Lecture 3 Internet Core

What we have covered so far… Hardware view of Internet

Components of Internet

Structural view Client-server model Peer-to-peer model

Number Systems and bits/bytes

Decimal, Binary and Hexadecimal Bits/Bytes and Electronic Prefixes

• (Giga/mega/kilo) • (milli/micro/nano/pico)

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What we will cover today

Network core circuit switching packet switching

Performance evaluation in Internet

Delay, loss and throughput

Protocol layers, service models

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The Network Core

Internet: mesh of interconnected routers

How is data transferred through networks?

Two methodologies

Circuit switching Packet switching

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Network Core: Circuit Switching

End-end resources reserved for “call”

dedicated circuit per call:

like telephone net

dedicated bandwidth resources: no sharing

Guaranteed performance

Overhead: call setup required

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Network Core: Circuit Switching Total network resources (e.g., bandwidth)

divided into “pieces” pieces allocated to each call resource piece idle if not used by owning call (no

sharing)

dividing link bandwidth into “pieces”…HOW? frequency division multiplexing (FDM)

• Users use different frequency channels time division multiplexing (TDM)

• Users use different time slots

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Circuit Switching: FDM and TDM

FDM

frequency

time TDM

frequency

time

4 users Example:

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Numerical example 1

You need to send a file of size 640,000 bits to your friend. You are using a circuit-switched network with TDM. Suppose, the circuit-switch network link has a total bit rate of 1.536 Mbps (1Mb = 106 bits) and uses TDM with 24 timeslots each for one user. How long does it take you to send the file to your friend?

Let’s work it out!

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Numerical example 2

You need to send a file of size 640,000 bits to your friend. You are using a circuit-switched network with FDM. Suppose, the circuit-switch network link has a total bit rate of 16 Mbps (1Mb = 106 bits) and uses FDM with 8 frequency channels (each channel for one user). How long does it take you to send the file to your friend?

Let’s work it out!

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Network Core: Packet Switching

each end-end data stream divided into packets

user A, B packets share network resources

each packet uses full link bandwidth

resources used as needed

Circuit switching Bandwidth division into “pieces”

Dedicated allocation Resource reservation

No flexibility

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Packet Switching

A

B

C 100 Mb/s Ethernet

1.5 Mb/s

D E

queue of packets waiting for output

link

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Packet switching versus circuit switching

Adv.: Packet switching allows users to use the network dynamically! Lot of flexibility, dynamic sharing No idle resource wastage simpler, no call setup

Disadv.: No dedicated resources for each user With excessive users: Excessive congestion packet delay and loss: performance degrade

How do delay and loss occur in Internet/network?

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How do delay and loss occur? packets queue in router buffers store and forward: packets move one hop at a time

Router receives complete packet before forwarding packets queue, wait for turn…DELAY

A

B

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Four sources of packet delay

1. nodal processing: check bit errors determine output link

A

B

propagation

transmission

nodal processing queueing

2. queueing time waiting at output

link for transmission depends on congestion

level of router

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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

4. Propagation delay: d = length of physical link s = propagation speed in

medium (~2x108 m/sec) propagation delay = d/s

A

B

propagation

transmission

nodal processing queueing

Note: s and R are very different quantities!

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Total delay

dproc = processing delay typically a few microsecs or less

dqueue = queuing delay depends on congestion

dtrans = transmission delay = L/R, significant for low-speed links

dprop = propagation delay a few microsecs to hundreds of msecs

proptransqueueproctotal ddddd +++=

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Numerical example 3

Example: A wants to send a packet to B. The packet size is, L = 7.5 Mb (1 Mb = 106 bits). The link speed is, R = 1.5 Mbps. How long does it take to send the packet from A to B? Assume zero propagation delay.

Let’s work it out!

R

L

A B

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Numerical example 4

Example: A wants to send a packet to B. The packet size is, L = 7.5 Mb (1 Mb = 106 bits). The link speed is, R = 1.5 Mbps. How long does it take to send the packet from A to B? Assume zero propagation delay.

What if there are three packets from A?

Let’s work it out!

R R

L

A B

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Packet loss

queue (aka buffer) preceding link in buffer has finite capacity

packet arriving to full queue dropped (aka lost) lost packet may be retransmitted by previous

node, by source end system, or not at all

A

B

packet being transmitted

packet arriving to full buffer is lost

buffer (waiting area)

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Network performance: Throughput Throughput: rate at which information bits

transferred between sender/receiver

Rs Rs

Rs

Rc Rc

Rc

R

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Numerical example 5: Throughput

Rs Rs

Rs

Rc Rc

Rc

A

B Example: A has requested for a

packet (size 640,000 bits) from server B. The packet will come through an intermediate router C. It takes 0.1 second for the packet from B to C and 0.4 seconds from C to A. (Note: 1Mb=106 bits). Assume zero propagation delay. What is the throughput from B

to C? What is the throughput from C

to A? What is the average

throughout from B to A?

Let’s work it out!

C