Lecture 2 1-1 Internet Overview: roadmap 1.1 What is the Internet? 1.2 Network edge end systems, access networks, links 1.3 Network core circuit switching,
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Lecture 2 1-1 Internet Overview: 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 Internet 1.5 Protocol layers, service models 1.6 Networks under attack: security
Quick Recap… Hardware view of Internet Components of Internet Structural view Client-server model Peer-to-peer model Lecture 2 1-2
Lecture 2 1-3 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 1-5 Network Core: Circuit Switching 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
Lecture 2 1-6 Circuit Switching: FDM and TDM FDM frequency time TDM frequency time 4 users Example:
Lecture 2 1-7 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 1-8 Packet Switching A B C 100 Mb/s Ethernet 1.5 Mb/s D E queue of packets waiting for output link
Lecture 2 1-9 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 store and forward: packets move one hop at a time Node receives complete packet before forwarding congestion: packets queue, wait for link use Circuit switching Bandwidth division into “pieces” Dedicated allocation Resource reservation
Lecture 2 1-10 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 How do delay and loss occur in Internet/network?
Lecture 2 1-11 How do delay and loss 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
Lecture 2 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 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 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 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 L A B
Lecture 2 1-16 Numerical example 3 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 L A B
Lecture 2 1-17 Numerical example 4 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. What if there are three packets from A? Let’s work it out! R R R L A B
Lecture 2 1-18 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 1-19 Throughput Throughput: rate at which information bits transferred between sender/receiver RsRs RsRs RsRs RcRc RcRc RcRc R
Lecture 2 1-20 Numerical example 5: 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