Networking I (Lecture 2).pdf

Networking I Lecture 2 Dr. Su Wai Phyo Professor Department of Computer Engineering and Information Technology Yangon Technological University 1  Out...

Networking I Lecture 2 Dr. Su Wai Phyo Professor Department of Computer Engineering and Information Technology Yangon Technological University 1  Outlines Introduction to Physical Layer - Data and Signals Transmission Media - Guided Media - Unguided Media: Wireless Switching - Introduction - Circuit-Switched Networks - Packet Switching 2  Lecture Objectives To introduce communication in physical layer To describe the physical characteristics of coaxial cable, STP, UTP, and fiber-optic media To describe radio waves, microwaves, and infrared waves of their characteristics and applications To describe switching methods: packet switching and circuit switching 3 Topic 1: Introduction to Physical Layer 4  Data and Signals Figure 1. Communication at the Physical Layer 5  Data and Signals (Continue) Analog and Digital Data ❑ Data can be analog or digital. ❑ Analog data refers to information that is continuous; digital data refers to information that has discrete states. Analog and Digital Signals ❑ Like the data they represent, signals can be either analog or digital. ❑ An analog signal has infinitely many levels of intensity over a period of time. ❑ A digital signal, can have only a limited number of defined values. ❑ Although each value can be any number, it is often as simple as 1 and 0. 6  Data and Signals (Continue) ❑ The simplest way to show signals is by plotting them on a pair of perpendicular axes. ❑ The vertical axis represents the value or strength of a signal. ❑ The horizontal axis represents time. ❑ Figure 2 illustrates an analog signal and a digital signal. Figure 2. Comparison of Analog and Digital Signals 7 Topic 2: Transmission Media 8  Transmission Media: Guided Media ❑ Transmission media are actually located below the physical layer and are directly controlled by the physical layer. ❑ Figure 22 shows the position of transmission media in relation to the physical layer. ❑ A transmission medium can be broadly defined as anything that can carry information from a source to a destination. Figure 22. Transmission medium and physical layer 9  Transmission Media: Guided Media (Continue) ❑ In telecommunications, transmission media can be divided into two broad categories: guided and unguided. ❑ Guided media include twisted-pair cable, coaxial cable, and fiber-optic cable. ❑ Unguided medium is free space. ❑ Figure 23 shows this taxonomy. Figure 23. Classes of transmission media 10  Transmission Media: Guided Media (Continue) Guided Media ❑ Guided media, which are those that provide a conduit from one device to another, include twisted-pair cable, coaxial cable, and fiber-optic cable. Twisted-Pair Cable ❑ A twisted pair consists of two conductors (normally copper), each with its own plastic insulation, twisted together, as shown in figure 24. ❑ One of the wires is used to carry signals to the receiver, and the other is used only as a ground reference. Figure 24. Twisted-pair cable 11  Transmission Media: Guided Media (Continue) Unshielded Versus Shielded Twisted-Pair Cable ❑ The most common twisted-pair cable used in communications is referred to as unshielded twisted-pair (UTP). Figure 25. UTP and STP Cables 12  Transmission Media: Guided Media (Continue) Table 3: Categories of Unshielded Twisted-Pair Cables 13  Transmission Media: Guided Media (Continue) Connector - The most common UTP connector is RJ45 , as shown in figure 26. Figure 26. UTP Connector Performance - One way to measure the performance of twisted-pair cable is to compare attenuation versus frequency and distance. - A twisted-pair cable can pass a wide range of frequencies. 14 Transmission Media: Guided Media (Continue) - Figure 27 shows that with increasing frequency, the attenuation, measured in decibels per kilometer (dB/km), sharply increases with frequencies above 100 kHz. - Gauge is a measure of the thickness of the wire. Figure 27. UTP performance 15 Transmission Media: Guided Media (Continue) Applications - Twisted-pair cables are used in telephone lines to provide voice and data channels. Coaxial Cable - Coaxial cable (or coax) carries signals of higher frequency ranges than those in twisted-pair cable, in part because the two media are constructed quite differently. Figure 28. Coaxial cable 16  Transmission Media: Guided Media (Continue) Coaxial Cable Connectors - The most common type of connector used today is the Bayonet Neill- Concelman (BNC) connector as shown in figure 29. Figure 29. BNC connectors Applications - Coaxial cable was widely used in analog telephone networks where a single coaxial network could carry 10,000 voice signals. 