Chapter 1: roadmap

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 models 1.6 Networks under attack: security Lecture 2 Introduction

2. 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” The Network Core Lecture 2 Introduction

3. End-end resources reserved for “call” link bandwidth, switch capacity dedicated resources: no sharing circuit-like (guaranteed) performance call setup required Network Core: Circuit Switching Lecture 2 Introduction

4. 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 Network Core: Circuit Switching Lecture 2 Introduction

5. Example: 4 users FDM frequency time TDM frequency time Circuit Switching: FDM and TDM Lecture 2 Introduction

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

7. D E Packet Switching 100 Mb/s Ethernet C A 1.5 Mb/s B queue of packets waiting for output link Lecture 2 Introduction

8. 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 allocation Resource reservation Network Core: Packet Switching 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 Lecture 2 Introduction

9. Packet switching allows users to use the network dynamically! resource sharing simpler, no call setup With excessive users: Excessive congestion packet delay and loss Packet switching versus circuit switching What are delay and loss in Internet/network? Lecture 2 Introduction

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

11. 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 packet being transmitted (delay) packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers How do loss and delay occur? A B Lecture 2 Introduction

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

13. 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 transmission A propagation B nodal processing queueing Delay in packet-switched networks Note: s and R are very different quantities! Lecture 2 Introduction

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 Lecture 2 Introduction

15. 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! Numerical example 2 L B A R R R Lecture 2 Introduction

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 all buffer (waiting area) packet being transmitted A B packet arriving to full bufferis lost Lecture 2 Introduction

17. Throughput • throughput: rate at which information bits transferred between sender/receiver Rs Rs Rs R Rc Rc Rc Lecture 2 Introduction

18. Numerical example 3: Throughput • 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! B Rs Rs Rs C Rc Rc Rc A Lecture 2 Introduction