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Network Devices
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Talk about network boxes
Packet headers
Different ethernet media
Different devices
Wires
Fiberlinks
Fast ethernet / gigabit ethernet
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)
Types of ethernet addresses
• Use ethernet addresses to be able to filter
packets in ethernet receiver hardware
(performance)
• 3 types of addresses:
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Unicast: single host
Multicast: group of hosts
Broadcast: all hosts
(tell by lsb of first byte of address)
Collisions (1)
• Ethernet is CSMA/CD (Carrier Sense
Multiple Access/Collision Detection)
• Listen before you transmit
• Listen while you transmit
Collisions (2)
• If there is a collision, both transmitters will:
– Jam
– Backoff (exponentially, randomly)
– Re-try sending the packet
• Minimal packet size: 64 bytes
• Collisions are harmless!
– Actually help scale / stabilize an ethernet
network
Thick ethernet (10base5)
10base5
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Thick, rigid cable
Separate transceivers, AUI cable
Inflexible
Vampire clamps (unreliable)
Cables do fall off
10base5 not used much anymore
AUI connector itself very flimsy
Thin ethernet (10base2/BNC)
10base2 (2)
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Thin cable (RG58/u) and BNC connectors
Max 185 metres
“Christmas tree problem”
Very cheap
Very unreliable
Getting obsolete, even in SOHO market
10base-T (TP)
Hub
10base-T (2)
• Classic ethernet used
coax
• Classic ethernet
daisychained
• Phone system uses
twisted pair
• Phone system starshaped
• 10base-T applies ‘phone culture’ to computers
10base-T (3)
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Hub is layer 1 device
Individual link integrity indicators
max 100 metres
Hub does not filter packets
Whole hub is one collision domain
Daisychaining of hubs limited
Cheap
•UTP ethernet (10baseT)
• Unshielded twisted pairs
• Two pairs of wires
– one pair transmit
– one pair receive
• If idle, send link pulses
• Simultaneous transmit/receive possible
without collisions
– Full duplex operation possible
•10baseT cables
Normal cable
Used from host to hub
Crossover cable
Used hub to hub
or host to host
Some hubs have built-in crossovers
•n-way autonegotiation
• link pulses now encoded
– carry information about sender’s capabilities
• full/half duplex, 10Mbps/100Mbps
• Announce your own capabilities
• Receive neighbour’s capabilities
• Automatically choose highest common set
of capabilities
• Parallel detection of link pulses for
backward compatibility
n-way autonegotiation
• Technology is mature now
– Early chips had serious issues however, so be
aware
• Failure scenario:
– Forced full-duplex on one side
– Autonegotiation on other side
• Causes full-duplex/half-duplex mismatch
Ethernet Repeaters
Repeater
Repeaters (2)
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Layer 1 device
‘Amplifies’ the ethernet signal
Allows for multiple ethernet segments
Does not filter packets
Whole ethernet still one collision domain
Number of repeaters on same ethernet
limited
Bridges
Bridge
A
B
C
D
E
F
Bridges (2)
• Layer 2 device
• Bridges look at ethernet address:
– left: A, B, C
– right: D, E, F
• Will retransmit packet from A to D
• Will not retransmit packet from A to B
Bridges (3)
• Left and right are separate collision
domains (less collisions)
• Store-and-forward: will wait for the whole
packet to arrive completely (forward delay)
• Not used much these days anymore
• Often used approach for dualspeed (10/100)
hubs
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
– like a bridge
• Cut-through forwarding
– starts output as soon as headers are known
•Ethernet Switch (3)
• Each port is a separate collision domain
• Usually multiple ports
• Often one host per port
– high performance
– works like a very expensive hub
• Security features
•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.
•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
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
– ST generally ‘older’ 10mbit stuff
– SC newer, generally 100mbit
– VF45
Fiber optics (3)
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10BaseF for 10Mbps ethernet
100BaseFX for 100Mbps fast ethernet
1000BaseSX for gigabit ethernet
Advice: run more fibers than you need, but
don’t terminate them (yet)
Faster ethernet
• Go from 10mbit/s to 100mbit/s
• 3 competing standards:
– 100base-TX
– 100base-T4
– 100VG-Anylan
Fast ethernet: the losers
• 100base-T4
– 100 mbit over cat3, 4 wire pairs (8 wires)
– One wire pair fixed in each direction,
two pairs can ‘switch’ direction
– Half-duplex only
• 100VG-Anylan
– 100mbit over cat3
Fast ethernet: the winner
• 100base-TX
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100 mbit over 2 wirepairs (just like 10base-T)
Requires cat5 wiring
Can run full-duplex
Defacto standard today
Very small price difference with 10mbit-only
equipment these days
– Has clearly won over 100baseT4 and 100VGAnylan by now
Fast ethernet over fiber
• 100base-FX for fiber
• No auto-negotiation, no speed selection
Still faster: gigabit ethernet
• 1 gbit/sec (100 times faster then 10mbit)
• Standard for fiber well established
(1000base-SX)
• Standard for copper defined recently, not
much equipment available yet
– Will run cat5, so don’t buy cat6/7/whatever
Gigabit ethernet
• Non-mature first generation products died
out now
– Firstgen products generally lack performance
• Requires special hardware to drive it
(64-bit PCI)
• Generally only full-duplex operation
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
10gbit ethernet
• Currently IEEE study group
• Pie-in-the-sky, no products yet