Lecture 2 1-1 Internet Overview: roadmap 1.1 What is the Internet? (A simple overview last week) Today, A closer look at the Internet structure! 1.2 Network.
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Lecture 2 1-1 Internet Overview: roadmap 1.1 What is the Internet? (A simple overview last week) Today, A closer look at the Internet structure! 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
Lecture 2 Recap: What are the components of Internet? End-users (Hosts) e.g. computers access networks, physical media: wired, wireless communication links network core: interconnected routers network of networks 1-2
Lecture 2 End-users (Hosts) End-users (hosts): run application programs e.g. Web, email Hosts further divided into Client Hosts Server Hosts Two different models of networking client/server model client host requests, receives service from always-on server e.g. Web browser/server; email client/server peer-peer model: minimal (or no) use of dedicated servers e.g. Skype, BitTorrent client/server peer-peer 1-3
Client/server model is the dominant design for Internet applications server - is the information provider client - is the information consumer example web server and a client running web browser a CNN web server simultaneously serves thousands of clients. Lecture 2 The Client/Server Model 1-4
Lecture 2 Hosts are not sufficient for networking! End-users (hosts): run application programs e.g. Web, email But, hosts alone would not be enough We need to connect the hosts HOW? 1-5
Lecture 2 Access networks and physical media Q: How to connect end systems to edge router? 1. residential access nets 2. institutional access networks (school, company) 3. mobile access networks 1-6
Lecture 2 Residential access: point to point access Dialup via modem up to 56Kbps direct access to router (conceptually) ADSL: asymmetric digital subscriber line up to 1 Mbps home-to-router up to 8 Mbps router-to-home ADSL deployment: happening 1-7
Lecture 2 Residential access: cable modems HFC: hybrid fiber coax asymmetric: up to 10Mbps upstream, 1 Mbps downstream network of cable and fiber attaches homes to ISP router shared access to router among home issues: congestion deployment: available via cable companies, e.g., MediaOne, CableVision 1-8
Lecture 2 Institutional access: local area networks company/univ local area network (LAN) connects end system to edge router Ethernet: shared or dedicated cable connects end system and router 10 Mbps, 100Mbps, Gigabit Ethernet deployment: institutions, home LANs happening now 1-9
Lecture 2 Wireless access networks shared wireless access network connects end system to router wireless LANs: radio spectrum replaces wire e.g., 802.11b/g (WiFi): 11 or 54 Mbps wider-area wireless access next up (?): WiMAX (10’s Mbps) over wide area base station mobile hosts router 1-10
Lecture 2 1-11 Internet Overview: roadmap 1.1 What is the Internet? (A simple overview last week) Today, A closer look at the Internet structure! 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
Lecture 2 1-12 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 1-14 Network Core: Circuit Switching Total 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 1-15 Circuit Switching: FDM and TDM FDM frequency time TDM frequency time 4 users Example:
Lecture 2 1-16 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 1-17 Disadvantages of Circuit-Switching Only static number of users This number must be fixed before the actual operation Each user gets only a “piece of the pie” even if the other users are possibly idle Prev. example: I get only 1/24 th of the entire time Resource wastage Impossible to admit new user in the middle of the operation
Lecture 2 1-18 Packet Switching A B C 100 Mb/s Ethernet 1.5 Mb/s D E queue of packets waiting for output link
Lecture 2 1-19 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 Bandwidth division into “pieces” Dedicated allocation Resource reservation
Lecture 2 1-20 Packet switching versus circuit switching Adv: Packet switching allows users to use the network dynamically! resource sharing simpler, no call setup New user can enter or leave inside the operation Is there any downside of packet switching? With excessive number of users packet delay and loss Efficiency of the system (measured in throughput) drops!
Lecture 2 1-21 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 packet being transmitted (delay) packets queueing (delay)
Lecture 2 1-22 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 1-23 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 1-24 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 1-25 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 1-26 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 1-27 Throughput throughput: rate at which information bits transferred between sender/receiver RsRs RsRs RsRs RcRc RcRc RcRc R
Lecture 2 1-28 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 throughput from B to A? Let’s work it out! C