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CCM 4300 Lecture 2 Computer Networks, Wireless and Mobile Communication Systems Dr E. Ever School of Engineering Information Sciences CCM4300_09-10 1 Group Presentation Topics Wi-Fi Bluetooth GPRS GSM Wired Equivalent Privacy (WEP) Wi-Fi Protected Access (WPA and WPA2) Kerberos is a computer network authentication protocol Beowulf Clusters Myrinet-based cluster Supercomputer (Cray, or any other one) CCM4300_Jan 10 2 CISCO Training the link http://www.cs.mdx.ac.uk/cnap/index.html User name: Cisco PWD: class CCM4300_Sep 08 3 Session Content Recap of last session Lesson Objectives Network Topologies Introduction – an example of human-to-human interaction What is a protocol? ISO OSI Reference Model - TCP/IP protocol Physical layer Data link layer and access control CCM4300 4 Recap of Last Session defined a computer network and identified some of the basic components. explored the history of computers and computer networks and how they have evolved. identified some of the advantages and disadvantages of using computer networks. introduced the various network standards, how they are created and by whom CCM 4300 5 Lesson objectives At the completion of this lesson you should be able to - understand network logical and physical topologies - understand the concept of layering and structure of the OSI Reference Model - describe the functionality of each layer in the OSI Reference Model - understand the difference between analogue and digital communication - understand the OSI Reference Model Physical Layer specification - understand the OSI Reference Model Data Link Layer specification CCM4300 6 Physical and Logical Topologies The word topology means maps of nodes (dots) and links (lines) that often contain patterns. There are two types physical and logical Physical locations describes the plan for wiring the physical devices. A logical topology of a network shows how the hosts communicate across the medium. Note that a network may have one type of physical topology and a totally different type of logical topology. CCM4300 7 Bus Each computer is connected to a single cable which connects all of the computers. This is the cheapest network topology as the smallest amount of cabling is required. If the network cable breaks anywhere then none of the computers can access the network. CCM4300 8 Ring Each computer is connected to the two computers on either side of it. The last computer is linked to the first to form a ring. If the network cable breaks anywhere then none of the computers can access the network. Long cable length CCM4300 9 Mesh Any computer can be connected to any other computer. There are multiple routes. If one link fails messages can go another way so this topology is very reliable. The Internet has a mesh topology. CCM4300 10 Star All communication takes place via a central computer. If the central computer fails the whole network will break down. If one of the network cables breaks only the computer connected to that cable is affected. CCM4300 11 Extended Star An extended star topology is like a star topology except that each device that links to the centre device is also the centre of another star. Each of these devices acts as the centre of another star. An apparent advantage of this is to extend the length and the size of the network. The number of devices and the numbers of cables needed to interconnect to the central device is still limited (QoS issues). CCM4300 12 The Network Core: CS vs PS mesh of interconnected routers the fundamental question: how is data transferred through net? circuit switching: dedicated circuit per call: telephone net packet-switching: data sent thru net in discrete “chunks” CCM4300 13 What is a Protocol? Represent the “rules” for communication: how do the computers initiate communication what features can be negotiated at the start the communication vocabulary - i.e. what requests (commands) can be given and what the valid responses are what kinds of data can be exchanged how the communication can be closed in an orderly way CCM4300 14 What is a protocol? - cont A human protocol and a computer network protocol: Hi TCP connection req. Hi TCP connection reply. Got the time? 2:00 Get http://www.mdx.ac.uk <file> time CCM4300 15 What is a protocol? - cont Protocols also define the format of the communication exchanges Header Control Information Data Message CCM4300 16 What is a protocol? - cont For two networked devices to communicate they must speak the same language (i.e. use the same protocol) The protocols must be able to: - indicate who (what address) they want to talk to - provide any required delivery assurances/recovery - control the flow of information Must specify how to initiate, maintain and conclude the communications exchange CCM4300 17 What is a protocol? - cont Several protocols are involved in a network Operate together in a layered manner - each layer builds upon the services of its lower layer(s) File Transfer Workstation End-to-end integrity Router Internetworking Remote Server WAN LAN CCM4300 18 What is a protocol? - cont A protocol (or more typically a protocol suite) is needed for communications - some organisations may use multiple suites (e.g. Novell and TCP/IP protocols) Protocols are specified in the form of documents and usually implemented in