computer network no3 physical layer from apcoms [ uandistar ]
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Computer Networks
Physical Layer Issues
The Network Core
‰ mesh ofinterconnected routers
‰ the fundamentalquestion: how is datatransferred through thenetwork?
™ circuit switching:dedicated circuit per call:telephone net
™ packet-switching: datasent through network indiscrete “chunks”
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Network Core: Circuit Switching Network Core: Circuit Switching
End-end resourcesreserved for “call”
‰ link bandwidth,switch capacity
‰ no sharing‰ guaranteedperformance‰ call setup required
‰ network resources(e.g., bandwidth) dividedand allocated to calls(Multiplexing)
‰ Resources remain idleif not used by owning call(no sharing)
‰ Techniques for dividingnetwork resources into“pieces”™ frequency division™ time division™ Any other
multiplexingtechniques?
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Circuit Switching: FDM and TDMExample:
Network Core: Packet Switchingeach end-end data stream
FDM
TDM
frequency
frequency
time
4 users divided into packets‰ packets from hostsshare
network resources‰ each packet uses full
link bandwidth forcomplete packet duration
‰ resources used asneeded
A
Bandwidth division into “pieces”Dedicated allocation Resource
reservation
C
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BComputer Science Department
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Network Core: Packet Switchingresource contention:‰ aggregate resource demand can exceed amount
available‰ congestion: packets queue-up, wait for link use‰ store and forward: packets move one hop at atime
™ Node receives complete packet before forwarding
C
A
Packet switching versus circuit switching
Is packet switching a “clear winner?”
‰ Great for bursty data™ resource sharing™ simpler, no call setup
‰ Excessive congestion: packet delay and loss™ protocols needed for reliable data transfer,
congestion control‰ Q: How to provide circuit-like behavior for a
packet switched network?™ bandwidth guarantees needed for audio/video apps™ QoS – Quality of Service (will be studied later in TCP)
B D
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Packet-switched networks: Types‰ Goal: move packets from source to destination‰ Two types of packet switch networks‰ datagram network:
™ destination address in packet determines next hop™ routes may change during session™ analogy: driving, asking directions
‰ virtual circuit network:™ each packet carries tag (virtual circuit ID), tag
determines next hop™ fixed path determined at call setup time, remains
fixed throughout the session/call™ Pro: routers can perform resource reservation™ Con: routers maintain per-call state (complex)
Delay, Loss and Throughput
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How do loss and delay occur? Four sources of packet delaypackets queue-up in router buffers‰ packet arrival rate to link exceeds output linkcapacity‰ packets queue-up, wait for turn
packet being transmitted (delay)
‰ 1. nodalprocessing:
™ check bit errors™ determine outputlink
‰ 2. queueing™ time spent waiting
at output link fortransmission
™ depends oncongestion level ofrouter
transmissionA A propagation
B
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packets queueing (delay)free (available) buffers: arriving packetsdropped (loss) if no free buffers
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B
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nodalprocessing queueing
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link capacityRs bits/sec
link capacityRc bits/sec
server, withfile of F bits
to send to client
Delay in packet-switched networks Nodal delaydnodal = dproc + dqueue + d trans + dprop3. Transmission delay:
‰ R=link bandwidth(bps)‰ L=packet length(bits)‰ time to send bits
into link = L/R
transmission
4. Propagation delay:‰ d = length of physicallink‰ s = propagation speed in
medium (~2-3x108 m/sec)‰ propagation delay = d/s
Note: s and R are verydifferent quantities!
‰ dproc = processing delay‰ dqueue = queuing delay‰ dtrans = transmission delay‰ dprop = propagation delay
A propagation
B
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nodalprocessing queueing Maj Faisal
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“Real” Internet delays and routes
‰ What do “real” Internet delay & loss look like?‰ Traceroute program: provides delay
measurement from source to router along end-endInternet path towards destination. For all i:™ sends three packets that will reach router i on
path towards destination™ router i will return packets to sender™ sender times interval between transmission and reply.
Packet loss
‰ queue (buffer) has finite capacity‰ when packet arrives to full queue, packet
is dropped (lost)‰ Other sources of packet loss?‰ lost packet may be retransmitted by source
(TCP), or not retransmitted at all (UDP)
buffer
3 probes 3 probesA (waiting area) packet being transmitted
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3 probes
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B
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packet arriving tofull buffer is lost Maj Faisal
Throughput‰ throughput: rate (bits/time unit) at which
bits transferred between sender/receiver™ instantaneous: rate at given point in time™ average: rate over long(er) period of time
Throughput‰ Rs < Rc What is end-end throughput?
Rs bits/sec Rc bits/sec
‰ Rs > Rc What is end-end throughput?
server sends bits(fluid) into pipe
pipe that can carrybits (fluid) at rate
(Rs bits/sec)
pipe that can carry(bits) fluid at rate
(Rc bits/sec)
Rs bits/sec Rc bits/sec
bottleneck linklink on end-end path that constrains end-end throughput
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Rs
Throughput: Internet scenario Network TaxonomyTelecommunication
networks
Rs‰ per-connection
end-end Rs
throughput:min(R ,R,R ) R
Circuit-switchednetworks
Packet-switchednetworks
c sNetworks Datagram
‰ Rc or Rs often the Rc Rcbottleneck link Rc
FDM TDMwith VCs Networks
‰ Is it the casenowadays?
• Internet is a datagram / packet-switched network• Internet provides both connection-oriented (TCP) andconnectionless services (UDP) to applications.
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Transmission Media
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