Lecture 2 Introduction 1-1

Chapter 1: roadmap

1.1 What is the Internet?1.2 Network edge

end systems, access networks, links

1.3 Network core circuit switching, packet switching

1.4 Delay, loss and throughput in packet-switched networks

1.5 Protocol layers, service models1.6 Networks under attack: security

Lecture 2 Introduction 1-2

The Network Core

Internet: mesh of interconnected routers

How is data transferred through net? circuit switching:

dedicated circuit per call: telephone net

packet-switching: data sent thru net in discrete “chunks”

Lecture 2 Introduction 1-3

Network Core: Circuit Switching

End-end resources reserved for “call”

link bandwidth, switch capacity

dedicated resources: no sharing

circuit-like (guaranteed) performance

call setup required

Lecture 2 Introduction 1-4

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)

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

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

• Users use different time slots

Lecture 2 Introduction 1-5

Circuit Switching: FDM and TDM

FDM

frequency

time

TDM

frequency

time

4 users

Example:

Lecture 2 Introduction 1-6

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 bit rate of 1.536 Mbps (1Mb = 106 bits) and uses TDM with 24 slots. How long does it take you to send the file to your friend?

Let’s work it out!

Lecture 2 Introduction 1-7

Packet Switching

A

B

C100 Mb/sEthernet

1.5 Mb/s

D E

queue of packetswaiting for output

link

Lecture 2 Introduction 1-8

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

resource contention: aggregate resource

demand can exceed amount available

congestion: packets queue, wait for link use

store and forward: packets move one hop at a time Node receives complete

packet before forwarding

Bandwidth division into “pieces”

Dedicated allocationResource reservation

Lecture 2 Introduction 1-9

Packet switching versus circuit switching Packet switching allows users to use the network

dynamically! resource sharing simpler, no call setup

With excessive users: Excessive congestion packet delay and loss

What are delay and loss in Internet/network?

Lecture 2 Introduction 1-10

Take home messages

Think, what would be the problem if excessive number of users are trying to access a circuit switch network?

Advantages and disadvantages between circuit-switch and packet-switch networks…

Lecture 2 Introduction 1-11

How do loss and delay 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

packet being transmitted (delay)

packets queueing (delay)

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

Lecture 2 Introduction 1-12

Four sources of packet delay

1. nodal processing: check bit errors determine output link

A

B

propagation

transmission

nodalprocessing queueing

2. queueing time waiting at output

link for transmission depends on

congestion level of router

Lecture 2 Introduction 1-13

Delay in packet-switched networks3. 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

nodalprocessing queueing

Note: s and R are very different quantities!

Lecture 2 Introduction 1-14

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

Lecture 2 Introduction 1-15

Numerical example 2

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 R R

L

A B

Lecture 2 Introduction 1-16

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 allA

B

packet being transmitted

packet arriving tofull buffer is lost

buffer (waiting area)

Lecture 2 Introduction 1-17

Throughput throughput: rate at which information

bits transferred between sender/receiver

Rs

Rs

Rs

Rc

Rc

Rc

R

Lecture 2 Introduction 1-18

Numerical example 3: 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