01-Introduction DCN.pdf

Sri Lanka Institute of Information Technology Faculty of Computing Introduction Ms. Shashika Lokuliyana Year 01 and Semester 01 Year 01 Semester 01 11 Lesson outline What is Data Communication? Analog and Digital data...

Sri Lanka Institute of Information Technology Faculty of Computing Introduction Ms. Shashika Lokuliyana Year 01 and Semester 01 Year 01 Semester 01 11 Lesson outline What is Data Communication? Analog and Digital data Components of a data communication system Message types Data flow Networks Network extent (sizes) Network topologies Circuit switching and packet switching Performance: Loss, Delay, Throughput Year 01 Semester 01 1-2 2 Key terms and concepts Data Communication Networks LAN/WAN/PAN/MAN Topilogies Circuit Switching Packet Switching 1-3 3 What is data communication? Communication is the process of exchanging or sharing of information or messages. Human-to-human communication Local communication usually occurs face to face Remote communication takes place over a distance Communicating entities can also be non-human (devices or systems) Data communication is the communication where the information or messages to be communicated are represented as “data” Usually means that the communicating entities are devices or systems The “data” can be analog or digital Analog data: Something that is varying continuously in time and amplitude Examples: Voice, temperature, pressure, … Digital data: Something that is represented using 1s and 0s Examples: Document, image, … In today’s world, data communication mostly involves digital data More about analog and digital later (Topic 3 - Physical Layer) 1-4 4 Components of data communication systems The five components of a data communication system are: Message, Sender, Receiver, Transmission medium and Protocol(s) 1-5 5  Components of data communication systems Message Content Message Type Text 0x0041 = A 0x0D85 = අ 0x0B85 = அ The message is the Numbers 0-9 information (data) to be Image communicated Examples: text, Video numbers, pictures, audio, and video Audio 1-6 6 Components of data communication systems Sender The sender is the device that sends the data message. It can be a computer, a telephone handset, a video camera, etc. 1-7 7 Components of data communication systems Receiver The receiver is the device that receives the message. It can be a computer, workstation, telephone handset, television set, etc 1-8 8 Components of data communication systems Transmission medium The transmission medium is the physical path by which a message travels from sender to receiver Actually, the message is transformed into a signal that can travel across the transmission medium Some examples of transmission media are twisted-pair wire, coaxial cable, fiber-optic cable, and free space (open air) 1-9 9 Components of data communication systems Protocols A protocol is a set of rules that govern data communications It represents an agreement between the communicating devices Without a protocol, two devices may be connected but not able to communicate just as a person speaking French cannot be understood by a person who knows only Japanese 1-10 10 Data flow Simplex Communication is unidirectional Only one of the two devices on a link can transmit; the other can only receive Keyboards and traditional monitors are examples of simplex devices The keyboard can only introduce input; the monitor can only accept output Half-Duplex Each station can transmit and receive, but not at the same time When one device is sending, the other can only receive, and vice versa Walkie-talkies are half-duplex systems Full-Duplex Both stations can transmit and receive simultaneously Signals going in the two directions share the communication channel In most cases, two dedicated “paths” are available for signals going in the two directions Example: Telephone network where it is possible for both parties to talk at the same time 1-11 11 More detailed communication model Data: Entities that convey information Signals: Electric or electromagnetic representations of data Transmission: Communication of data by the propagation and processing of signal 1-12 12 Networks A network is an interconnection of a set of devices capable of communication Examples: A large computer, desktop, laptop, workstation, cellular phone, or a security system Use the term “host” or “end-system” to refer to an endpoint device There are other devices that perform additional functions Switch: interconnects endpoints to form a small network Router: interconnects networks to other networks 1-13 13 “Fun” Internet-connected devices Tweet-a-watt: monitor energy use bikes Pacemaker & Monitor Amazon Echo Web-enabled toaster + IP picture frame weather forecaster Internet refrigerator Slingbox: remote cars control cable TV Security Camera AR devices sensorized, scooters bed Fitbit Others? mattress Gaming devices Internet phones diapers 1-14 14 The Internet: a “nuts and bolts” view Billions of connected mobile network computing devices: national or global ISP § hosts = end systems § running network apps at Internet’s “edge” Packet switches: forward packets (chunks of data) local or Internet regional § routers, switches ISP home network content Communication links provider network datacenter § fiber, copper, radio, satellite network § transmission rate: bandwidth Networks enterprise § collection of devices, routers, network links: managed by an organization 1-15 15 Network extent Personal area network (PAN) Local area network (LAN) Campus network Metropolitan area network (MAN) Wide area network (WAN) Intranet / Extranet / Internet 1-16 16 Personal area network (PAN) Network connections between devices used by a single person or a small group of persons Bluetooth Phone connected to a headset Mouse connected to a computer … Ad-hoc Wi-Fi Devices connecting to each other via Wi-Fi without an access point More about this later (Topic 4 – Local Area Networks) Network diameter: 1 ~ 5 meters 1-17 17 Local area network (LAN) Wired and/or wireless Covers a home, small office, single floor or part of a floor in a building Diameter: 10m - 200m 1-18 18 Campus network Interconnected to off campus set of LANs border border covering multiple buildings of a Core core single site Diameter: 1 ~ 10 km Agg1 Agg2 Agg3 Agg4 WiFi... firewall data center............ Wireless Wireless building Controller Controller closets 1-19 19 Metropolitan area network (MAN) Covers a single metropolitan area (city) Usually owned by a service provider Diameter: 10 ~ 50 km 1-20 20 Wide area network (WAN) Usually built using point-to-point connections between multiple sites Diameter: 10 km ~ 1000km or more 1-21 21 Network topologies Point-to-point Bus Ring Star (hub and spoke) Tree Mesh 1-22 22 Point-to-point A point-to-point connection provides a dedicated link between two devices. The entire capacity of the link is reserved for transmission between those two devices. 1-23 23 Bus All stations attach directly to a linear transmission medium, or bus Usually through appropriate hardware interfacing known as a tap A transmission from any station propagates the length of the medium in both directions and can be received by all other stations. At each end of the bus is a terminator, which absorbs any signal, removing it from the bus 1-24 24 Ring Each device has a dedicated point-to-point connection with only the two devices on either side of it A signal is passed along the ring in one direction, from device to device, until it reaches its destination Each device in the ring incorporates a repeater, which regenerates the bits and passes them along 1-25 25 Star (hub and spoke) Each device has a dedicated point-to-point link only to a central controller, usually called a hub The devices are not directly linked to one another. Star topology does not allow direct traffic between devices The controller acts as an exchange If one device wants to send data to another, it sends the data to the controller, which then relays the data to the other connected device 1-26 26 Tree Multiple levels of star type networks Usually, end stations are at the bottom level At the upper levels, interconnection devices such as switches and routers are used 1-27 27 Mesh Devices have dedicated point-to- point links to one or many other devices Full-mesh connectivity: every device is connected to every other device If the number of nodes is N, full- mesh connectivity requires N(N-1) links Usually full-mesh connectivity is avoided as it is expensive in terms of the number of links 1-28 28 Data flow in networks Unicast From the sender to one destination node Broadcast From the sender to all destinations (nodes) in the network Useful in making “announcements” Multicast From the sender to selected destinations Usually, the destinations must subscribe to a particular multicast 1-29 29 Circuit Switching Uses a dedicated path between two stations Can be inefficient Establish Channel capacity dedicated for duration of connection If no data, capacity is wasted Set up (connection) takes time Once connected, transfer is transparent Transfer Has three phases Connection establishment Data transfer Disconnection Disconnect 1-30 30  Long-distance Long-distance office office End Office End Office Digital PBX Figure 9.2 Example Connection Over a Public Circuit-Switching Network 1-31 31 Circuit switching end-end resources allocated to, reserved