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The network core:

CSE 413: C omputer Network Circuit Switching and Packet Switching Networks Md. Kamrul Hasan 09-03-2010. The network core:. mesh of interconnected routers the fundamental question: how is data transferred through net? circuit switching: dedicated circuit per call: telephone net

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The network core:

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  1. CSE 413: Computer NetworkCircuit Switching and Packet Switching NetworksMd. Kamrul Hasan09-03-2010

  2. The network core: • mesh of interconnected routers • the fundamental question: how is data transferred through net? • circuit switching: dedicated circuit per call: telephone net • packet-switching: data sent through net in discrete “chunks” (packets) on shared media

  3. The network core:Circuit Switching End-to-end resources reserved for “call” • link bandwidth, switch capacity • dedicated resources: no sharing • circuit-like (guaranteed) performance • call setup required

  4. Circuit Switching Boston Switch LA Switch • It’s the method used by the telephone network • A call has three phases: • Establish circuit from end-to-end (“dialing”), • Communicate, • Close circuit (“tear down”). • If circuit not available: “busy signal” Caller Callee processing delay at switch propagation delay between caller and Boston switch DATA (1) (2) (3)

  5. Circuit Switching: Multiplexing/Demultiplexing Switch Frames 0 1 2 4 5 0 1 2 4 5 3 3 Slots = One way for sharing a circuit is TDM: • Time divided into frames and frames divided into slots • Relative slot position inside a frame determines which conversation the data belongs to • E.g., slot 0 belongs to the red conversation • Need synchronization between sender and receiver Lecture notes use the word “frame” for slot

  6. The network core:Circuit Switching network resources (e.g., bandwidth) divided into “pieces” • pieces allocated to calls • resource piece idle if not used by owning call (no sharing) • Consumers are charged on a per-minute basis • 2 ways of dividing the link bandwidth into “pieces” • frequency division multiplexing (FDM) • time division multiplexing (TDM)

  7. Example: 4 users Frequency Division Multiplexing (FDM) frequency time Time Division Multiplexing. (TDM) frequency time Circuit Switching: FDM and TDM

  8. Numerical example • How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network? • The link’s transmission rate = 1.536 Mbps • Each link uses TDM with 24 slots/sec • 500 msec to establish end-to-end circuit Figure it out … • Solution: • Bandwidth of circuit = 1.536/24 = 64 kbps • Time to send: 640 kbits/64 kbps + 0.5s = 10.5s What would be different if we use FDM instead of TDM?

  9. Common mistake/confusion : Question: • A) Express transmission rate of 1Kbits/sec in bits/sec • B) Express the file size of 1KBytes in bits Answer: • A) 1000 bits/sec (in throughput, K = 103=1000) • B) 1024 Bytes = 8192 bits (in data size, K = 210=1024) • Electronic speeds/times: K = 103, M = 106, G = 109 • Computer file/memory sizes: K = 210 , M = 220, G = 230 • Common computer notation: • b(bits) Kb, Mb, Gb • B(Bytes) KB, MB, GB • Better computer notation: • b(bits) Kib, Mib, Gib • B(Bytes) KiB, MiB, GiB

  10. Host 1 Host 2 Node 1 Node 2 Packet 1 Packet 1 Packet 1 Packet 2 Packet 2 Packet 2 Packet 3 Packet 3 Packet 3 Packet Switching • Used in the Internet • Data is sent in Packets (header contains control info, e.g., source and destination addresses) • Per-packet routing • At each node the entire packet is received, stored, and then forwarded (store-and-forward networks) • No capacity is allocated propagation delay between Host 1 & Node 2 Header Data transmission time of Packet 1 at Host 1 processing delay of Packet 1 at Node 2

  11. Packet Switching: Multiplexing/Demultiplexing Router Queue • Multiplex using a queue • Routers need memory/buffer • Demultiplex using information in packet header • Header has destination • Router has a routing table that contains information about which link to use to reach a destination

  12. Packet switching also show reordering Packets in a flow may not follow the same path (depends on routing as we will see later)  packets may be reordered Host C Host D Host A Node 1 Node 2 Node 3 Node 5 Host B Host E Node 7 Node 6 Node 4

  13. Bandwidth division into “pieces” Dedicated allocation Resource reservation The network core:Packet Switching • all streams share network resources • each packet uses full link bandwidth • resources used as needed • Resource contention: • aggregate resource demand can exceed amount available • congestion: packets queue, wait for link

  14. The network core:Packet switching • Data transmitted in small, independent pieces • Source divides outgoing messages into packets • Destination recovers original data • Each packet travels independently • Includes enough information for delivery • May follow different paths • Can be retransmitted if lost

  15. The network core:Functions of packet-switching networks • Packet construction • encode/package data at source • Packet transmission • send packet from source to destination • Packet interpretation • unpack/decode data from packet at destination • acknowledge receipt

  16. D E The network core:statistical multiplexing 100 Mb/s Ethernet C A statistical multiplexing 1.5 Mb/s B queue of packets waiting for output link statistical multiplexing Sequence of A & B packets does not have fixed pattern; shared on demand. Compare: in TDM, each host gets same slot (periodically) in FDM, each host gets same bandwidth (continuously)

  17. Differences Between Circuit & Packet Switching

  18. Network performance metrics End-to-end delay (nodal delay) : • Total time from initiating “send” (from source) to completed “receive” (at destination) Throughput : • Rate (bits/sec) at which bits are actually being transferred between sender/receiver • instantaneous: rate at given point in time • average: rate over longer period of time

  19. transmission A propagation B nodal processing queueing Four sources of packet delay • 1. nodal processing: • check bit errors • determine output link • 2. queueing delay • time waiting at output link for transmission • depends on congestion level of router

  20. transmission A propagation B nodal processing queueing Four sources of packet delay • 4. Propagation delay: • d = length of physical link (in meters) • s = propagation speed in medium (~2.5 x 108m/sec) • propagation delay = d/s • 3. Transmission delay: • R=link bandwidth (speed in bits per second, i.e. “bps”) • L=packet length (in bits) • transmission delay = L/R Note: R and s are very different quantities!

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