Some slides are in courtesy of J. Kurose and K. Ross Review of Previous Lecture Course Administrative Trivia Internet Architecture Network Protocols Network.

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Presentation transcript:

Some slides are in courtesy of J. Kurose and K. Ross Review of Previous Lecture Course Administrative Trivia Internet Architecture Network Protocols Network Edge A taxonomy of communication networks

Overview Homework 1 out, due 1/22 Project 1 ready to go on Tlab, should have found partners Network access and physical media Internet structure and ISPs Delay & loss in packet-switched networks Protocol layers, service models

Access networks and physical media Q: How to connection end systems to edge router? residential access nets institutional access networks (school, company) mobile access networks Keep in mind: bandwidth (bits per second) of access network? Shared or dedicated?

Residential access: point to point access Dialup via modem –up to 56Kbps direct access to router (often less) –Can’t surf and phone at same time: can’t be “always on” ADSL: asymmetric digital subscriber line –up to 1 Mbps upstream (today typically < 256 kbps) –up to 8 Mbps downstream (today typically < 1 Mbps) –FDM: 50 kHz - 1 MHz for downstream 4 kHz - 50 kHz for upstream 0 kHz - 4 kHz for ordinary telephone

Residential access: cable modems HFC: hybrid fiber coax cable –Asymmetric: up to 1Mbps upstream, 10 Mbps downstream Network of cable and fiber attaches homes to ISP router –shared access to router among home –issues: congestion, dimensioning

Residential access: cable modems Diagram:

Cable Network Architecture: Overview home cable headend cable distribution network (simplified) Typically 500 to 5,000 homes

Cable Network Architecture: Overview home cable headend cable distribution network (simplified)

Cable Network Architecture: Overview home cable headend cable distribution network Channels VIDEOVIDEO VIDEOVIDEO VIDEOVIDEO VIDEOVIDEO VIDEOVIDEO VIDEOVIDEO DATADATA DATADATA CONTROLCONTROL FDM:

Company access: local area networks company/univ local area network (LAN) connects end system to edge router Ethernet: –shared or dedicated link connects end system and router –10 Mbs, 100Mbps, Gigabit Ethernet deployment: institutions, home LANs happening now

Wireless access networks shared wireless access network connects end system to router –via base station aka “access point” wireless LANs: –802.11b (WiFi): 11 Mbps –802.11a, g 54Mbps wider-area wireless access –provided by telco operator –3G ~ 384 kbps Will it happen?? base station mobile hosts router

Home networks Typical home network components: ADSL or cable modem router/firewall/NAT Ethernet wireless access point wireless access point wireless laptops router/ firewall cable modem to/from cable headend Ethernet (switched)

Physical Media Bit: propagates between transmitter/rcvr pairs Physical link: what lies between transmitter & receiver Guided media: –signals propagate in solid media: copper, fiber, coax Unguided media: –signals propagate freely, e.g., radio (WaveLAN) Twisted Pair (TP) two insulated copper wires –Category 3: traditional phone wires, 10 Mbps Ethernet –Category 5 TP: 100Mbps Ethernet

Physical Media: coax, fiber Coaxial cable: two concentric copper conductors baseband: –single channel on cable –legacy Ethernet broadband: – multiple channel on cable – HFC Fiber optic cable: glass fiber carrying light pulses, each pulse a bit high-speed operation: –high-speed point-to-point transmission (e.g., 5 Gps) low error rate: repeaters spaced far apart ; immune to electromagnetic noise

Overview Network access and physical media Internet structure and ISPs Delay & loss in packet-switched networks Protocol layers, service models

Internet structure: network of networks Roughly hierarchical At center: “tier-1” ISPs (e.g., UUNet, BBN/Genuity, Sprint, AT&T), national/international coverage –treat each other as equals, near-clique Tier 1 ISP Tier-1 providers interconnect (peer) privately NAP Tier-1 providers also interconnect at public network access points (NAPs) POP

Tier-1 ISP: e.g., Sprint Sprint US backbone network

Internet structure: network of networks “Tier-2” ISPs: smaller (often regional) ISPs –Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs –E.g.: UUNet Europe, Singapore telecom Tier 1 ISP NAP Tier-2 ISP Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet  tier-2 ISP is customer of tier-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-3: Turkish Telecom, Minnesota Regional Network Tier 1 ISP NAP Tier-2 ISP local ISP local ISP local ISP local ISP local ISP Tier 3 ISP local ISP local ISP local ISP Local and tier- 3 ISPs are customers of higher tier ISPs connecting them to rest of Internet