software A separate software package is needed for each different protocol suite to be supported CCM4300 19 ISO OSI Reference Model Open Systems Interconnection Developed by International Standards Organisation (ISO) - 1981 - revised in 1994 Described in ISO-7498 standard Proposes 7 layers Provides: - a common terminology - a framework for networking CCM4300 20 Principles of OSI Layering A layer should be created where a different level of abstraction is needed Each layer should perform a well-defined function The function of each layer should be chosen with a view to defining internationally standardised protocols The layer boundaries should be chosen to minimise the information flow across the interfaces The number of layers should be: - large enough that distinct functions are not thrown together - small enough that the architecture is not unwieldy CCM4300 21 OSI Reference Model Source : PDU: Protocol Data Unit CCM4300_Sep 08 Computer Networks 1996 22 OSI Layers - cont Application contains a number of standard protocols of general use -e.g. file transfer protocols, email, virtual terminals - any service program may define an application level protocol that clients must use. - CCM4300 23 OSI Layers - cont Presentation - concerned with the representation of data (e.g. between different hardware, Operating Systems etc.) Session - allows clients of an OS on one machine to establish and use sessions with clients of an OS on another machine CCM4300 24 OSI Layers - cont Transport - concerned with transmission from end system to end system Network - concerned with transmitting data from a source to destination across networks - must determine route for data packets and attempt to avoid congestion by controlling the number transmitted CCM4300 25 OSI Layers Data Link - Concerned with taking a raw transmission facility and turning it into a link that appears to be free from errors Physical - Concerned with transmitting uninterrupted bits from on computer to another and managing the connection A common mnemonic device for remembering the layers in the right order is All People Seem To Need Data Processing CCM4300 26 OSI Layer Services Layer Number 7 6 5 4 3 2 1 Layer Name Layer Services Application Presentation Session Transport Network Data Link Support for e-mail, file transfer etc. Data representation Control the dialog End-to-end data integrity Internet addressing, routing and segmentation Package bits into frames and control their delivery Adapt bits for transmission over the medium Physical CCM4300 27 Question? If OSI is a Reference Model then what is a network architecture? A set of layers and protocol is called a network architecture. It defines communication protocols, message formats, and standards required for interoperability. CCM4300 28 OSI Central Concepts Service vs. Interface Vs. Protocol Service – what is done - defines what the layer does (but not how entities above access it or how it works) Interface – how it is called above - tells the processes above how to access it - specifies what the parameters are and the results to expect Protocol – how it “talks” to its peer layer - how a layer works (i.e. provides the offered services) CCM4300 29 Advantages of Layering Standard interfaces between layers - allows internal developments within a particular layer to evolve Alternative services may be offered at a given layer - via different options or routes through the layer Internal mechanisms of each layer are invisible to the other layers Layers may be completely removed if not required, or substituted by simpler versions CCM4300 30 Problems of OSI The following factors limited adoption of OSI in practice: Timing - TCP/IP protocols were already in widespread use Technology - the 7 layers are not optimal (e.g. Session and Presentation layers hardly perform any function) CCM4300 31 Problems of OSI - cont Implementation - initial implementations were unwieldy and slow led to a lasting bad reputation Politics - perceived as bureaucratic organisations attempting to impose inferior standard BUT valuable as a conceptual architecture - a ‘reference model’ for comparison purposes CCM4300 32 Mapping onto OSI Source : Computer Networks 2003 CCM4300 33 TCP/IP Protocols Source : Computer Networks 2003 CCM4300 34 OSI & TCP/IP Protocols OSI - model useful as a means of discussing computer networks and educational purposes - protocols have not become popular yet TCP/IP - model practically non-existent - protocols very widely used CCM4300 35 Question? A. application, transport, internet, host-to-network B. application, internet, transport, host-to-network C. application, presentation, session, network, transport, data link, physical D. application, presentation, session, transport, network, data link, physical CCM4300_09-10 36 Client/Server Paradigm • A client server system is more structured than general distributed computing • A client sends request to servers to execute tasks • The tasks may be just to provide information, or to perform a complex computation (perhaps returning data, results, etc) • A client and servers are asymmetric • A server may be a client of another server CCM4300 37 Client/Server Properties • Clients and servers are separate processes • They may run on the same or different machines • Each process can hide internal information • Each process