# Lecture 2 Introduction 1-1 Chapter 1: roadmap 1.1 What is the Internet? 1.2 Network edge  end systems, access networks, links 1.3 Network core  circuit.

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Lecture 2 Introduction 1-1 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 1-2 The Network Core  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”

Lecture 2 Introduction 1-3 Network Core: Circuit Switching End-end resources reserved for “call”  link bandwidth, switch capacity  dedicated resources: no sharing  circuit-like (guaranteed) performance  call setup required

Lecture 2 Introduction 1-4 Network Core: Circuit Switching  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

Lecture 2 Introduction 1-5 Circuit Switching: FDM and TDM FDM frequency time TDM frequency time 4 users Example:

Lecture 2 Introduction 1-6 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 = 10 6 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 1-7 Packet Switching A B C 100 Mb/s Ethernet 1.5 Mb/s D E queue of packets waiting for output link

Lecture 2 Introduction 1-8 Network Core: Packet Switching 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 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 Bandwidth division into “pieces” Dedicated allocation Resource reservation

Lecture 2 Introduction 1-9 Packet switching versus circuit switching  Packet switching allows users to use the network dynamically!  resource sharing  simpler, no call setup  With excessive users:  Excessive congestion  packet delay and loss What are delay and loss in Internet/network?

Lecture 2 Introduction 1-10 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 1-11 How do loss and delay occur? 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 A B packet being transmitted (delay) packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers

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

Lecture 2 Introduction 1-13 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 nodal processing queueing Note: s and R are very different quantities!

Lecture 2 Introduction 1-14 Total delay  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

Lecture 2 Introduction 1-15 Numerical example 2  Example: A wants to send a packet to B. The packet size is, L = 7.5 Mb (1 Mb = 10 6 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! R R R L A B

Lecture 2 Introduction 1-16 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 A B packet being transmitted packet arriving to full buffer is lost buffer (waiting area)

Lecture 2 Introduction 1-17 Throughput  throughput: rate at which information bits transferred between sender/receiver RsRs RsRs RsRs RcRc RcRc RcRc R

Lecture 2 Introduction 1-18 Numerical example 3: Throughput RsRs RsRs RsRs RcRc RcRc RcRc A B  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=10 6 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! C

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