17  Transmission Media: Guided Media (Continue) Performance - It can measure the performance of a coaxial cable. - The attenuation is much higher in coaxial cable than in twisted-pair cable. - In other words, although coaxial cable has a much higher bandwidth, the signal weakens rapidly and requires the frequent use of repeaters. Figure 30. Coaxial cable performance 18  Transmission Media: Guided Media (Continue) Fiber-Optic Cable - A fiber-optic cable is made of glass or plastic and transmits signals in the form of light. - Light travels in a straight line as long as it is moving through a single uniform substance. - If a ray of light traveling through one substance suddenly enters another substance (of a different density), the ray changes direction. - Optical fibers use reflection to guide light through a channel. - A glass or plastic core is surrounded by a cladding of less dense glass or plastic. - The difference in density of the two materials must be such that a beam of light moving through the core is reflected off the cladding instead of being refracted into it as shown in figure 31. 19  Transmission Media: Guided Media (Continue) Figure 31. Optical fiber Propagation Modes - Current technology supports two modes (multimode and single mode) for propagating light along optical channels, each requiring fiber with different physical characteristics. - Multimode can be implemented in two forms: step-index or graded- index as shown in figure 32. 20  Transmission Media: Guided Media (Continue) Figure 32. Propagation modes Multimode - Multimode is so named because multiple beams from a light source move through the core in different paths. - How these beams move within the cable depends on the structure of the core, as shown in figure 33. 21 Transmission Media: Guided Media (Continue) - In multimode step-index fiber, the density of the core remains constant from the center to the edges. - The term step-index refers to the suddenness of this change, which contributes to the distortion of the signal as it passes through the fiber. - A second type of fiber, called multimode graded-index fiber, decreases this distortion of the signal through the cable. Single -Mode - Single-mode uses step-index fiber and a highly focused source of light that limits beams to a small range of angles, all close to the horizontal. 22 Transmission Media: Guided Media (Continue) Figure 33.Modes 23  Transmission Media: Guided Media (Continue) Fiber-optic Cable Connectors - There are three types of connectors for fiber-optic cables as shown in figure 34. - The subscriber channel (SC) connector is used for cable TV. - The straight-tip (ST) connector is used for connecting cable to networking devices. - MT-RJ is a connector that is the same size as RJ45. Figure 34. Fiber-optic cable connectors Applications - Fiber-optic cable is often found in backbone networks because its wide bandwidth is cost-effective. 24  Transmission Media: Guided Media (Continue) Performance - The plot of attenuation versus wavelength in Figure 35 shows a very interesting phenomenon in fiber-optic cable. - Attenuation is flatter than in the case of twisted-pair cable and coaxial cable. - The performance is such that we need fewer (actually onetenth as many) repeaters when we use fiber-optic cable. Figure 35. Optical fiber performance 25  Unguided Media: Wireless Unguided medium transport electromagnetic waves without using a physical conductor. This type of communication is often referred to as wireless communication. Figure 36. Electromagnetic spectrum for wireless communication 26  Unguided Media: Wireless (Continue) Unguided signals can travel from the source to the destination in several ways: ground propagation, sky propagation, and line-of-sight propagation, as shown in figure 37. Figure 37. Propagation methods 27  Unguided Media: Wireless (Continue) In ground propagation, radio waves travel through the lowest portion of the atmosphere, hugging the earth. In sky propagation, higher-frequency radio waves radiate upward into the ionosphere (the layer of atmosphere where particles exist as ions) where they are reflected back to earth. In line-of-sight propagation, very high-frequency signals are transmitted in straight lines directly from antenna to antenna. Radio Waves - Electromagnetic waves ranging in frequencies between 3 kHz and 1 GHz are normally called radio waves. 