Lecture 2 1-1

Internet Overview: roadmap

1.1 What is the Internet? (A simple overview last week)

Today, A closer look at the Internet structure!1.2 Network edge

end systems, access networks, links

1.3 Network core circuit switching, packet switching

1.4 Delay, loss and throughput in Internet

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

Lecture 2

Recap: What are the components of Internet?

End-users (Hosts) e.g. computers

access networks, physical media: wired, wireless

communication links

network core: interconnected

routers network of networks

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Lecture 2

End-users (Hosts) End-users (hosts):

run application programs e.g. Web, email

Hosts further divided into Client Hosts Server Hosts

Two different models of networking client/server model

• client host requests, receives service from always-on server

• e.g. Web browser/server; email client/server

peer-peer model:• minimal (or no) use of dedicated servers• e.g. Skype, BitTorrent

client/server

peer-peer

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Client/server model is the dominant design for Internet applications server - is the information provider client - is the information consumer

example web server and a client running web browser a CNN web server simultaneously serves

thousands of clients.

Lecture 2

The Client/Server Model

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Lecture 2

Hosts are not sufficient for networking!

End-users (hosts): run application programs e.g. Web, email

But, hosts alone would not be enough We need to connect the

hosts

HOW?

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Lecture 2

Access networks and physical media

Q: How to connect end systems to edge router?

1. residential access nets

2. institutional access networks (school, company)

3. mobile access networks

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Lecture 2

Residential access: point to point access

Dialup via modem up to 56Kbps direct access

to router (conceptually)

ADSL: asymmetric digital subscriber line up to 1 Mbps home-to-router up to 8 Mbps router-to-home ADSL deployment:

happening

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Lecture 2

Residential access: cable modems

HFC: hybrid fiber coax asymmetric: up to 10Mbps upstream, 1

Mbps downstream network of cable and fiber attaches homes to

ISP router shared access to router among home issues: congestion

deployment: available via cable companies, e.g., MediaOne, CableVision

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Lecture 2

Institutional access: local area networks

company/univ local area network (LAN) connects end system to edge router

Ethernet: shared or dedicated

cable connects end system and router

10 Mbps, 100Mbps, Gigabit Ethernet

deployment: institutions, home LANs happening now

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Lecture 2

Wireless access networks

shared wireless access network connects end system to router

wireless LANs: radio spectrum replaces

wire e.g., 802.11b/g (WiFi):

11 or 54 Mbps

wider-area wireless access next up (?): WiMAX

(10’s Mbps) over wide area

basestation

mobilehosts

router

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Lecture 2 1-11

Internet Overview: roadmap

1.1 What is the Internet? (A simple overview last week)

Today, A closer look at the Internet structure!1.2 Network edge

end systems, access networks, links

1.3 Network core circuit switching, packet switching

1.4 Delay, loss and throughput in Internet

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

Lecture 2 1-12

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 1-13

Network Core: Circuit Switching

Telephone call like mechanism End-end resources

reserved for “call”

dedicated resources: no sharing (link bandwidth)

circuit-like (guaranteed) performance

call setup required

Lecture 2 1-14

Network Core: Circuit Switching

Total 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 1-15

Circuit Switching: FDM and TDM

FDM

frequency

time

TDM

frequency

time

4 users

Example:

Lecture 2 1-16

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 1-17

Disadvantages of Circuit-Switching

Only static number of users This number must be fixed before the actual

operation Each user gets only a “piece of the pie” even if the

other users are possibly idle Prev. example: I get only 1/24th of the entire time

Resource wastage Impossible to admit new user in the middle of the

operation

Lecture 2 1-18

Packet Switching

A

B

C100 Mb/sEthernet

1.5 Mb/s

D E

queue of packetswaiting for output

link

Lecture 2 1-19

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

Bandwidth division into “pieces”

Dedicated allocationResource reservation

Lecture 2 1-20

Packet switching versus circuit switching Adv: Packet switching allows users to use the

network dynamically! resource sharing simpler, no call setup New user can enter or leave inside the

operation

Is there any downside of packet switching? With excessive number of users packet delay and loss Efficiency of the system (measured in throughput)

drops!

Lecture 2 1-21

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

packet being transmitted (delay)

packets queueing (delay)

Lecture 2 1-22

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 1-23

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 1-24

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 1-25

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 1-26

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 1-27

Throughput throughput: rate at which information

bits transferred between sender/receiver

Rs

Rs

Rs

Rc

Rc

Rc

R

Lecture 2 1-28

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

throughput from B to A?

Let’s work it out!

C