for “call” between source and destination in diagram, each link has four circuits. call gets 2nd circuit in top link and 1st circuit in right link. dedicated resources: no sharing circuit-like (guaranteed) performance circuit segment idle if not used by call (no sharing) § commonly used in traditional telephone networks * Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive 1-32 32 Packet Switching Circuit switching was designed for voice Packet switching was designed for data Data transmitted in small packets Packets contain user data and control info User data may be part of a larger message Control information includes routing (addressing) Packets are received, stored briefly (buffered) and passed on to the next node 1-33 33  Packet-switching: store-and-forward L bits per packet 3 2 1 source destination R bps R bps packet transmission delay: takes L/R seconds One-hop numerical example: to transmit (push out) L-bit packet into link at § L = 10 Kbits R bps (bits per second) § R = 100 Mbps store and forward: entire packet must arrive at § one-hop transmission delay router before it can be transmitted on next link = 0.1 msec 1-34 34 Packet-switching: queueing R = 100 Mb/s A C D B R = 1.5 Mb/s E queue of packets waiting for transmission over output link Queueing occurs when work arrives faster than it can be serviced: 1-35 35 Packet-switching: queueing R = 100 Mb/s A C D B R = 1.5 Mb/s E queue of packets waiting for transmission over output link Packet queuing and loss: if arrival rate (in bps) to link exceeds transmission rate (bps) of link for some period of time: packets will queue, waiting to be transmitted on output link packets can be dropped (lost) if memory (buffer) in router fills up 1-36 36 How do packet delay and loss occur? § packets queue in router buffers, waiting for turn for transmission § queue length grows when arrival rate to link (temporarily) exceeds output link capacity § packet loss occurs when memory to hold queued packets fills up packet being transmitted (transmission delay) A B packets in buffers (queueing delay) free (available) buffers: arriving packets dropped (loss) if no free buffers 1-37 37 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 buffer is lost * Check out the Java applet for an interactive animation (on publisher’s website) of queuing and loss 1-38 38  Throughput § throughput: rate (bits/time unit) at which bits are being sent from sender to receiver instantaneous: rate at given point in time average: rate over longer period of time link capacity pipe that can carry linkthat pipe capacity can carry Rsfluid bits/sec at rate Rfluid c bits/sec at rate serverserver, sends with bits (fluid) into pipe (Rs bits/sec) (Rc bits/sec) file of F bits to send to client 1-39 39 Lesson summary Components of a data communication system Networks Network extent (sizes) Network topologies Circuit switching and packet switching Performance: Loss, Delay, Throughput 1-40 40 References Chapter 1 - Introduction James F. Kurose and Keith W. Ross, Computer Networking – A Top-Down Approach, (8th Edition), Pearson, 2020 Chapter 1 - Data Communications, Data Networks, and the Internet William Stallings, Data and Computer Communications (10th Edition), Pearson, 2014 1-41 41 42 Thank you Q&A 1-42 42 43 Further Information and Helpful Resources 1-43 43 Packet delay: four sources transmission A propagation B nodal processing queueing dnodal = dproc + dqueue + dtrans + dprop dproc: nodal processing dqueue: queueing delay § check bit errors § time waiting at output link for § determine output link transmission § typically < microsecs § depends on congestion level of router 1-44 44 Packet delay: four sources transmission A propagation B nodal processing queueing dnodal = dproc + dqueue + dtrans + dprop dtrans: transmission delay: dprop: propagation delay: § L: packet length (bits) § d: length of physical link § R: link transmission rate (bps) § s: propagation speed (~2x108 m/sec) § dtrans = L/R § dprop = d/s dtrans and dprop very different 1-45 45 Packet queueing delay (revisited) § a: average packet arrival rate average queueing delay § L: packet length (bits) § R: link bandwidth (bit transmission rate) L.a arrival rate of bits “traffic : R service rate of bits intensity” traffic intensity = La/R 1 § La/R ~ 0: avg. queueing delay small La/R ~ 0 § La/R -> 1: avg. queueing delay large § La/R > 1: more “work” arriving is more than can be serviced - average delay infinite! La/R -> 1 1-46 46 Throughput Rs < Rc What is average end-end throughput? Rs bits/sec Rc bits/sec Rs > Rc What is average end-end throughput? Rs bits/sec Rc bits/sec bottleneck link link on end-end path that constrains end-end throughput 1-47 47