Overview-Part2

Physical Media• Bit: propagates between

transmitter/receiver pairs• physical media: what lies

between transmitter & receiver

• guided media: – signals propagate in solid

media: copper, fiber, coax

• unguided media: – signals propagate freely,

e.g., radio

Twisted Pair (TP)• two insulated copper

wires– Category 3 UTP:

traditional phone wires, 10 Mbps Ethernet

– Category 5 UTP: 100Mbps Ethernet

Physical Media: coax, fiberCoaxial cable:• higher bit rates than

twisted pair• baseband:

– single channel on cable– legacy Ethernet

• broadband:– multiple channels on

cable– Cable TV systems

Fiber optics:• glass fiber carrying light pulses,

each pulse a bit• high bit rate:

– Terabits/sec with wavelength division multiplexing (WDM)

• low signal attenuation; immune to electromagnetic interference

Physical media: radio

• signal carried in electromagnetic spectrum

• no physical “wire”• propagation

environment effects:– reflection – obstruction by objects– interference

Radio link types:• terrestrial

– local area (802.11b)– wide-area (WAP, i-

mode, 3G)

• satellite– Geostationary: 250

msec end-end delay– Low-altitude: many

satellites needed

Internet structure: network of networks

• roughly hierarchical• at the top: “tier-1” ISPs (Internet backbones)

– international coverage– directly connected to other tier-1 ISPs– e.g., UUNet, BBN/Genuity, Sprint, AT&T

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

Tier-1 providers interconnect (peer) privately

NAP

Tier-1 providers also interconnect at public network access points (NAPs)

Internet structure: network of networks

• “Tier-2” ISPs: smaller (often national/regional) ISPs– Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet.Tier-2 ISP is customer oftier-1 provider

Tier-2 ISPs also peer privately with each other, interconnect at NAP

Internet structure: network of networks

• “Tier-3” ISPs and local ISPs – last hop (“access”) network (closest to end systems)

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

Local and tier- 3 ISPs are customers ofhigher tier ISPsconnecting them to rest of Internet

How do loss and delay occur?packets queue in router buffers, wait for turn

A

B

packet being transmitted (delay)

packets queueing (delay)

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

Four sources of packet delay

• 1. nodal processing: – check bit errors– determine output link

A

B

propagation

transmission

nodalprocessing queueing

• 2. queuing– time waiting at output

link for transmission

– depends on congestion level of router

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

nodalprocessing queueing

Note: s and R are very different quantities!

Caravan analogy

• Cars “propagate” at 100 km/hr

• Toll booth takes 12 sec to service a car (transmission time)

• car~bit; caravan ~ packet• Q: How long until caravan

is lined up before 2nd toll booth?

• Time to “push” entire caravan through toll booth onto highway = 12*10 = 120 sec

• Time for last car to propagate from 1st to 2nd toll both: 100km/(100km/hr)= 1 hr

• A: 62 minutes

toll booth

toll booth

ten-car caravan

100 km

100 km

Caravan analogy (more)

• Cars now “propagate” at 1000 km/hr

• Toll booth now takes 1 min to service a car

• Q: Will cars arrive to 2nd booth before all cars serviced at 1st booth?

• Yes! After 7 min, 1st car at 2nd booth and 3 cars still at 1st booth.

• 1st bit of packet can arrive at 2nd router before packet is fully transmitted at 1st router!– See Ethernet applet at

AWL Web site

toll booth

toll booth

ten-car caravan

100 km

100 km

Nodal 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

proptransqueueprocnodal ddddd

Queueing delay (revisited)

• R=link bandwidth (bps)• L=packet length (bits)• a=average packet arrival

rate (packets/sec)

traffic intensity = La/R

• La/R ~ 0: average queueing delay small• La/R -> 1: delays become large• La/R > 1: more “work” arriving than can be

serviced, average delay infinite!

Packet loss

• queue (aka buffer) preceding a link has finite capacity

• when packet arrives to full queue, packet is dropped (aka lost)

• lost packet may be retransmitted by previous node, by source end system, or not retransmitted at all

Internet History

• 1961: Kleinrock - queueing theory shows effectiveness of packet-switching

• 1967: ARPAnet conceived by Advanced Research Projects Agency (ARPA)

• 1969: first ARPAnet node operational

• 1972: – ARPAnet

demonstrated publicly– NCP (Network Control

Protocol): first host-to-host protocol

– first e-mail program– ARPAnet has 15 nodes

1961-1972: Early packet-switching principles

Internet History

• 1970: ALOHAnet microwave network in Hawaii

• 1973: Metcalfe’s PhD thesis proposed Ethernet

• Proprietary architectures: DECnet, SNA, XNA

• 1974: Cerf and Kahn - architecture for interconnecting networks

• 1979: ARPAnet has 200 nodes

1972-1980: Internetworking, new and proprietary nets

Internet History

• 1983: deployment of TCP/IP

• 1982: SMTP e-mail protocol defined

• 1983: DNS defined for name-to-IP-address translation

• 1985: FTP protocol defined

• 1988: TCP congestion control

• new national networks: CSNET, BITnet, NSFnet

• 100,000 hosts connected to confederation of networks

1980-1990: new protocols, a proliferation of networks

Internet History

• Early 1990’s: ARPAnet decommissioned

• 1991: NSF lifts restrictions on commercial use of NSFnet (decommissioned, 1995)

• early 1990s: Web– HTML, HTTP: Berners-

Lee– late 1990’s:

commercialization of the Web

Late 1990’s – 2000’s:• more killer apps: instant

messaging, peer2peer file sharing

• est. 50 million host, 100 million+ users

• backbone links running at Gbps

1990, 2000’s: commercialization, the Web, new apps