Ethernet LANs

1
Ethernet LANs

• Chapter 4

2
Perspective

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

3
Ethernet History

• Developed at Xerox Palo Alto Research Center in
  the 1970s
• After a trip to the University of Hawaiis
  Alohanet project
• Taken over by the IEEE
• 802 LAN/MAN Standards Committee is in charge of
  LAN Standards
• 802.3 Working Group develops Ethernet standards
• Other working groups create other standards

4
Ethernet Standards are OSI Standards

• Ethernet standards are LAN standards
• LANs (and WANs) are single networks
• Single networks are based on Layer 1 (physical)
  and Layer 2 (data link) standards
• OSI dominates standards at these layers
• Ethernet standards are OSI standards
• Must be ratified by ISO, but this is a mere
  formality

5
Figure 4-1 Ethernet Physical Layer Standards
Physical Layer Standard
Medium
Maximum Run Length
Speed
UTP


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


100Base-FX
62.5/125 multimode, 1300 nm, switch
2 km
100 Mbps
7
Ethernet Physical Layer Standards, Continued
Physical Layer Standard
Medium
Maximum Run Length
Speed
1000Base-SX
62.5/125 micron multimode, 850 nm, 160
MHz-km modal bandwidth
220 m
1 Gbps
1000Base-SX
62.5/125 micron multimode, 850 nm, 200 MHz-km
275 m
1 Gbps
1000Base-SX
50/125 micron multimode, 850 nm, 400 MHz-km
500 m
1 Gbps
1000Base-SX
50/125 micron multimode, 850 nm 500 MHz-km
550 m
1 Gbps
Gigabit Ethernet, 850 nm, various core sizes and
modal bandwidths
8
Figure 4-1 Ethernet Physical Layer Standards,
Continued
Physical Layer Standard
Medium
Maximum Run Length
Speed
1000Base-LX
62.5/125 micron multimode, 1310 nm
550 m
1 Gbps
1000Base-LX
9/125 micron single mode, 1310 nm
5 km
1 Gbps
Gigabit Ethernet, 1300 nm, multimode versus
single mode
9
Perspective

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

10
Figure 4-1 Ethernet Physical Layer Standards,
Continued
Physical Layer Standard
Medium
Maximum Run Length
Speed
10GBase-SR/SW
62.5/125 micron multimode, 850 nm
65 m
10 Gbps
10GBase-LX4
62.5/125 micron multimode, 1300 nm, WDM with 4
lambdas
300 m
10 Gbps
10 Gbps Ethernet, multimode S 850 nm, L
1300 nm RLAN, WWAN
11
Figure 4-1 Ethernet Physical Layer Standards,
Continued
Physical Layer Standard
Medium
Maximum Run Length
Speed
10GBase-LR/LW
9/125 micron single mode, 1300 nm.
10 km
10 Gbps
10GBase-ER/EW
9/125 micron single mode, 1550 nm.
40 km
10 Gbps
10 Gbps Ethernet, for wide area networks L
1300 nm, E 1550 nm R LAN, W WAN
12
Figure 4-1 Ethernet Physical Layer Standards,
Continued
Physical Layer Standard
Medium
Maximum Run Length
Speed
40 Gbps Ethernet
9/125 micron single mode.
Under Development
40 Gbps
13
Figure 4-1 Ethernet Physical Layer Standards,
Continued

• Notes
• For 10GBase-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)
• The 40 Gbps Ethernet standards are still under
  preliminary development

14
Figure 4-2 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
15
Figure 4-2 Baseband Versus Broadband
Transmission, Continued
Broadband Transmission
Modulated Signal
Radio Channel
Source
Radio Tuner
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)
100Base-TX Switch
Two links provide 200 Mbps of trunk capacity
between the switches No need to buy a more
expensive Gigabit Ethernet port Switch must
support link aggregation (trunking)
UTP Cord
UTP Cord
100Base-TX Switch
17
Figure 4-4 Data Link Using Multiple Switches
Original Signal
Received Signal
Regenerated Signal
Switches regenerate signals before sending them
out this removes errors
18
Figure 4-4 Data Link Using Multiple Switches,
Continued
Received Signal
Original Signal
Received Signal
Received Signal
Regenerated Signal
Regenerated Signal
Thanks to regeneration, signals can travel far
acrossa series of switches
19
Figure 4-4 Data Link Using Multiple Switches,
Continued
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
100Base-TX (100 m maximum) Physical Link
1000Base-SX (220 m maximum) Physical Link
Each transmission line along the way has a
distance limit.
20
Figure 4-5 Layering in 802 Networks, Continued
TCP/IP Internet Layer Standards (IP, ARP, etc.)
Other Internet Layer Standards (IPX, etc.)
Internet Layer
802.2
Logical Link Control Layer
Data Link Layer
Ethernet 802.3 MAC Layer Standard
Media Access Control Layer
Other MAC Standards (802.5, 802.11, etc.)
Physical Layer
10Base-T
1000 Base- SX

