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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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slide1
CSE 413: Computer NetworkCircuit Switching and Packet Switching NetworksMd. Kamrul Hasan09-03-2010
slide2

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
slide3

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
slide4

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)

slide5

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

slide6

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)
slide7

Example:

4 users

Frequency Division Multiplexing (FDM)

frequency

time

Time Division Multiplexing. (TDM)

frequency

time

Circuit Switching: FDM and TDM

slide8

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?

slide9

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
slide10

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

slide11

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
slide12

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

slide13

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
slide14

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
slide15

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
slide16

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)

slide18

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
slide19

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
slide20

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!