Machines communicating 12: Communications Distributed Network - PDF document

Machines communicating 12: Communications Distributed Network (Internet backbone) Star (Ethernet) Bus ( may have terminator(s) and/or Bus Master ) (Ethernet) Links • Point-to-point Ring (Token Ring, Cambridge Ring) – Serial - two wires – Parallel – lots of wires • Broadcast • Wired • Wireless • Local-Area Networks (LANs) short distance high speed Broadcast (WiFi) • Wide-Area Networks (WANs) long distance lower speed • distinctions blurred with new technologies • Interplanetary network – backbone for space probes +others Tree (USB) Serial and parallel Things that communicate • Serial: bits sent down a wire one after the other • Motherboard to hard disk, CD etc. – Parallel (IDE ribbon cable – lots of wires) • Parallel: bits all set up together one per wire • (new “S-ATA” disks are serial, that’s the “S”) then a control signal tells the far end that the • Motherboard to old-style printer data is valid. – Parallel • USB, Firewire, PS/2 Mouse, keyboard , • Parallel communication used to be faster but RS232 connection to modem required bulky/expensive cables and – Serial connectors. 1

Protocol What goes down the wire? • “The rules & formalities of any procedure” • Telephone (speech) • If I want to communicate with you we have – An analogue signal – a varying voltage that represents your voice. to agree how to do it – Degrades over long or poor lines • Who speaks first? How do I signal I’ve – Can’t reconstruct original signal exactly finished and you can start? What language • Computer data do we speak? How do I indicate I’ve not heard or understood? – Digital information (0 and 1) • Bidding in Bridge; dance Modulation / Demodulation + • Just ‘sending bits’ down the wire i.e. trying to send a square wave down the wire doesn’t really work – it degrades and is inefficient. Instead you modulate a carrier = then de-modulate it at the far end • box that does this = Modem From: Introduction to Computer Communication Dr. D. Koren / University of Tel Aviv So now you can send bits Framing It is rather important to know where your data starts and • Actually your data will have structure ends … – Need to send data and control information “yadayadaMaryhadalittlelambitsfleecewaswhiteassnowzzz” – Bytes (groups of 8 bits) usually sent as .. Or: – Packets – groups of a few bytes up to maybe 2000 Caesar entered, with control information at the beginning and end. On his head a helmet – Why? On each foot a sandal. • Error recovery In his hand he held his trusty sword, to boot. • Flow control [“to boot” = “as well”] • Need framing convention – packet boundaries 2

Framing .. Errors happen Caesar entered on his head • You have to detect them and recover from them a helmet on each foot • Simplest error detection mechanism – parity bit chosen so even number of bits are ‘1’ a sandal in his hand he held – Not foolproof (2 errors?) his trusty sword to boot • Can’t recover data with a single redundant bit but if you have a protocol you can ask for it to be retransmitted. Protocol gives rules for recognising start and • Works but inefficient end of data Error Detection & Recovery Flow control • Data is sent in packets . The last bytes in the • What if the sending machine is faster than packet are a Cyclic Redundancy Check (CRC) – the receiver? a complex sum of all bytes in the packet. The sum is recomputed as the packet is received • Receiver may lose data (inefficient) and if the computed sum doesn’t match the CRC – Better to say “wait a bit!” when it gets an error has occurred. congested then “OK, off you go” when it • The protocol then gives a rule on how to request clears a retransmission. • This is flow control • Can include enough redundancy in the packet to recover the data (CD-ROM, deep space comms) Packet length Data Compression • Trade-off: • Sending long sequences of the same • Short: know quickly if something is wrong information is inefficient and recover quickly but high overheads • Send an example of the data plus a count • Long: more efficient but slower reaction • = Simple compression • Complex compression algorithms • Throughput versus response. – Video, streaming video, VoIP audio • File transfer requires high throughput – Compression-Decompression = codec • Interaction requires fast response 3

Getting packets across a network Addressing • You need to know where the packet is to • Now you know how to send data down a go. point-to-point link • Addressing is part of Internet Protocol (IP). • www.inf.ed.ac.uk – name • What about getting across a network? • 129.215.128.35 – address • Hierarchy of name servers is responsible for tables of names vs. addresses • Routers are responsible for getting packets to destination Circuit and packet switching Datagrams • Traditional wired telephone network is a “circuit • A datagram is a self-contained packet of switched” network. You request a circuit to the data like a letter – it is addressed with a to: far end then it is yours for the duration of the and from: address and is sent into the call. [actually it isn’t any more but you don’t see network to be sent to the far end. It will be that] • Computer networks are “packet switched” – your routed by the best available route at that data is sent as packets and shares connections instant, may not get to the far end at all with other packets like cars on the road. and if it does it may overtake packets • Wired and bus networks are “broadcast” – your which have been routed differently. packet has the network to itself when you send it but someone else might send at the same time TCP and IP Layered protocols • TCP – maintains reliable data stream by sending • “IP is responsible” for getting datagrams to (unreliable) datagrams the destination “best effort” • IP – responsible for getting datagrams to • Need something more for file transfer etc. destination if possible • TCP is responsible for turning this into a • Error recovery and flow control ensure reliable transmission down a single wire reliable data stream – keeping track of • Framing conventions determine how we missing and out-of-sequence packets, assemble bits into bytes and packets requesting retransmission etc. • Modulation techniques let us send data over wires, wireless and fibre 4