Other Physical Layer Standards (802.11, etc.)
21
Figure 4-6 The Ethernet Frame
Field
Preamble (7 Octets) 10101010
Start of Frame Delimiter (1 Octet) 10101011
Computers use raw 48-bit MAC addresses Humans
use Hexadecimal notation (A1-23-9C-AB-33-53), Whic
h is discussed Later.
Destination MAC Address (48 bits)
Source MAC Address (48 bits)
22
Figure 4-6 The Ethernet Frame, Continued
Field
Length (2 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.
LLC Subheader (Usually 8 Octets)
Data Field (Variable Length)
Packet (Variable Length)
PAD Field
If an error is found, the frame is discarded.
Frame Check Sequence (4 Octets)
23
Figure 4-7 Hexadecimal Notation
4 Bits (Base 2)
Decimal (Base 10)
Hexadecimal (Base 16)
Begin Counting at Zero
0000
0
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
2416 combinations
24
Figure 4-7 Hexadecimal Notation, Continued
4 Bits (Base 2)
Decimal (Base 10)
Hexadecimal (Base 16)
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
25
Figure 4-7 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-BB-3C-D7-23-FF

26
Figure 4-8 Multiswitch Ethernet LAN
The Situation A1 Sends to E5
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
C3-2D-55-3B-A9-4F Switch 2, Port 5
B2-CD-13-5B-E4-65 Switch 1, Port 7
A1-44-D5-1F-AA-4C Switch 1, Port 2
E5-BB-47-21-D3-56 Switch 3, Port 6
D4-47-55-C4-B6-9F Switch 3, Port 2
27
Figure 4-8 Multi-Switch Ethernet LAN, Continued
On Switch 1
Switch 2

• Switching Table Switch 1
• Port Station
• 2 A1-44-D5-1F-AA-4C
• 7 B2-CD-13-5B-E4-65
• C3-2D-55-3B-A9-4F
• 5 D4-47-55-C4-B6-9F
• 5 E5-BB-47-21-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-47-21-D3-56 Switch 3, Port 6
28
Figure 4-8 Multi-Switch Ethernet LAN, Continued
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
C3-2D-55-3B-A9-4F Switch 2, Port 5
Switch 1
Switch 3

• Switching Table Switch 2
• Port Station
• A1-44-D5-1F-AA-4C
• 3 B2-CD-13-5B-E4-65
• C3-2D-55-3B-A9-4F
• D4-47-55-C4-B6-9F
• 7 E5-BB-47-21-D3-56

E5-BB-47-21-D3-56 Switch 3, Port 6
29
Figure 4-8 Multi-Switch Ethernet LAN, Continued
On Switch 3
Switch 2
Port 7 on Switch 2 to Port 4 on Switch 3

• Switching Table Switch 3
• Port Station
• 4 A1-44-D5-1F-AA-4C
• B2-CD-13-5B-E4-65
• 4 C3-2D-55-3B-A9-4F
• 2 D4-47-55-C4-B6-9F
• 6 E5-BB-47-21-D3-56

Switch 3
A1-44-D5-1F-AA-4C Switch 1, Port 2
D4-55-C4-B6-9F Switch 3, Port 2
E5-BB-47-21-D3-56 Switch 3, Port 6
30
Figure 4-9 Hub Versus Switch Operation
Ethernet Hub
Hub Broadcasts Each Bit If A Is Transmitting to
C, B Must Wait to Transmit
X
C
D
A
B
31
Figure 4-9 Hub Versus Switch Operation, Continued
Ethernet Switch
Switch Sends Frame Out One Port. If A Is
Transmitting to C, Frame Only Goes Out Cs Port.
C
D
A
B
32
Figure 4-9 Hub Versus Switch Operation, Continued
Ethernet Switch
Switch Sends Frame Out One Port If A Is
Transmitting to C, B Can Transmit to
D Simultaneously
C
D
A
B
33
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 PC1
34
Figure 4-10 Hierarchical Ethernet LAN, Continued

