1CS519: Computer NetworksLecture 1: Jan 26, 2004Intro to Computer NetworkingCS519Lets start at the beginning…| What is a network for?z To allow two or more endpoints to communicate| What is a network?z Nodes connected by linksCS519Lets start at the beginning…| Is this a network?CS519Lets start at the beginning…| Is this a network?| Of course it is! z Just not very interesting2CS519Other “networks” (network topologies)CS519What is a data network?| The answer is NOT “a network that carries data”z Cause you can send “data” (e.g. a fax) over the “voice network”| “Data network” is often a euphemism for “packet network”z And “voice network” is often a euphemism for “circuit network”CS519Packet network versus circuit network| Historically, a circuit network was a network that literally established a physical wired connection between two pointsz With relays, plus amplifiers and stuff| Before computers, this was the only way to do networksCS519Packet network versus circuit network| But these days voice is modulated and digitized in numerous ways as it works through the networkz Very few physical circuits| So nowadays we consider a circuit network one that appears to establish a fixed “pipe” (amount of bandwidth) between two points3CS519Types of circuits| Synchronous time-division multiplexing (STDM)z Each circuit is given a slice of time| Frequency-division multiplexing (FDM)z Each circuit is given a transmission frequencyCS519Packet network versus circuit network| By contrast, a packet network allows small units of data (packets) to be individually sent to different destinationsCS519Packet network versus circuit networkCS519Packet network versus circuit network| So clearly packet switched is better than circuit switched, right?4CS519Packet network versus circuit network| So clearly packet switched is better than circuit switched, right?| Well, as with so much in this world, it depends| What if A and C try to talk exclusively to B at high speed at the same time?CS519Delay and packet loss in packet networksCS519Delay and packet loss in packet networks| Can happen any time multiple links feed into a single linkz And incoming volume exceeds outgoing volume| Larger queues can reduce packet loss at the expense of more delay| Ultimately the sources have to slow down (congestion control)| By contrast, circuit networks can block (busy tone)CS519Also Jitter5CS519Also JitterCS519Also JitterCS519Circuits versus packets| Circuits are an all or nothing propositionz Give good quality, if you can get yourself a circuit in the first placez Efficient only if the application keeps the circuit full (I.e. a voice stream)| Packets are more flexiblez Can send a little or a lotz But other traffic can interfere at any timez More efficient when traffic is burstyCS519Can a packet network emulate a circuit?| After all, our STDM circuit sent data over the wire in “chunks”6CS519Can a packet network emulate a circuit?| After all, our STDM circuit sent data over the wire in “chunks”| The answer is yes, it can| And indeed, the first packet networks offered “services” that very much emulated circuitsCS519One way to do itCS519One way to do itCS519But this has complications too7CS519“Datagram” versus “virtual circuit” networks| Both are packet networksz (We won’t discuss pure circuit networks any more in this course)| Virtual circuit networks have the notion of call setup and blockingz But much more complex traffic models than our simple two-queue example| Datagram networks is how the Internet ultimately got built!CS519But virtual circuit networks still important| We don’t see virtual circuit networks to our desktopz Though this was the vision for many folks| But virtual circuit networks formed the unpinning of the Internetz Something called ATMz Though this is fadingCS519This class focuses on the Internet| Which is a datagram network| One big topic will be how queues in the Internet manage not to become hopelessly overloadedz Many of you know, the answer is TCP, but we’ll look at this in detailCS519Some terms introduced so far| Network, node, link, queue| Circuit and packet networksz a.k.a. data and voice networks| Virtual circuit and datagram networks| Delay, latency, loss, drop, jitter, blocking8CS519Bandwidth and Latency| We looked at delay due to queuing| But there are three main components to delay:z Propagation delayz Transmit delayz Queuing delayCS519Queuing, transmit, and propagation delaysCS519Queuing, transmit, and propagation delaysCS519Total latency| Total latency = z Propagation + Transmit + Queue| Propagation = z Distance / Speed of light| Transmit = z Packet size / Bandwidth9CS519Delay x Bandwidth Product| Refers to the number of bits you can have “in the pipe” at the same timez Or, how many bits you can stuff in the pipe before the first bit comes out the other endz Like hot water getting from the water heater to your shower!| As bandwidth increases (and distance doesn’t change) this is becoming an issueCS519An extreme (but realistic) Delay x Bandwidth Example| Coast-to-coast propagation delay = 15ms| OC192 link = 10 Gbps| 10 Gbps x 15ms = 150,000,000 bits = 19 Mbytes = 7 songs (MP3 files)| You could stuff 7 songs into an OC192 pipe at Boston before the first song starting arriving in LA!!!CS519A more common Delay x Bandwidth Example| 50ms coast to coast delay (mainly from queuing)| 100 Mbps Ethernet| This is about 600Kbytes…still a decent sized file| Delay x Bandwidth is starting to dominate our thinking about protocol performanceCS519Common provider bandwidth units | DSO = 64 Kbps | DS1 = 1.544 Mbps| DS3 = 44.736 Mbps| OC3 = 155.52 Mbps| OC12 = 622.08 Mbps| OC48 = 2.488 Gbps| OC192 = 9.953 Gbps| OC768 = 39.813 Gbps10CS519Bandwidth and throughput and goodput| Bandwidth is the maximum theoretical speed of a pipe| Throughput is the actual measured speedz Vague term because depends on where you measure| Goodput is the throughput seen by the applicationz Throughput over the pipe can be more than goodput because of dropped and retransmitted packets, control packets, and