can implement its own set of business rules (integrity) • They communicate by peer-to-peer protocols CCM4300 38 Physical Layer It defines everything that is required to support the transmission and reception of signals (i.e. 1s and 0s) The Physical layer has four functional areas: 1. Electrical – signal type, amplitude, etc 2. Mechanical – connectors, cabling, etc 3. Procedural – control and timing 4. Functional - requirements for activating, maintaining, and deactivating a physical link between end systems. CCM4300 39 Physical Layer - cont The physical layer is usually a combination of software and hardware programming and may include electromechanical devices. All wiring, power, cabling and connections are part of the physical layer. Without the physical layer functioning properly none of the upper layers will respond correctly. It has no mechanism for determining the significance of the bits it transmits or receives. The onus for this is passed on to higher layer protocols CCM4300 40 Question? If you were given one word to describe the physical layer what would it be? hint. Think like an electrical engineer! CCM4300 41 Physical Layer Function For transmission, the physical layer generally: convert framed data from Data Link Layer to a binary stream transmit framed data serially (that is, one bit at a time) as a binary system For reception, the physical layer generally: listens for inbound transmission that are addressed to its host device accept appropriately addressed streams pass the binary stream up to the Data Link Layer for reassembly into frames 42 Analogue or Digital? The term analogue refers to any physical device or signal that can continuously vary in strength or quantity, for example, voltage in a circuit CCM4300 43 Analogue or Digital? - cont The term digital refers to any physical device or signal that is coded in a binary form (i.e. 1s and 0s) CCM4300 Source : Data Communications, Computer Networks and Open Systems 44 1992 What is “speed” or capacity? In analogue communication bandwidth is the total capacity (or theoretical capacity) of a communication channel. bandwidth = highest frequency – lowest frequency The greater the bandwidth, the more signals that can be carried Example: Typical ordinary telephone lines (often called a voicegrade line) transmit frequencies from 300Hz to 3300Hz. bandwidth = 3300Hz – 300Hz => 3000Hz, or 3kHz CCM4300 45 What is “speed” or capacity? - cont In digital communication, bandwidth is referred to as data rate Data rate – amount of data that can be transmitted over a communications medium in a given period. Data rates measured in bits per second (bps) an can vary considerably from one type of channel to another. For example, the bandwidth of dialup connections using a modem ranges from 300bps to 33,600bps (33.6kbps) or 56kbps. CCM4300 46 What is “speed” or capacity? - cont If we measure data rates in bits per second, then what is baud rate? The speed in baud (symbol rate) is equal to the number of times the line condition (i.e. frequency, amplitude, voltage, or phase) changes per second. I.e., the number of distinctive events per sec. Named after French engineer Jean Maurice Emile Baudot (1845 – 1903) For example a communication channel transmitting at 2400 baud. If each signal is used to represent one bit, then the baud rate is equal to the data rate - 2400bps. If each signal represents four bits, then the baud rate – 2400, but the data rate is 4 X 2400bps = 9600bps. CCM4300 47 What is “speed” or capacity? - cont Is there any difference between bandwidth and throughput? Bandwidth represents a theoretical capacity of a communications channel. The “reality rate” is known as throughput. Just because a medium or LAN architecture is specified to operate at a certain data rate, it is not a valid assumption to assume that this rate will be the actual throughput achieved. CCM4300 48 Transmission medium Transmission medium can be: Simplex - transmission in one direction only Half-duplex - transmission in both direction; but not at the same time Full-duplex (duplex) - simultaneous transmission in both directions. CCM4300 49 Twisted Pair Twisted pair cabling comes in two varieties: shielded and unshielded. Unshielded twisted pair (UTP) is the most popular Two insulated wires are twisted around each other, and combined with others into cable Each pair is twisted with a different number of twists per inch eliminates interference from adjacent pairs and other electrical devices. RJ-45 Connector Unshielded twisted pair CCM4300 50 Twisted Pair - cont Several techniques can be used to improve throughput: Increase the thickness of the conductor Increase the twist of rate Use several different twist rates in bundle of multiple pairs Shield the pairs with a metallic barrier Type Use Category 1 Voice Only (Telephone Wire) Category 2 Data to 4 Mbps (LocalTalk) Category 3 Data to 10 Mbps (Ethernet) Category 4 Data to 20 Mbps (16 Mbps Token Ring) Category 5 Data to 100 Mbps (Fast Ethernet) Cat 6a: Suitable for 10GBase-T. Defined up to 500 MHz. 51 Coaxial Cable Coaxial cabling has a single copper conductor at its center A plastic layer provides insulation between the center conductor and a braided metal shield. The metal shield helps to block any outside interference from fluorescent lights, motors, and other computers Bayone-Neill-Concelman (BNC) Coaxial cable CCM4300_09-10 52 Coaxial Cable The two types of coaxial cabling are thick coaxial and thin coaxial (refers to diameter - 0.25inch and 0.5inch). - Thin coaxial cable is also referred to as thinnet. 