Computer Networks 0100-Ethernet Gergely Windisch

Channel allocation problem Point to point communication is easy – direct link between the communicating parties – anyone can talk whenever they wish Broadcast communication – Not so easy – too many people might want to talk – who can? – if two messages overlap, both are destroyed

How do we allocate the channel to each user? – Static channel allocation - dedicated frequency for each user – good for a small number of users who transmit a large amount – like radio stations not so good for large number of users with burst traffic – Dynamic channel allocation channel is allocated to whoever wants to use it – better for large number of users, but some problems may arise – single channel – collision is bad (but observable) – carrier sense – no carrier sense » whether the sender can listen in on the channel first

ALOHA – 18% Slottet ALOHA – 35% Carrier Sense Multiple Access Protocol – goood – instead of just sending data whenever the urge drives us, let us listen first and if there is silence, then talk

Carrier Sense Multiple Access Protocol Listen first, talk later – but how? – if two computers want to send when a third is talking – they will collide – better: wait a random amount of time before sending Carrier Sense Multiple Access / Collision Detection – like above, but look out for collision and stop sending as soon as it is noted – the message is runined anyway, why waste bandwidth Basis of the classic Ethernet

History of ethernet Invented 40 years ago at Xerox

Ethernet topology Used to be a but Star nowdays

Transport medium Anything goes

CSMA/CD protocol Collisions should be avoided at all cost

Ethernet implementation Thick ethernet – 500 m – 100 hosts Thin ethernet – 100 m – 30 hosts UTP (switched ethernet)

Ethernet frames Bits are transferred, but the bits need some organization

Errors Error detection vs. error correction – error detection can detect errors and the offending message is retransmitted – error correction is better as faulty messages can be corrected, but at what price? Error detection is used on reliable connections, error correction is used on unreliable connections – on unreilable channels even the retransmitted messages will be corrupt – no use having them retransmitted Error correction codes – hamming code – reed solomon code Error detection codes – parity (normal and interleaved) – checksum – cyclic redundancy check (polinomial code)

Devices in an etherenet network hub – first there was the hub bridge – bridge connects two networks switch – switch is like a bridge, but with more legs – (they are essentially the same)

Interesting questions concerning switches What do we want from bridges/switches? – Let them be plug and play – They should send the data only to the appropriate port – When multiple switches are connected they should not form loops as loops are normally fun*, but not in this case When do we want them? – Now How do we do it? – Backward learning – Spanning tree protocol spanning tree is a type of graph without a cycle *: Source:

Switch backward learning Hash table stores computers and ports At first there are no entries in the hash table – messages destined at a host not in the hash table will be sent to all ports – whenever a computer sends a message the source port is extracted and stored Hash table entries are timestamped at each use, old entries are removed to keep the system dynamic and up-to-date Switching can be done as soon as the destination address is available – good thing it is the first one to arrive then – wormhole switching

Switch backward learning Protocol processing in the bridge

Spanning tree protocol Redundant magic Imagine two networks – each have their own switches, and the switches are connected to form an UBER LAN. What happens if a hamster chews the cable between the two? – disaster

Spanning tree protocol Redundant magic Imagine two networks – each have their own switches, and the switches are connected to form an UBER LAN. What happens if a hamster chews the cable between the two? – disaster Let’s connect the switches by multiple cables! – now we would need multiple hamsters, but as there is just one hamster to do the chewing, we are fine

Spanning tree protocol Redundant magic Imagine two networks – each have their own switches, and the switches are connected to form an UBER LAN. What happens if a hamster chews the cable between the two? – disaster Let’s connect the switches by multiple cables! – now we would need multiple hamsters, but as there is just one hamster to do the chewing, we are fine Or are we? – What happens if the machines start communicating?

Spanning tree protocol

How to build spanning tree – Periodically send out messages to all ports, and analyze messages coming from othter routers – Select a root bridge (based on the MAC – smallert MAC ID will be the root) – Find out the shortest paths to the root – Turn off all other links

Spanning tree protocol

Layers and devices Repeaters and hubs do not care about frames, they only care about voltages Standard allows 4 repeaters to extend cable length to 2.5 km – enhance the signal Hubs don’t enhance signal Devices extend the collision domain – CSMA/CD

Layers and devices Switches isolate networks and hosts – no collision domains, no CSMA/CD Originally bridges were created to connect different technologies (eg. Ethernet and token ring) – not used for that any more, but to connect similar LANs

Layers and devices Routers strip the frame and look at the payload, extract IP addresses and do their magic to forward packages. – We’ll look behind the curtains later this semester hopefully

Layers and devices High level stuff, will not even touch with a stick – Application gateway is an -sms gateway for example

Ethernet speeds Original Ethernet – – 10Mbps – Like heaven when first introduced, but even heaven gets boring in a few short years Fast Ethernet – 1995 (IEEE committee – 802.3u) – 100Mbps – compatible to the original in every aspect Gigabit Ethernet – 1999 – 802.3ab – 1000Mbps – compatible to the original in every aspect 10 Gigabit Ethernet, 40 Gigabit, 100 Gigabit – not so compatible anymore (hubs are not allowed)

VLAN VLANs are good – Topology and LAN membership can be independent of geographical location Definition Traffic filtering Access control

VLAN

VLAN ID The ethernet frame had to be modified – frames must be identified as part of a VLAN – but how? – vlan id

Wireless lan Ethernet Wireless network topologies Network security WPA/WPA2