Internet structure: network of networks a packet passes through many networks! Tier 1 ISP NAP Tier-2 ISP local ISP local ISP local ISP local ISP local ISP Tier 3 ISP local ISP local ISP local ISP

Overview Network access and physical media Internet structure and ISPs Delay & loss in packet-switched networks Protocol layers, service models

How do loss and delay occur? packets queue in router buffers packet arrival rate to link exceeds output link capacity packets queue, 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. processing: –check bit errors –determine output link A B propagation transmission processing queueing 2. queueing –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 (~2x10 8 m/sec) propagation delay = d/s A B propagation transmission processing 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

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! toll booth toll booth ten-car caravan 100 km

Nodal delay: Total delay at each node along the path d proc = processing delay –typically a few microsecs or less d queue = queuing delay –depends on congestion d trans = transmission delay –= L/R, significant for low-speed links d prop = propagation delay –a few microsecs to hundreds of msecs

Queueing delay (revisited) R=link bandwidth (bps) L=packet length (bits) a=average packet arrival rate 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!

“Real” Internet delays and routes What do “real” Internet delay & loss look like? Traceroute program: provides delay 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. 3 probes

“Real” Internet delays and routes mpl-idf-vln-122.northwestern.edu ( ) ms ms ms 2 lev-mdf-6-vln-54.northwestern.edu ( ) ms ms ms 3 abbt-mdf-1-vln-902.northwestern.edu ( ) ms ms ms 4 abbt-mdf-4-ge northwestern.edu ( ) ms ms ms 5 starlight-lsd6509.northwestern.edu ( ) ms ms ms ( ) ms ms ms ( ) ms ms ms ( ) ms ms ms 9 sl-gw25-stk-1-2.sprintlink.net ( ) ms ms ms 10 sl-bb21-stk-8-1.sprintlink.net ( ) ms ms ms 11 sl-bb21-hk-2-0.sprintlink.net ( ) ms ms ms 12 sl-gw10-hk-14-0.sprintlink.net ( ) ms ms ms 13 sla-cent-3-0.sprintlink.net ( ) ms ms ms ( ) ms ms ms ( ) ms ms ms 16 shnj4.cernet.net ( ) ms ms ms 17 hzsh3.cernet.net ( ) ms ms ms 18 zjufw.zju.edu.cn ( ) ms ms ms 19 * * * 20 * * * 21 ( ) ms ms ms traceroute: zappa.cs.nwu.edu to Three delay measements from Zappa.cs.cs.nwu.edu to 1890mpl-idf-vln-122.northwestern.edu * means no reponse (probe lost, router not replying) trans-oceanic link

Packet loss Queue (aka buffer) preceding link in buffer 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

Overview Network access and physical media Internet structure and ISPs Delay & loss in packet-switched networks Protocol layers, service models

Protocol “Layers” Networks are complex! many “pieces”: –hosts –routers –links of various media –applications –protocols –hardware, software Question: Is there any hope of organizing structure of network? Or at least our discussion of networks?

Why layering? Dealing with complex systems: Explicit structure allows identification, relationship of complex system’s pieces –layered reference model for discussion Modularization eases maintenance, updating of system –change of implementation of layer’s service transparent to rest of system –e.g., change in gate procedure doesn’t affect rest of system Layering considered harmful?

Internet protocol stack application: supporting network applications –FTP, SMTP, HTTP transport: host-host data transfer –TCP, UDP network: routing of datagrams from source to destination –IP, routing protocols link: data transfer between neighboring network elements –PPP, Ethernet physical: bits “on the wire” application transport network link physical

Layering: logical communication application transport network link physical application transport network link physical application transport network link physical application transport network link physical network link physical Each layer: distributed “entities” implement layer functions at each node entities perform actions, exchange messages with peers

Layering: logical communication application transport network link physical application transport network link physical application transport network link physical application transport network link physical network link physical data E.g.: transport take data from app add addressing, reliability check info to form “datagram” send datagram to peer wait for peer to ack receipt analogy: post office data transport ack

Layering: physical communication application transport network link physical application transport network link physical application transport network link physical application transport network link physical network link physical data

Protocol layering and data Each layer takes data from above adds header information to create new data unit passes new data unit to layer below application transport network link physical application transport network link physical source destination M M M M H t H t H n H t H n H l M M M M H t H t H n H t H n H l message segment datagram frame

Summary Network access and physical media Internet structure and ISPs Delay & loss in packet-switched networks Protocol layers, service models More depth, detail to follow! Homework 1 out, due 1/22. Project 1 ready to go on Tlab, should have found partners.