10Base2 refers to the specifications for thin coaxial cable carrying Ethernet signals. The 2 refers to the approximate maximum segment length being 200 meters. - Thick coaxial cable is also referred to as thicknet. 10Base5 refers to the specifications for thick coaxial cable carrying Ethernet signals. CCM4300_09-10 53 Optical Fibre Optical fiber is a thin, flexible medium capable of conducting an optical ray It transmits light rather than electronic signals eliminating the problem of electrical interference Very high bandwidth (currently up to 100Gbps) Used for long-distance trunks, local area networks, highspeed transmissions Fibre Optic Cable SC Connector CCM4300_09-10 ST Connector 54 Question? Which of the following is not defined at the physical layer of the OSI reference model? A. hardware addresses B. bitstream transmission C. voltage levels D. physical interface CCM4300_09-10 55 Data Link Layer Regulates and format transmission of data from software on a node to the network cabling facilities. It acts like a “Glue” between the wire and the software on a node. Some of the services the data link layer provides to the network layer include: framing – involves partitioning data into frames with recognized frame boundaries and exchange these frames over the link frame sequencing – involves maintaining the correct ordering of frames as they are being exchanged establishing and maintaining an acceptable level of flow control as frames are being exchanged across a link CCM4300_09-10 56 Data Link Layer - cont detecting (and possibly correcting) errors in the physical layer, which includes error notification when errors are detected but not corrected selecting quality of services (QoS) parameters associated – ensuring sufficient bandwidth is available and that transmission delays (i.e latency) are predictable and guaranteed. the data link layer enables data frames to be transmitted error-free between two end nodes over the physical layer CCM4300_09-10 57 Data Link Layer - cont How is the data link layer implemented within a network? Typically implemented on a node as device drive (i.e. firmware layer of the network interface card), which is a software component that is specified to both a piece of hardware (e.g. network interface card), and the operating system of the computer in which it is installed. CCM4300_09-10 58 OSI Reference Model vs IEEE 802 Model IEEE initiated its development of the LAN standards with an architectural model, defined in IEEE 802.1 The architectural model corresponds to the two lowest layers of the OSI Model with the following differences: The IEEE divides OSI’s data link layer into two parts – the logical link control (LLC) and the medium access control (MAC) sublayers Note: The MAC sublayer has nothing to do with Apple Computer’s Machintosh CCM4300_09-10 59 OSI Reference Model vs IEEE 802 Modle - cont Network Layer Network Layer Logical Link Control Data Link Layer Physical Layer Media Access Control OSI Model Physical Layer IEEE 802 Model CCM4300_09-10 60 LLC and MAC The LLC sublayer (i.e. upper half of data link layer) encompass several functions – framing, flow control and error control The MAC sublayer (i.e. lower half of data link layer) provides media access management protocols for accessing a shared medium. CCM4300_09-10 61 Logical Link Layer - Framing Framing enables synchronize the transmission and reception of data since frames have detectable boundaries. Integrity of frames - detection and correction Data set to be transmitted: 1101110011011011 Thus, the frame to be transmitted is: 01111110 Start of frame 1101110011011011 User Data CCM4300_09-10 01111110 End of Frame 62 Logical Link Layer – Error Control The term error control refers to the process of guaranteeing reliable data delivery Two basic strategies: error control through retransmission (also known as errordetecting codes) – provides enough information in the data stream to detect errors during transmission (e.g. parity, cyclic redundancy check (CRC)) autonomous error correction (also known as errorcorrecting codes) – provides redundant information in the data stream to detect and correct any errors autonomously (e.g. hamming distance) CCM4300_09-10 63 Media Access Control The MAC sublayer provides the protocol that define the manner in which nodes share the single physical transmission medium. The IEEE 802 specifications recognizes three different forms of media access: Contention Demand priority (not so common anymore) Token passing CCM4300_09-10 64 Media Access Control - Contention Contention based media access is embodied in the Carrier Sense, Multiple Access with Collision Detection (CSMA/CD) scheme 802.3 As its name implies: - requires station to check the wire to determine whether any other station is already sending data - If the station can sense an available carrier signal on a wire, it is free to transmit The sensing of carrier signal does not necessarily guarantee a free transmission media – collide with previously transmitted signal on what appeared to be an idle medium. CCM4300_09-10 65 Media Access Control – Token