 Port 7 on Switch 2 to Port 4 on Switch 3 E 5 -BB-47 -21 -D 3 -56 Switch 3, Port 6 32

Figure 4 -8: Multi-Switch Ethernet LAN, Continued On Switch 3 Switch 2 Switching Table Switch 3 Port Station 4 A 1 -44 -D 5 -1 F-AA-4 C 4 B 2 -CD-13 -5 B-E 4 -65 5 4 C 3 -2 D-55 -3 B-A 9 -4 F 2 D 4 -47 -55 -C 4 -B 6 -9 F 6 E 5 -BB-47 -21 -D 3 -56 A 1 -44 -D 5 -1 F-AA-4 C Switch 1, Port 2 Port 7 on Switch 2 to Port 4 on Switch 3 D 4 -55 -C 4 -B 6 -9 F Switch 3, Port 2 Switch 3 E 5 -BB-47 -21 -D 3 -56 Switch 3, Port 6 33

Figure 4 -9: Hub Versus Switch Operation Ethernet Hub Broadcasts Each Bit If A Is Transmitting to C, B Must Wait to Transmit X A B C D 34

Figure 4 -9: Hub Versus Switch Operation, Continued Ethernet Switch Sends Frame Out One Port. If A Is Transmitting to C, Frame Only Goes Out C’s Port. A B C D 35

Figure 4 -9: Hub Versus Switch Operation, Continued Ethernet Switch Sends Frame Out One Port If A Is Transmitting to C, B Can Transmit to D Simultaneously A B C D 36

Advanced Ethernet Considerations STP and RSTP VLANs Momentary Traffic Peaks

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 D Ethernet Switch F Ethernet Switch E Server X Client PC 1 Server Y 38

Figure 4 -10: Hierarchical Ethernet LAN, Continued n Only one possible path between stations Therefore only one entry per MAC address in switching table n The switch can find the one address quickly, with little effort n This makes Ethernet switches inexpensive per frame handled Port Station n Low cost has led 2 A 1 -44 -D 5 -1 F-AA-4 C to Ethernet’s 7 B 2 -CD-13 -5 B-E 4 -65 5 E 5 -BB-47 -21 -D 3 -56 LAN dominance n 39

Figure 4 -10: Hierarchical Ethernet LAN, Continued Core and Workgroup Switches Core Ethernet Switch A Core Ethernet Switch B Workgroup Ethernet Switch D Core Ethernet Switch C Workgroup Ethernet Switch F Workgroup Ethernet Switch E 40

Figure 4 -10: Hierarchical Ethernet LAN, Continued n Workgroup switches connect to stations via access lines n Core switches higher in the hierarchy connect switches to other switches via trunk lines n The core is the collection of all core switches n Core switches need more capacity than workgroup switches because they have to handle the traffic of many conversations instead of just a few 41

Figure 4 -11: Single Point of Failure in a Switch Hierarchy Switch Fails No Communication Switch 1 C 3 -2 D-55 -3 B-A 9 -4 F B 2 -CD-13 -5 B-E 4 -65 A 1 -44 -D 5 -1 F-AA-4 C Switch 2 No Communication Switch 3 D 4 -47 -55 -C 4 -B 6 -9 F E 5 -BB-47 -21 -D 3 -56 42

Figure 4 -12: 802. 1 D Spanning Tree Protocol Normal Operation Loop, but Spanning Tree Protocol Deactivates One Link Switch 2 Activated Switch 1 Activated Deactivated C 3 -2 D-55 -3 B-A 9 -4 F B 2 -CD-13 -5 B-E 4 -65 A 1 -44 -D 5 -1 F-AA-4 C Switch 3 D 4 -47 -55 -C 4 -B 6 -9 F E 5 -BB-47 -21 -D 3 -56 43

Figure 4 -12: 802. 1 D Spanning Tree Protocol, Continued Switch 2 Fails Deactivated Switch 2 Deactivated Reactivated Switch 1 C 3 -2 D-55 -3 B-A 9 -4 F B 2 -CD-13 -5 B-E 4 -65 A 1 -44 -D 5 -1 F-AA-4 C Switch 3 D 4 -47 -55 -C 4 -B 6 -9 F E 5 -BB-47 -21 -D 3 -56 44

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 45

Figure 4 -13: Virtual LAN (VLAN) with Ethernet Switches, Continued With VLANs, Broadcasts Only Go To a Server’s VLAN Clients; Less Latency Server Broadcast No No Client C on VLAN 1 Client A on VLAN 1 Client B on VLAN 2 Server D on VLAN 2 Server E on VLAN 1 46

Figure 4 -14: Tagged Ethernet Frame (Governed By 802. 1 Q) Basic 802. 3 MAC Frame By looking Tagged 802. 3 MAC Frame at the value Preamble (7 octets) in the 2 octets after Start-of-Frame Delimiter the (1 Octet) addresses, Destination Address the switch (6 Octets) can tell if this frame Source Address (6 Octets) is a basic frame (value less Tag Protocol ID (2 Octets) Length (2 Octets) 100000000 than 1, 500) Length of Data Field in or a tagged 81 -00 hex; 33, 024 decimal. Octets Larger than 1, 500, So not (value is 1, 500 (Decimal) Maximum a Length Field 33, 024). 47

