Lecture 3 Internet Core

1. Lecture 3Internet Core

2. What we have covered so far… • Hardware view of Internet • Components of Internet • Structural view • Client-server model • Peer-to-peer model • Number Systems and bits/bytes • Decimal, Binary and Hexadecimal • Bits/Bytes and Electronic Prefixes • (Giga/mega/kilo) • (milli/micro/nano/pico) Lecture 3

3. What we will cover today • Network core • circuit switching • packet switching • Performance evaluation in Internet • Delay, loss and throughput • Protocol layers, service models Lecture 3

4. Internet: mesh of interconnected routers How is data transferred through networks? Two methodologies Circuit switching Packet switching The Network Core Lecture 3

5. End-end resources reserved for “call” dedicated circuit per call: like telephone net dedicated bandwidth resources: no sharing Guaranteed performance Overhead: call setup required Network Core: Circuit Switching Lecture 3

6. Total network resources (e.g., bandwidth) divided into “pieces” pieces allocated to each call 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 3

7. Example: 4 users FDM frequency time TDM frequency time Circuit Switching: FDM and TDM Lecture 3

8. 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 total bit rate of 1.536 Mbps (1Mb = 106 bits) and uses TDM with 24 timeslots each for one user. How long does it take you to send the file to your friend? Let’s work it out! Lecture 3

9. Numerical example 2 • You need to send a file of size 640,000 bits to your friend. You are using a circuit-switched network with FDM. Suppose, the circuit-switch network link has a total bit rate of 16 Mbps (1Mb = 106 bits) and uses FDM with 8 frequency channels (each channel for one user). How long does it take you to send the file to your friend? Let’s work it out! Lecture 3

10. 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 Network Core: Packet Switching Circuit switching Bandwidth division into “pieces” Dedicated allocation Resource reservation No flexibility Lecture 3

11. D E Packet Switching 100 Mb/s Ethernet C A 1.5 Mb/s B queue of packets waiting for output link Lecture 3

12. Adv.: Packet switching allows users to use the network dynamically! Lot of flexibility, dynamic sharing No idle resource wastage simpler, no call setup Disadv.: No dedicated resources for each user With excessive users: Excessive congestion packet delay and loss: performance degrade Packet switching versus circuit switching How do delay and loss occur in Internet/network? Lecture 3

13. 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 How do delay and loss occur? A B Lecture 3

14. 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 3

15. 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 3

16. 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 3

17. 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 3 L B A R Lecture 3

18. 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. What if there are three packets from A? Let’s work it out! Numerical example 4 L B A R R Lecture 3

19. 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 3

20. Network performance: Throughput • Throughput: rate at which information bits transferred between sender/receiver Rs Rs Rs R Rc Rc Rc Lecture 3

21. Numerical example 5: 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 3