• Only one possible path between stations
• Therefore only one entry per MAC address in
  switching table
• The switch can find the one address quickly, with
  little effort
• This makes Ethernet switches inexpensive per
  frame handled
• Low cost has ledto EthernetsLAN dominance

Port Station
2 A1-44-D5-1F-AA-4C 7 B2-CD-13-5B-E4-65 5 E5-BB-4
7-21-D3-56
35
Figure 4-10 Hierarchical Ethernet LAN, Continued
Core and Workgroup Switches
Core
Core Ethernet Switch A
Core Ethernet Switch C
Core Ethernet Switch B
Workgroup Ethernet Switch F
Workgroup Ethernet Switch D
Workgroup Ethernet Switch E
36
Figure 4-10 Hierarchical Ethernet LAN, Continued

• Workgroup switches connect to stations via access
  lines
• Core switches higher in the hierarchy connect
  switches to other switches via trunk lines
• The core is the collection of all core switches
• Core switches need more capacity than workgroup
  switches because they have to handle the traffic
  of many conversations instead of just a few

37
Figure 4-11 Single Point of Failure in a Switch
Hierarchy
Switch Fails
Switch 2
No Communication
No Communication
C3-2D-55-3B-A9-4F
Switch 1
Switch 3
B2-CD-13-5B-E4-65
D4-47-55-C4-B6-9F
E5-BB-47-21-D3-56
A1-44-D5-1F-AA-4C
38
Figure 4-12 802.1D Spanning Tree Protocol
Normal Operation
Loop, but Spanning Tree Protocol Deactivates One
Link
Activated
Switch 2
Activated
Deactivated
C3-2D-55-3B-A9-4F
Switch 1
Switch 3
B2-CD-13-5B-E4-65
D4-47-55-C4-B6-9F
E5-BB-47-21-D3-56
A1-44-D5-1F-AA-4C
39
Figure 4-13 Virtual LAN (VLAN) with Ethernet
Switches
Server Broadcasting without VLANS
Servers Sometimes Broadcast Goes To All
Stations Latency Results
40
Figure 4-13 Virtual LAN (VLAN) with Ethernet
Switches, Continued
Server Broadcasting with VLANS
With VLANs, Broadcasts Only Go To a Servers
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
41
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 802.3 MAC Frame
Tagged 802.3 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) 1000000100000000 81-00
hex 33,024 decimal. Larger than 1,500, So not a
Length Field
42
Figure 4-14 Tagged Ethernet Frame (Governed By
802.1Q), Continued
Basic 802.3 MAC Frame
Tagged 802.3 MAC Frame
Tag Control Information (2 Octets) Priority Level
(0-7) (3 bits) VLAN ID (12 bits) 1 other bit
Data Field (variable)
PAD (If Needed)
Length (2 Octets)
Data Field (variable)
Frame Check Sequence (4 Octets)
PAD (If Needed)
Frame Check Sequence (4 Octets)
43
Figure 4-15 Handling Momentary Traffic Peaks
with Overprovisioning and Priority
Congestion and Latency
Traffic
Momentary Traffic Peak Congestion and Latency
Network Capacity
Time
44
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
45
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
Time
46
Figure 4-16 Switch Purchasing Considerations

• Number and Speeds of Ports
• Decide on the number of ports needed and the
  speed of each
• Often can buy a prebuilt switch with the right
  configuration
• Modular switches can be configured with
  appropriate port modules before or after purchase

47
Figure 4-16 Switch Purchasing Considerations,
Continued

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

48
Figure 4-17 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
Input Queue(s)
100Base-TX Input Ports
100Base-TX Output Ports
1
2
3
4
Note Input Port 1 and Output Port 1 are the same
port
49
Figure 4-16 Switch Purchasing Considerations,
Continued

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

50
Figure 4-18 Store-and-Forward Versus Cut-Through
Switching
Cut-Through Based On MAC Destination Address (14
Octets)
Preamble
Start-of-Frame Delimiter
Destination Address
Source Address
Cut-Through for Priority or VLANs (24 Octets)
Tag Fields if Present
Store-and- Forward Processing Ends
Here (Often Hundreds Of Bytes)
Length
Data (and Perhaps PAD)
Cut-Through at 64 Bytes (Not a Runt)
Cyclical Redundancy Check
51
Figure 4-19 Jitter

