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ATM, Infrastructure

ATM, Infrastructure. Getting around the world if you’re a network packet. How those layers work. Networks have up to 7 (8!) Layers: Application: software Presentation: translation Session: start and stop comms sessions Transport: make connection good enough Network: routing and switching

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ATM, Infrastructure

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  1. ATM, Infrastructure Getting around the world if you’re a network packet

  2. How those layers work • Networks have up to 7 (8!) Layers: • Application: software • Presentation: translation • Session: start and stop comms sessions • Transport: make connection good enough • Network: routing and switching • Data Link (2): Link control and access control (IEEE 802.2 LLC and MAC) • Physical: the wire, radio, etc.

  3. Where they happen • In the main computer software • Application, Presentation, Transport, some routing • In the computer’s hardware • Data Link (LLC and MAC), Physical • In the LAN or WAN • Routing, Transport, Session • In the global network • Routing, switching, big time!

  4. From Physical to Network • Data link controlled by network card, accesses physical network. • Sharing (“multiplexing”) • Collision detection • Hardware address (eg Ethernet/MAC) has to be mapped onto IP number! • ARP server - software on computers that announces and communicates IP numbers on LAN

  5. MAC/ARP is important, can go wrong! • Computers only occasionally announce their IP address • Switches need MAC to know it to send to right place - re-connections! • Hubs can make it hard to detect disconnections • Wireless systems need to switch and act like hubs, often.

  6. Going Local, Going Internet • If it’s a local machine, ARP tells me the MAC address: send it there (can tell from the IP number and subnet mask) • If it’s not a local machine: • Work out which gateway/router to send it to (IP number) • Work out the MAC address of the gateway (ARP!) • Send it THERE!

  7. Routing • Each computer can route a little: sending to the main routers on the LAN. • When a main router receives a packet, it does exactly the same thing the other computers do: • Work out which router to send it to • Send it via the MAC address (not always ethernet)!

  8. More Routing (IMPORTANT!) • Packets bounce from one network to another: Internetworking • Often don’t have enough bandwidth to transmit all the packets on: • Send those packets first that are important • Throw away less important packets when you have too much to do! • Quality of Service: making things work appropriately

  9. Even More Routing • Need to communicate which route to go, and make sure things don’t go in circles (RIP, BGP, and more) • The routing rules determine how the internet behaves. Very iffy in IPv4. • Video needs to get through fast • Audio musn’t vary too much in speed • Important stuff MUST get through! • How do you decide - they’re all alike!

  10. Quality of Service (QoS) • Bandwidth: how fast • Latency: how long (not the same as how fast) • Packet Loss: how often you drop the information. • Bandwidth, Packet Loss, Latency variation: how constant is all this?

  11. The Big Problem with IP • Packets all look alike - really need to analyze ‘em to work out what you should do with them. Often just let them all go through, or all die! • Packets all different sizes - hard to predict how to fit them all together in the right order to make things efficient.

  12. The Telco Solution: ATM • Asynchronous Transfer Mode • Chop up all the information into the same-sized little box: 53 bytes CELLS! • Forget source and destination: label by what path the cell takes • Set up path depending on what the user wants: QoS • Simple decisions to keep it moving: Only THREE layers!

  13. IP vs ATM • IP: I want to go from 1131 Barclay St to 105 W. Broadway. Route Route Route. • ATM: I’m going on path 10943 • 10943 = Take a left out of my door, turn right onto Thurlow, Turn left on Nelson, Right on Burrard, take the bridge, Turn right on Broadway. • “Fabric” remembers the path from the path numbers.

  14. QoS and ATM • I ask for how fast I want to go, where I want to go, how long I want to take going there, how much I’m willing to put up with variations • Fabric finds the path that matches my needs • ATM switches make sure the right cells, just going on the path number, get switched/dumped in the right way.

  15. IP over ATM • Big IP pipes are carried by defining virtual IP networks over ATM • Other protocols can also be carried. • A lot of the “broadcast” mode of IP is missing though. IP lets me send to more than one place at once. • ATM can be awfully expensive, but uses links way more efficiently.

  16. Other QoS stuff • Older systems like Frame: “synchronous” version of ATM. TDMA is one of these! Big for older satellite comms, links to remote parts of Canada, and used for communication with Mars by JPL and others. QoS big for space stuff! • RSVP: Resource reSerVation Protocol: trying to tell the routers how to treat packets • TOS: oldest and dumbest: simple way of labelling certain packets as more important than others.

  17. Why should I care? • Each jitter in video, or weirdness in sound for IP Telephony or Voice mode in Messenger, etc, is due to the QoS problems of IPv4. • Mail loops, denial of service attacks, etc: cases where we can’t make sure the important information gets through • Bad QoS wastes links, time, money: how much am I willing to pay?

  18. Big Pipes • Network QoS problems often solved by making pipes bigger: if everything gets through fast, everything is fine • Bit like saying we can turn the entire country into big highways. REALLY big highways. • What happens if you can’t afford big? • What happens if you can’t afford ATM? • What happens if you DIE without QoS?

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