Computer Network No.3 (physical Layer) From Apcoms

The Network Core

Computer Networks

‰ mesh of interconnected ‰

Physical Layer Issues

routers the fundamental question: how is data transferred through the network? ™

™

circuit switching: dedicated circuit per call: telephone net packet-switching: data sent through network in discrete “chunks”

Maj Faisal Computer Science Department

Maj Faisal Computer Science Department

Network Core: Circuit Switching

Network Core: Circuit Switching ‰ network resources (e.g.,

End-end resources reserved for “call”

bandwidth) divided and allocated to calls (Multiplexing) ‰ Resources remain idle if ‰ Techniques for dividing not used by owning call network resources into (no sharing) “pieces” ™ frequency division ™ time division ™ Any other multiplexing techniques?

‰ link bandwidth, switch

capacity

‰ no sharing ‰ guaranteed performance ‰ call setup required

Maj Faisal Computer Science Department

Maj Faisal Computer Science Department

Circuit Switching: FDM and TDM

Network Core: Packet Switching each end-end data stream divided into packets ‰ packets from hosts share network resources ‰ each packet uses full link bandwidth for complete packet duration ‰ resources used as needed

Example: FDM

4 users frequency time

TDM

Bandwidth division into “pieces” Dedicated allocation Resource reservation

C A

frequency time

B Maj Faisal

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D Maj Faisal

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Network Core: Packet Switching resource contention: ‰ aggregate resource demand can exceed amount available ‰ congestion: packets queue-up, wait for link use ‰ store and forward: packets move one hop at a time ™

Node receives complete packet before forwarding

C

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?

A

™

D

B

™

bandwidth guarantees needed for audio/video apps QoS – Quality of Service (will be studied later in TCP)

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Maj Faisal Computer Science Department

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

Delay, Loss and Throughput

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)

Maj Faisal Computer Science Department

Maj Faisal Computer Science Department

How do loss and delay occur?

Four sources of packet delay

packets queue-up in router buffers ‰ packet arrival rate to link exceeds output link capacity ‰ packets queue-up, wait for turn

‰ 1. nodal processing: ™ check bit errors ™ determine output link

packet being transmitted (delay)

A

transmission

A B

free (available) buffers: arriving packets dropped (loss) if no free buffers Computer Science Department

propagation

B

packets queueing (delay)

‰ 2. queueing ™ time spent waiting at output link for transmission ™ depends on congestion level of router

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nodal processing

queueing Maj Faisal

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Delay in packet-switched networks 3. Transmission delay: ‰ R=link bandwidth (bps) ‰ L=packet length (bits) ‰ time to send bits into link = L/R

transmission

A

Nodal delay

4. Propagation delay:

d nodal = d proc + d queue + d trans + d prop

‰ d = length of physical link ‰ s = propagation speed in

medium (~2-3x108 m/sec)

‰ dproc = processing delay

‰ propagation delay = d/s

‰ dqueue = queuing delay

Note: s and R are very different quantities!

‰ dprop = propagation delay

‰ dtrans = transmission delay

propagation

B

nodal processing

queueing

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Computer Science Department

Maj Faisal Computer Science Department

“Real” Internet delays and routes

Packet loss

‰ What do “real” Internet delay & loss look like?

‰ queue (buffer) has finite capacity

‰ Traceroute program: provides delay

‰ when packet arrives to full queue, packet is

measurement from source to router along end-end Internet 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.

dropped (lost)

‰ Other sources of packet loss? ‰ lost packet may be retransmitted by source

(TCP), or not retransmitted at all (UDP)

3 probes

3 probes

buffer (waiting area)

A

3 probes

B

packet arriving to full buffer is lost

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Throughput

‰ throughput: rate (bits/time unit) at which

‰ Rs

bits transferred between sender/receiver ™

< Rc What is end-end throughput?

instantaneous: rate at given point in time average: rate over long(er) period of time

Rs bits/sec

‰ Rs

link capacity pipe that can carry bitsR(fluid) at rate s bits/sec (Rs bits/sec)

link that capacity pipe can carry Rc bits/sec (bits) fluid at rate (Rc bits/sec)

Rc bits/sec

bottleneck link link on end-end path that constrains end-end throughput

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Rc bits/sec

> Rc What is end-end throughput? Rs bits/sec

server, with server sends bits file ofinto F bits (fluid) pipe to send to client

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Throughput

™

packet being transmitted

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Throughput: Internet scenario

Network Taxonomy Telecommunication networks

‰ per-connection

end-end throughput: min(Rc,R,Rs) ‰ Rc or Rs often the bottleneck link ‰ Is it the case nowadays?

Rs

Rs Rs

Circuit-switched networks

Packet-switched networks

R Rc

Rc Rc

FDM

TDM

Networks with VCs

Datagram Networks

• Internet is a datagram / packet-switched network • Internet provides both connection-oriented (TCP) and connectionless services (UDP) to applications. Maj Faisal

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Transmission Media

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