• Jitter
• Variability in latency from cell to cell. Makes
  voice sound jittery

High Jitter (High Variability in Latency)
Low Jitter (Low Variability in Latency)
52
Figure 4-16 Switch Purchasing Considerations,
Continued

• Manageability
• Manager controls many managed switches (Figure
  4-20 Managed Switches)
• Polling to collect data and problem diagnosis
• Fixing switches remotely by changing their
  configurations
• Providing network administrator with summary
  performance data

53
Figure 4-20 Managed Switches
Get Data
Data Requested
Managed Switch
Manager
Command to Change Configuration
Managed Switch
54
Figure 4-16 Switch Purchasing Considerations,
Continued

• Manageability
• Managed switches are substantially more expensive
  than unmanageable switches
• To purchase and even more to operate
• However, in large networks, the savings in labor
  costs and rapid response are worth it

55
Figure 4-21 Physical and Electrical Features

• Form Factor
• Switches fit into standard 19 in (48 cm) wide
  equipment racks
• Sometimes, racks are built into enclosed
  equipment cabinets
• Switch heights usually are multiples of 1U (1.75
  inches or 4.4 cm)

19 inches (48 cm)
56
Figure 4-21 Physical and Electrical Features,
Continued

• Port Flexibility
• Fixed-port switches
• No flexibility number of ports is fixed
• 1U or 2U tall
• Most workgroup switches are fixed-port switches

57
Figure 4-21 Physical and Electrical Features,
Continued

• Port Flexibility
• Stackable Switches
• Fixed number of ports
• 1U or 2U tall
• High-speed interconnect bus connects stacked
  switches
• Ports can be added in increments as few as 12

58
Figure 4-21 Physical and Electrical Features,
Continued

• Port Flexibility
• Chassis switches
• Several U tall
• Contain several expansion slots
• Each expansion board contains 6 to 12 slots
• Most core switches are chassis switches

59
Figure 4-21 Physical and Electrical Features,
Continued

• 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)
• If you connect two normal ports on different
  switches, they will not be able to communicate
• 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

60
Figure 4-21 Physical and Electrical Features,
Continued

• 802.3af brings electrical power over the
  stations ordinary UTP cord
• Limited to 12.95 watts (at 48 volts)
• Sufficient for wireless access points (Chapter 5)
• Sufficient for IP telephones (Chapter 6)
• Not sufficient for computers
• Automatic detection of compatible devices will
  not send power to incompatible devices

61
Topics Covered

• Who develops Ethernet standards?
• Many physical layer standards (100Base-TX,
  1000Base-SX, etc.)
• Baseband versus broadband transmission
• Link aggregation
• Switch signal regeneration allows maximum
  distances spanning several UTP and fiber links

62
Topics Covered

• MAC and LLC layers
• Ethernet Frame
• Preamble and Start of Frame Delimiter fields
• 48-bit Source and Destination Address fields
• Length field (length of data field)
• Data field
• LLC subheader
• Packet
• PAD if needed to make data field PAD 64 bits
  long

63
Topics Covered

• Ethernet Frame
• Frame check sequence field
• Discard if detect error unreliable
• Hexadecimal Notation
• For humans, not computers
• Multi-Switch LAN Operation with Switching Tables

64
Topics Covered

• Hubs versus Switches
• Hierarchical Topology
• Only one possible path between any two end
  stations
• Makes switching decisions easy and fast
• This makes the cost per frame handled low
• Key to Ethernets LAN dominance
• Core and Workgroup Switches

65
Topics Covered

• VLANs to reduce congestion due to server
  broadcasting
• Handling Momentary Traffic Peaks
• Overprovisioningleast expensive Ethernet choice
  today
• Priority is more efficient but more expensive to
  do
• Tagged Frames for VLANs and Priority

66
Topics Covered

• Switch Purchasing Decisions
• Number and speeds of ports
• Switch matrix capacity and nonblocking switches
• Store-and-forward versus cut-though switches
• Jitter
• Manageability
• Form factor (U)
• Port flexibility
• UTP uplink ports
• 802.3af for electrical power