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Introduction to Networks

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1 Introduction to Networks
CSEN 404 Introduction to Networks Amr El Mougy Ali Saudi ** Slides are attributed to J. F. Kurose

2 Roadmap 1.1 What is the Internet? 1.2 Network edge 1.3 Network core
end systems, access networks, links 1.3 Network core circuit switching, packet switching, network structure 1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models Introduction

3 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 packet being transmitted (delay) A free (available) buffers: arriving packets dropped (loss) if no free buffers packets queuing (delay) B Introduction

4 Four sources of packet delay
1. nodal processing: check bit errors determine output link 2. queuing: time waiting at output link for transmission depends on congestion level of router A B propagation transmission nodal processing queuing Introduction

5 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 Note: s and R are very different quantities! A B propagation transmission nodal processing queuing Introduction

6 Caravan analogy toll booth ten-car caravan 100 km cars “propagate” at km/hr toll booth takes 12 sec to service 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 Introduction

7 Caravan analogy (more)
toll booth ten-car caravan 100 km Cars now “propagate” at 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! Introduction

8 Nodal delay dproc = processing delay dqueue = queuing delay
typically a few microsecs or less dqueue = queuing delay depends on congestion. Characterized using statistical measures dtrans = transmission delay = L/R, significant for low-speed links dprop = propagation delay a few microsecs to hundreds of msecs Introduction

9 Queuing delay (revisited)
average queuing delay R=link bandwidth (bps) L=packet length (bits) a=average packet arrival rate traffic intensity = La/R La/R ~ 0: average queuing delay small La/R < 1: delays depends on nature of arrivals La/R ~1: more “work” arriving than can be serviced, average delay infinite! Introduction

10 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 buffer (waiting area) packet being transmitted A B packet arriving to full buffer is lost Introduction

11 Throughput throughput: rate (bits/time unit) at which bits transferred between sender/receiver instantaneous: rate at given point in time average: rate over longer period of time pipe that can carry fluid at rate Rc bits/sec pipe that can carry fluid at rate Rs bits/sec link capacity Rs bits/sec link capacity Rc bits/sec server sends bits (fluid) into pipe server, with file of F bits to send to client Introduction

12 Throughput (more) Rs < Rc What is average end-end throughput?
Rc bits/sec Rs bits/sec Rs > Rc What is average end-end throughput? Rs bits/sec Rc bits/sec link on end-end path that constrains end-end throughput bottleneck link Introduction

13 Throughput: Internet scenario
Rs per-connection end- to-end throughput: min(Rc,Rs,R/10) in practice: Rc or Rs is often bottleneck Rs Rs R Rc Rc Rc 10 connections (fairly) share backbone bottleneck link R bits/sec Introduction

14 Roadmap 1.1 What is the Internet? 1.2 Network edge 1.3 Network core
end systems, access networks, links 1.3 Network core circuit switching, packet switching, network structure 1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models Introduction

15 Protocol “Layers” Question: hosts routers links of various media
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? Introduction

16 Organization of air travel
ticket (purchase) baggage (check) gates (load) runway takeoff airplane routing ticket (complain) baggage (claim) gates (unload) runway landing a series of steps Introduction

17 Layering of airline functionality
ticket (purchase) baggage (check) gates (load) runway (takeoff) airplane routing departure airport arrival intermediate air-traffic control centers ticket (complain) baggage (claim gates (unload) runway (land) ticket baggage gate takeoff/landing Layers: each layer implements a service via its own internal-layer actions relying on services provided by layer below In a distributed way Introduction

18 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 Introduction

19 Internet protocol stack
application: supporting network applications FTP, SMTP, HTTP transport: process-process 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 Introduction

20 Encapsulation source destination application transport network link
message M application transport network link physical segment Ht M Ht datagram Ht Hn M Hn frame Ht Hn Hl M link physical switch destination network link physical Ht Hn M Ht Hn Hl M M application transport network link physical Ht Hn M Ht M Ht Hn M router Ht Hn Hl M Introduction

21 Introduction: Summary
Internet overview what’s a protocol? network edge, core, access network packet-switching versus circuit-switching Internet structure performance: loss, delay, throughput layering, service models You now have: context, overview, “feel” of networking more depth, detail to follow! Introduction

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