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2a. Introduction to Data Communications and Networking. 1. Communication Link. Data Communication Link. 2. General Definitions. Information is the meaning that a human being assigns to data by means of the conventions applied to those data.
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1. Communication Link Data Communication Link
2. General Definitions • Information is the meaning that a human being assigns to data by means of the conventions applied to those data. • Datais a representation of facts, concepts, or instructions in a formalized manner suitable for communications. • Signals are the physical encoding of data, electric, or electromagnetic means. Signals can be: • Analog(continuous in time and amplitude), • Discrete(discrete in time, continuous in amplitude), or • Digital. A digital signal (discrete in time and amplitude),it is a sequence of digital values which changes once every interval.
4. Networking • Communication networks enable many users to transfer information in different form of voice, video, electronic mail, and computer files. • a.InTelephone Network: Circuit switching.“Circuit" reefers-one telephone conversation along one link. • Circuit switching occurs at the beginning of new telephone call. • An electronic interface, coder/decoder (codec) in the switch converts the analog signal traveling on the link from the telephone set to the switch into digital signal-a bit stream. • Since the 1980s the transmission links of the telephone network have been changing to the SONET, or Synchronous Optical Network, standard. SONET rates are arranged in the Synchronous Transfer Signal (STS).
Phone connection to digital network Analog signal Codec Switch
Carrier Signal Rate in Mbps • OC-1 STS-1 51,840 • OC-3 STS-3 155,520 • OC-9 STS-9 466,560 • OC-12 STS-12 622,080 • OC-18 STS-18 933,120 • OC-24 STS-24 1244,160 • OC-36 STS-36 1866,240 • OC-48 STS-48 2488,320 • OC-192 STS-192 9853,280 • OC-768 STS-768 39,413,120
b. Computer or Data Communication Networks • Data packets • Packet switching + Rules of operations (protocols) =The ARPANET -single packet format and addressing scheme. Through the ARPANET was evolved into the Internet. ARPANET architecture was formalized layered model of OSI • The packet switching technique in networks is based on Multiplexing and/or MultipleAccess methods of computer interconnections. • Multiplexing:TDM FDM (WDM / DWDM) • MultipleAccess: Ethernet Network. Token Ring Network. Fiber Distributed Data Interface - FDDI-Timed-token mechanism-fixed time of the arrivals token
Model of the information system Server Network Clients Broadcast links. Point-to-point (unicast) links
LANTopology of the:Ethernet (a) and Token Ring networks (b). A a B IEEE 802.3 b IEEE 802.5
Metropolitan Area Networks (MAN) Wide Area Network (WAN) MAN represents as a interconnected LANs by point-to-point communication links. The interconnection is controlled by switches, Subnet Router (Switch) Client Links Host (Server)
Cable Television, originally known as Community Antenna Television or CATV. In CATV the signal from one master antenna distributed over a large area using coaxial cable and amplifiers. The key innovations in cable TV are optical fiber links, digital compression techniques, and service integration. • Today cable TV uses frequency-division multiplexing to transmit up to 69 analog TV channels, each 4.5 MHz wide. Transmission is over coaxial cables arranged as a unidirectional tree. • Amplifiers used to compensate for the attenuation of the cable signal. The number of TV a channels is limited by the bandwidth of coaxial cables. • Optical fibers are used to transmit the TV signals over longer distance. Transmission over the fiber is still analog. The signal is fed into the coaxial cable network at various points, where the optical signal is converted into electrical signals. This hybrid fiber/coaxial cable distribution system has a longer span and better signal quality than a coaxial cable network. This network called fiber-to-the-curb (FTTC) network.
To increase the number of channels, digital transmission technology. • Before transmitting the TV signals, the CATV company uses a TV codec that converts each signal into a bit stream. • Using Motion Pictures Expert Group (MPEG) algorithms, the codec compresses the bit stream to reduce its rate. • The bit streams are transmitted over fibers to the curb and then distributed by the neighborhood coaxial network. • The compression gain now allows-transmit about 500 TV channels. MPEG1 standard, TV signal is encoded-1.5 Mbps bit stream, which can be modulated in a signal that has a bandwidth of about 600 kHz. • Set-up boxes at the user residence perform the decompression. This CATV network is still unidirectional. Video on demand, Internet access, and telephony, the CATV industry is organizing bidirectional networks. Such a network connects video servers to users by means of control messages. • The user choices these messages to select the video program, and the video program is sent over the network to the user.
