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Network Devices & Wiring
Geert Jan de Groot
Network Wiring
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Half/full duplex Ethernet
Network Wiring
Autonegotiation
Structured wiring
Optics
Half duplex
• Ethernet is CSMA/CD (Carrier Sense
Multiple Access/Collision Detection)
• Only one station transmitting
simultaniously
• Listen before you transmit
• Listen while you transmit
Half duplex (2)
• If there is a collision, both transmitters will:
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Jam
Backoff (exponentially, randomly)
Re-try sending the packet
Minimal packet size: 64 bytes
• Collisions are harmless!
– Actually help scale / stabilize an ethernet
network
Full duplex
• Allows transmission/reception
simultaniously
– Impossible on older coax media
– Easy to implement on twisted pair/fiber media
• No collisions
• More bandwith available
Copper Ethernet media
• Coax (10 Mbit/sec)
– now obsolete
• Twisted pair
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Uses pair 1-2, 3-6, 4-5, 7-8
10 Mbit/sec (10BASE-T)
100 Mbit/sec
1000 Mbit/sec (‘gigabit ethernet’)
(10.000 Mbit/sec)
A few words on 10BASE-T
• Available everywhere today
• Uses pair 1-2, 3-6
– Unshielded twisted pairs
– Often miswired
• Typically half-duplex
– Full duplex possible
• Max length 100 meters!
10BASE-T cables
Normal cable
Used from host to hub
Crossover cable
Used hub to hub
or host to host
Some hubs have built-in crossovers
100 Mbit copper networks
• 3 standards:
– 100BASE-TX <<-- Winner
– 100BASE-T4
– 100VG-Anylan
• Requires Category 5 (‘CAT5’) cabling
– No more, no less
• 100BASE-TX runs on 2 wire pairs
– 1-2, 3-6, like 10BASE-T
• Cheap today
Faster: Gigabit Ethernet
• 1000 Mbit/sec, 1000BASE-T
• Still works on CAT5 cabling
– Tight fit
• Uses all 4 wire pairs
– In both directions
• About to get cheap and common
– $35 SRP for a gigabit network card
• Automatic crossover additional new feature
Gigabit Ethernet and packet size
• Faster ethernet historically kept 1500 byte
packet size
– Made bridging between speeds possible
• Part of gigabit industry wants bigger
packets
– Lower overhead, hence higher performance
• Bigger packets not part of standard
– Unresolved issues
– I believe wrong way to go
Future: 10 Gigabit Ethernet
• 10 Gigabit originally only on fiber
• 10 Gigabit on copper currently being
investigated by IEEE 802.3 standards
committee
Link pulses
• 10BASE-T sends link pulses when idle
– used to test integrity of link
– link light
• 100BASE-T uses faster link pulses
– Automatic detection between 10 and 100
possible
• Manual setting of half/full duplex
– Settings must match
Autonegotiation
• Method to automatically select ‘best’
transmission method between link partners
• Link pulse now becomes pulse train
• Automatically sets speed, duplex etc
Autonegotiation (2)
• ‘Parallel detection’ for devices w/o
autonegotiation
– 10BASE-T and 100BASE-TX only
– Only HDX
• Autonegotiation mandatory for gigabit
ethernet
• Technology is mature now
– Early chips had serious issues, so be aware
Autonegotiation failure mode
100Mbit FDX
Autonegotiation
• One end set to fixed 100Mbit, Full-Duplex
• Other end uses autonegotiation
• What happens?
Structured wiring
“Everything over the same wiring”
Wire Types
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Category 3: 10 Mbps
Category 4: 16 Mbps (for token ring)
Category 5: 100 Mbps / 1000 Mbps
Shielded or unshielded
Advice: use Cat 5 UTP (unshielded twisted
pair)
• Category 6, 7 and higher marketing hype
– Not official IEEE spec
Structured wiring pitfalls
• High installation cost
– so install enough the first time
• Use materials that are qualified for Cat 5
• Get guarantee from installer
Fiber optics
• Must use fiber between buildings
– Cable length restrictions
– Lightning protection
• Multi mode: short hauls
• Single mode: long hauls
Fiber optics (2)
• Different fiber diameters
• Different connector types
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ST generally ‘older’ 10mbit stuff
SC newer, generally 100mbit
VF45
Others
Fiber optics (3)
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10BASE-F for 10Mbps ethernet
100BASE-FX for 100Mbps fast ethernet
1000BASE-SX/LX for gigabit ethernet
No autonegotiation
– Colours are different
– HDX doesn’t make sense
• Advice: run more fibers than you need, but
don’t terminate them (yet)
Hubs, Switches and Routers
How do they work?
What are the differences?
Packet headers
Ether
IP
TCP
Data
Edst Esrc T
IPsrc IPdst
Network devices may use {ether, IP} headers to do it’s job
(sometimes in twisted ways)
Ethernet hub
Hub
Ethernet hub (2)
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Hub is layer 1 device
Hub does not filter packets
Whole hub is one collision domain
Daisychaining of hubs limited
Cheap
• Doesn’t exist for Gigabit Ethernet
Ethernet Switch
Switch
Ethernet Switch (2)
• Switch looks at ethernet headers (layer 2)
– ethernet to ethernet only
• Learns what addresses are connected to
which ports
• If destination of packet known, the packet is
only sent to the destination port
Ethernet Switch (3)
• Each port is a separate collision domain
– no daisychain limit
• Often one host per port
– high performance
• Security features
• Dualspeed ‘hub’ includes switch
Router
Router
Router
Router (2)
• Router works on IP header (layer 3)
• Can use almost any underlying media
– LAN or WAN
• Can have several ports
• Useful for long distance connections
(backbone)
• Must be configured
– IP addresses etc.
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