Figure 4 -14: Tagged Ethernet Frame (Governed By 802. 1 Q), Continued Basic 802. 3 MAC Frame Tagged 802. 3 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) 48

Figure 4 -15: Handling Momentary Traffic Peaks with Overprovisioning and Priority Congestion and Latency Traffic Network Capacity Momentary Traffic Peak: Congestion and Latency Time 49

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 Time 50

Figure 4 -15: Handling Momentary Traffic Peaks with Overprovisioning and Priority, Continued Priority in Ethernet Traffic Network Capacity Momentary Peak High-Priority Traffic Goes Low-Priority Waits Time 51

Purchasing Switches

Figure 4 -16: Switch Purchasing Considerations n Number and Speeds of Ports n Decide on the number of ports needed and the speed of each n Often can buy a prebuilt switch with the right configuration n Modular switches can be configured with appropriate port modules before or after purchase 53

Figure 4 -16: Switch Purchasing Considerations, Continued n Switching Matrix Throughput (Figure 4 -17) n Aggregate throughput: total speed of switching matrix n Nonblocking capacity: switching matrix sufficient even if there is maximum input on all ports n Less than nonblocking capacity is workable n n For core switches, at least 80% For workgroup switches, at least 20% 54

Figure 4 -17: Switching Matrix 100 Mbps 1 2 3 4 100 Base-TX Input Ports Queue(s) Port 1 to Port 3 400 Mbps Aggregate Capacity to Be Nonblocking 1 2 3 4 Any-to-Any Switching Matrix 100 Base-TX Output Ports Note: Input Port 1 and Output Port 1 are the same port 55

Figure 4 -16: Switch Purchasing Considerations, Continued n Store-and-Forward Versus Cut-Through Switching (Figure 4 -18) n Store-and-forward Ethernet switches read whole frame before passing it on n Cut-through Ethernet switches read only some fields before passing it on n Perspective: Cut-through switches have less latency, but this is rarely important 56

Figure 4 -18: Store-and-Forward Versus Cut-Through Switching Preamble Start-of-Frame Delimiter Cut-Through Based On MAC Destination Address (14 Octets) Destination Address Source Address Store-and. Forward Processing Ends Here (Often Hundreds Of Bytes) Tag Fields if Present Cut-Through for Priority or VLANs (24 Octets) Length Data (and Perhaps PAD) Cyclical Redundancy Check Cut-Through at 64 Bytes (Not a Runt) 57

Figure 4 -19: Jitter n Variability in latency from cell to cell. Makes voice sound jittery High Jitter (High Variability in Latency) Low Jitter (Low Variability in Latency) 58

Figure 4 -16: Switch Purchasing Considerations, Continued n Manageability n Manager controls many managed switches (Figure 420: Managed Switches) n Polling to collect data and problem diagnosis n Fixing switches remotely by changing their configurations n Providing network administrator with summary performance data 59

Figure 4 -20: Managed Switches Get Data Requested Managed Switch Manager Command to Change Configuration Managed Switch 60

Figure 4 -16: Switch Purchasing Considerations, Continued n Manageability n Managed switches are substantially more expensive than unmanageable switches n To n purchase and even more to operate However, in large networks, the savings in labor costs and rapid response are worth it 61

Figure 4 -21: Physical and Electrical Features n Form Factor n Switches fit into standard 19 in (48 cm) wide equipment racks n Sometimes, racks are built into enclosed equipment cabinets n Switch heights usually are multiples of 1 U (1. 75 inches or 4. 4 cm) 19 inches (48 cm) 62

Figure 4 -21: Physical and Electrical Features, Continued n Port Flexibility n Fixed-port switches n No flexibility: number of ports is fixed n 1 U or 2 U tall n Most workgroup switches are fixed-port switches 63

Figure 4 -21: Physical and Electrical Features, Continued n Port Flexibility n Stackable Switches n Fixed number of ports n 1 U or 2 U tall n High-speed interconnect bus connects stacked switches n Ports can be added in increments as few as 12 64

Figure 4 -21: Physical and Electrical Features, Continued n Port Flexibility n Modular Switches n n n 1 U or 2 U tall Contain one or a few slots Each slot module contains 1 to 4 ports 65

Figure 4 -21: Physical and Electrical Features, Continued n Port Flexibility n Chassis switches n Several U tall n Contain several expansion slots n Each expansion board contains 6 to 12 slots n Most core switches are chassis switches 66

Figure 4 -21: Physical and Electrical Features, Continued n UTP Uplink Ports Normal Ethernet RJ-45 switch ports transmit on Pins 3 and 6 and listen on Pins 1 and 2 (NICs do the reverse) n If you connect two normal ports on different switches, they will not be able to communicate n Most switches have an uplink port, which transmits on Pins 1 and 2. You can connect a UTP uplink port on one switch to any normal port on a parent switch n 67

Figure 4 -21: Physical and Electrical Features, Continued n 802. 3 af brings electrical power over the station’s ordinary UTP cord Limited to 12. 95 watts (at 48 volts) n Sufficient for wireless access points (Chapter 5) n Sufficient for IP telephones (Chapter 6) n Not sufficient for computers n Automatic detection of compatible devices; will not send power to incompatible devices n 68