Residential access: cable modems Diagram: http://www.cabledatacomnews.com/cmic/diagram.html
Cable Network Architecture: Overview Typically 500 to 5,000 homes cable headend home cable distribution network (simplified)
Cable Network Architecture: Overview cable headend home cable distribution network (simplified)
server(s) Cable Network Architecture: Overview cable headend home cable distribution network
C O N T R O L D A T A D A T A V I D E O V I D E O V I D E O V I D E O V I D E O V I D E O 5 6 7 8 9 1 2 3 4 Channels c. Cable Network Architecture: Overview FDM: cable headend home cable distribution network
d. Wireless Networks. The first packet-switched wireless network- in 1971- Alohanet, interconnected computers on four islands in a star topology: A first approximation wireless network- three main categories: 1. Components interconnection. Short-range radio. Bluetooth network. 2. Wireless LANs. Wireless LAN permitting per-to-per communications networks. LANs called IEEE 802.11, Wireless LAN can operate at bit rates up to about 50 Mbps over distances of tens of meters. 3. Wireless WANs. The radio network used for cellular telephones is an example of a low-bandwidth wireless wide area system. This system has already gone through three generations: a. The first- analog and voice only. b. The second- digital and for voice only. c. The third- digital and is for both voice and data. Cellular systems operate below 1 Mbps, but the distances between the base station and the computer or telephone is measured in kilometers.
router base station mobile hosts Wireless access networks • Shared wireless access network connects end system to router • via base station “access point” • wireless LANs: • 802.11b: • 50 Mbps, tens of meters • wireless WAN • -Cellular systems • - < 1Mbps, several km
Telephone Networks Computer Networks Cable TV Wireless Networks Circuit switching, separation of call control from voice transfer. ISDN and service integration. Optical links. SONET. ATM. Packet-switched networks. Multiple-access Networks. Layered architecture, ARPANET. Internet. OSI model Integrated services. ATM. Digitization and compression using signal processing techniques. Fiber-to-the-curb network. Two-way links. Service integration. Radio and TV Broadcast. Cellular. Telephones. Wireless LANs. Voice, data Integration. Bluetooth.
5. System interconnection • An important concept in data communications is the interconnections between the communication system components. The interconnection could be done if: • Physical characteristics of the interconnected equipment are fitted to each other. • It allows manufacturers of different systems to interconnect their equipment through standard interfaces. • It also allows software and hardware to integrate well and be portable on differing systems. • So, standards of hardware and software for interconnections in systems are necessary.
Communication Standards • Computer communication uses different standard for different approach. • The RS-232-Cstandard is used for the serial port of computer devices. This standard is for low bit rate transmissions (up to 38 Kbps) over short distances (less than 30 m). Transmissions take place over untwisted wires. • A serial link is often used to attach a computer to a modem. A modem transmits data by converting bits into tones that can be transported by the telephone network. The receiving modem then converts these tones back into bits, thus enabling two computers with compatible modems to communicate over the telephone network as if they were directly connected by a serial link. Modems conforming to new V.90standard can transmit 56,000 bps. • The Synchronous Transmission Standard increases the transmission rate. These standards are known as Synchronous Data Link Control (SDLC). The main idea of SDLC is to avoid the time wasting by RS-232-C. • SDLC groups many data bits into packets.
With an open system of standards any company can manufacture equipment or write software. Companies must cooperate on standards. • Standard organizations create and administer standards. Often competing companies will form a committee to create a standard acceptable to all interested parties. Then the companies will ask a standard organization for formal recognition of that standard. • An example: Ethernet, a Local Area Network (LAN) system created by Xerox, Intel, and Digital Equipment Corporation. These companies asked the Institute of Electrical and Electronics Engineers (IEEE)to formalize Ethernet, and this becomes standard IEEE 802.3. • United States major standards from industry are: The American National Standard Institute (ANSI),the IEEE,andthe Electronic Industries Association (EIA). The major governmental standards organization is the National institute of Standardsand Technology (NIST). NIST major standards concerns are the standard Volt, standard Ampere, time, and dimensions for manufactures.
Open System Interconnection Reference Model Each layer is a kind of virtual machine, offering certain services to the layer above Communication subnet
Protocol Hierarchies • The philosopher-translator-secretary architecture.
Why layering? • Each layer is a kind of virtual machine, offering certain services to the layer above it. • Layer n on one machine carries on a conversation with layer n on another machine. The rules and conversations are known as the layer n protocol. • A protocol is an agreement between the communicating parties on how communication is to proceed. Dealing with complex systems: • Clear structure allows identification, relationship of complex system’s pieces • modularization eases maintenance, updating of system change of implementation of layer’s service transparent to rest of system
Thefunction of layer: what task the layer is perform, but not how the layer performs its task. • The functionof interface: how a layer will communicate with the layer above it and the layer below it. • For software interfaces, information may be passed in a manner similar to parameter passing. The information must be in a particular format (a. length, b. the order in which individual fields appear within a frame, c. the bit order within individual frames). • The hardware interfaces (physical level) may be: a.voltages, b. impedance, and c. mechanical dimensions. • Bottom three layers - Communications Subnet. They are: 1. the Physical Layer, (is hardware) 2. the Data Link Layer (DLL), (can be a mixture of hardware and software). 3. the Network Layer. The Communication Subnet is one of the major subjects of CS 117 and CS M 171L classes to study.