Passing Project 802’s specifications include three different tokenbased protocols – 802.4 Token Bus, 802.5 Token Ring, and Fiber Distributed Data Interface (FDDI); generally pronounced as a word; fiddy. A token is a special frame that is passed from device to device, in sequence along the ring. It can circulate only when the ring is idle A device must have this token to place data frames on the network. CCM4300_09-10 66 MAC Protocols: a taxonomy Access Method Taking-turn Protocols / Round Robin Random Access Protocols / Contention Slotted Aloha Aloha CSMA/CD Channel Partitioning / Reservation TDMA FDMA CDMA Token Token Slotted Ring Ring Bus Goal: efficient, fair, simple, decentralised CCM4300_09-10 67 Desirable MAC characteristic ||| When only one node tx – throughput R bps ||| When M nodes tx – each node throughput R/M bps (average tx rate over suitable defined interval of time) ||| Decentralised protocol – no master to bring system down ||| Simple protocol – inexpensive to implement CCM4300_09-10 68 Random Access protocols ||| When node has packet to send - transmit at full channel data rate R. - no a priori coordination among nodes ||| two or more transmitting nodes -> “collision”, ||| random access MAC protocol specifies: - how to detect collisions - how to recover from collisions (e.g., via delayed retransmissions) ||| Examples of random access MAC protocols: - pure ALOHA - slotted ALOHA - CSMA and CSMA/CD CCM4300_09-10 69 Pure (unslotted) ALOHA ||| Norman Abramson – surfing & packet switching ||| unslotted Aloha: simpler, no synchronization ||| pkt needs transmission: - send without awaiting for beginning of slot ||| collision probability increases: - pkt sent at t0 collide with other pkts sent in [t0-1, t0+1] ||| If collision occurs – transmit after different time intervals CCM4300_Sep 08 70 Slotted Aloha ||| time is divided into equal size slots (= pkt trans. time) ||| node with new arriving pkt: transmit at beginning of next slot ||| if collision: retransmit pkt in future slots with probability p, until successful. ||| decentralized – independently decides when to transmit but needs synchronization Success (S), Collision (C), Empty (E) slots CCM4300_09-10 71 Recap: Pure/Slotted ALOHA ||| Pure ALOHA access protocol - Data from remote terminals to the central computer site share a common transmission medium, a uhf radio channel. - Any terminals with data to transmit simply sends a packet. Hence there are occasional collisions. ||| Slotted ALOHA access protocol - Similar to pure ALOHA except that packet transmission occurs only in agreed time slot - This doubles the maximum throughput compared to pure ALOHA, because a collision occupies no more than one time slot CCM4300_09-10 72 Recap ….. Cont. ||| Nodes decision to transmit is made independent of the activity of the other nodes attached to the broadcast channel ||| Nodes neither pays attention to whether another node begins transmitting nor stops transmission if another node begins to interfere with transmission CCM4300_09-10 73 Channel Partitioning MAC protocols What is multiplexing? ||| A mux (acronym for multiplexer) enables data of multiple transmission channels to share a common link. ||| In Its simplest form, multiplexing involves combining data from several relatively low-speed input channels and transmitting these across a single high-speed circuit. Which layer of the OSI does multiplexing occurs? Layer 1 - Physical Layer CCM4300_09-10 74 Network Core: Circuit Switching Capacity of medium exceeds the capacity required for transmission of a single signal How can we improve “efficiency”? Let’s multiplex! Divide link bandwidth into “pieces”: frequency division - FDMA time division – TDMA CCM4300_09-10 75 Channel Partitioning MAC protocols: TDM TDM: time division multiplexing ||| access to channel in "rounds" ||| each station gets fixed length slot (length = pkt trans time) in each round ||| unused slots go idle ||| example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle CCM4300_09-10 76 Channel Partitioning MAC protocols: FDM frequency bands FDM: frequency division multiplexing ||| channel spectrum divided into frequency bands ||| each station assigned fixed frequency band ||| unused transmission time in frequency bands go idle ||| example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle CCM4300_09-10 77 Example: FDMA and TDMA Example: FDMA 4 users frequency time TDMA frequency time CCM4300_09-10 78 Advantages / Disadvantages ||| Advantages - eliminates collision - perfectly fair; each node gets dedicated transmission ||| Disadvantages - node is limited to average rate even when it is the only node with frames to send - node must always wait for its turn in transmission sequence even when it is the only node with a frame to send – only for TDM CCM4300_09-10 79 Channel Partitioning (CDMA) CDMA (Code Division Multiple Access) ||| unique “code” assigned to each user; ie, code set partitioning ||| used mostly in wireless broadcast channels (cellular, satellite,etc) ||| all users share same frequency, but each user has own “chipping” sequence (ie, code) to encode data ||| encoded signal = (original data) X (chipping sequence) ||| decoding: inner-product of encoded signal and chipping sequence ||| allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are CCM4300_09-10 “orthogonal”) 80