Communications Subnet. • the Physical Layer, is hardware The Physical is the actual medium that conveys the bit stream. This connects the networks together and carries the "ones" and "zeros" (voltage or light pulses). Typical questions here are how many volts should be used to represent a “1” and how many for “0”. How many nanoseconds a bit lists,
Layer 1: Physical Layer. Transmitted signals are modulated electromagnetic waves that propagate through medium. • The medium can be fiber optics, twisted pair copper wire, coaxial cable, microwaves, satellite, laser beams, or radio waves. Layer 1 also includes the antennas, cables, satellites, and connectors. • The transmitter converts the bits into signals, and the physical layer in the receiver converts the signals back into bits. The receiver must be synchronized to be able to recover the arrival bits. To assist the synchronization, the transmitter inserts a specific bit pattern, called a preamble, at the beginning of the packet. • The physical layer transmits bits by converting them into electrical, electromagnetic waves, or optical signal. • Generally, wireless links are slower than copper links, and copper links are slower than optical links.
Physical Media: coax, fiber Coaxial cable: • copper conductors • bidirectional • baseband: • single channel on cable • broadband: • multiple channel on cable Fiber optic cable: • glass fiber carrying light pulses, each pulse a bit • high-speed operation: • high-speed point-to-point transmission (e.g., 5 Gps) • low error rate: repeaters spaced far apart ; immune to electromagnetic noise
Physical media: radio • signal carried in electromagnetic spectrum • no physical “wire” • bidirectional • propagation environment effects: • reflection • obstruction by objects • interference Radio link types: • terrestrial microwave: • e.g. up to 45 Mbps channels • LAN (e.g., WaveLAN) • 2Mbps, 11Mbps • wide-area (e.g., cellular) • e.g. 3G: hundreds of kbps • Satellite: • up to 50Mbps channel (or multiple smaller channels) • 270 msec end-end dela
Communications Subnet (cont) 2. the Data Link Layer (DLL):1. Error control; 2. Flow control; 3 Synchronizes the receiver to the incoming bit stream; 4. Decodes the bit stream. • Sublayer 2a: Media Access Control (MAC). regulate the access to that shared link • Sublayer 2b: Logical Link Control (LLC). Implement error detection or reliable packet transmission between computers attached to a shared link. • The MAC and LLC together constitute the data link layer to implement a packet transmission service with error detection or a reliable packet transmission service
Communications Subnet (cont) • Layer 3: Network Layeris concerned with routing the frame. The three steps of routing are: 1. Establishing the connection, 2. Maintaining the connection, 3. Terminating the connection after the data transfer is complete. • Routing is the function to find the path the packets must follow. • The network layer appends unique network addresses of the source and destination computers. • Addressing scheme in packet-switched networks is that used by the Internet. • Circuit-switch networks, like the telephone network, use different addressing schemes. • The network layer uses the transmission over point-to-point links provided by the data link layer to transmit packets between any two computers attached in a network.
The network edge: • End systems (hosts): • Run application programs • Web, email • at “edge of network” • Client/server model • Client host requests, receives service from always-on server • Web browser/server; email client/server
The Network Core • Mesh of interconnected routers • the fundamental question: how is data transferred through network? • --circuit switching: dedicated circuit per call: telephone net • packet-switching: data sent thru net in packets
Circuit Switching End-to-end resources reserved for “call” • link bandwidth, switch capacity • dedicated resources: no sharing • circuit-like (guaranteed) performance • call setup required
Packet Switching Each end-end data stream divided into packets • users share network resourcesdynamically • each packet uses full link bandwidth • resources used as needed Resource contention: • aggregate resource demand can exceed amount available bandwidth • congestion: packets queue, wait for link use
D E Packet Switching: Statistical Multiplexing 10 Mbs Ethernet C A statistical multiplexing 1.5 Mbs B queue of packets waiting for output link Sequence of A & B packets does not have fixed pattern statistical multiplexing.
router workstation server mobile local ISP regional ISP company network What’s the Internet: • Millions of connected computing devices: hosts, end-systems • PCs workstations, servers; running network applcts • communication links • fiber, copper, radio, satellite • transmission rate = bandwidth • Routers (gateways): forward packets (chunks of data)
TCP connection response Got the time? 2:00 <file> time What’s a protocol? Hi TCP connection req Hi
What’s a protocol? • network protocols: • machines rather than humans • all communication activity in Internet governed by protocols • human protocols: • “what’s the time?” • “I have a question” • introductions • … specific msgs sent • … specific actions taken when msgs received, or other events protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt
Protocol “Layers” Networks are complex! • many “pieces”: • hosts • routers • links of various media • applications • protocols • hardware, software
A closer look at network structure: • Network edge: applications and hosts • Network core: • routers • network of networks • Access networks, Physical media: communication links
application transport network link physical Internet protocol stack • application: supporting network applications • FTP, SMTP, STTP • transport: host-host data transfer -TCP, UDP (user datagram protocol) • Network: routing of datagrams from source to destination • IP, routing protocols • Data link: data transfer between neighboring network elements • PPP, Ethernet • Physical: bits “on the wire”
Network SoftwareProtocol Hierarchies • Layers, protocols, and interface services.
The relationship between aservice and aprotocol A set of layers and protocols is called network architecture. A list of protocols used by a certain system, one protocol per layer, is called a protocol stack.