28  Unguided Media: Wireless (Continue) Omnidirectional Antenna - Radio waves use omnidirectional antennas that send out signals in all directions as shown in figure 38. - Radio waves are used for multicast communications, such as radio and television, and paging systems. Figure 38. Omnidirectional Antenna 29  Unguided Media: Wireless (Continue) Microwaves - Electromagnetic waves having frequencies between 1 and 300 GHz are called microwaves. - Microwaves are unidirectional. - The characteristics of microwave propagation: 1. Microwave propagation is line-of-sight. 2. Very high-frequency microwaves cannot penetrate walls. 3. The microwave band is relatively wide, almost 299 GHz. 4. Use of certain portions of the band requires permission from authorities. 30  Unguided Media: Wireless (Continue) Unidirectional Antenna - Microwaves need unidirectional antennas that send out signals in one direction. - Two types of antennas are used for microwave communications: the parabolic dish and the horn as shown in figure 39. Figure 39. Unidirectional antennas 31  Unguided Media: Wireless (Continue) Applications - Microwaves are used for unicast communication such as cellular telephones, satellite networks, and wireless LANs. Infrared - Infrared waves, with frequencies from 300 GHz to 400 THz (wavelengths from 1 mm to 770 nm), can be used for short-range communication. Infrared waves, having high frequencies, cannot penetrate walls. 32 Topic 3: Switching 33  Switching: Introduction A switched network consists of a series of interlinked nodes, called switches. Switches are devices capable of creating temporary connections between two or more devices linked to the switch. In a switched network, some of these nodes are connected to the end systems (computers or telephones, for example). Others are used only for routing. Figure 40 shows a switched network. The end systems (communicating devices) are labeled A, B, C, D, and so on, and the switches are labeled I, II, III, IV, and V. Each switch is connected to multiple links. 34  Switching: Introduction (Continue) Figure 40. Switched Network Three methods of switching are circuit switching, packet switching, and message switching as shown in figure 41. Figure 41. Taxonomy of switched networks 35  Circuit-Switched Networks A circuit-switched network is made of a set of switches connected by physical links, in which each link is divided into n channels. Figure 42. A trivial circuit-switched network The end systems, such as computers or telephones, are directly connected to a switch. When end system A needs to communicate with end system M, system A needs to request a connection to M that must be accepted by all switches as well as by M itself. 36  Circuit-Switched Networks (Continue) This is called the setup phase; circuit (channel) is reserved on each link, and the combination of circuits or channels defines the dedicated path. After the dedicated path made of connected circuits (channels) is established, the data-transfer phase can take place. After all data have been transferred, the circuits are torn down. Three Phases - The actual communication in a circuit-switched network requires three phases: connection setup, data transfer, and connection teardown. Setup Phase - Before the two parties (or multiple parties in a conference call) can communicate, a dedicated circuit (combination of channels in links) needs to be established. 37  Circuit-Switched Networks (Continue) - The end systems are normally connected through dedicated lines to the switches, so connection setup means creating dedicated channels between the switches. Data-Transfer Phase - After the establishment of the dedicated circuit (channels), the two parties can transfer data. Teardown Phase - When one of the parties needs to disconnect, a signal is sent to each switch to release the resources. 38  Circuit-Switched Networks (Continue) Delay - Although a circuit-switched network normally has low efficiency, the delay in this type of network is minimal. - During data transfer the data are not delayed at each switch; the resources are allocated for the duration of the connection. - Figure 43 shows the idea of delay in a circuit-switched network when only two switches are involved. Figure 43. Delay in a Circuit-Switched Network 39  Packet Switching In data communications, it is needed to send messages from one end system to another. If the message is going to pass through a packet-switched network, it needs to be divided into packets of fixed or variable size. Two types of packet-switched networks are datagram networks and virtual- circuit networks. 40  Packet Switching (Continue) Datagram Network - In a datagram network, each packet is treated independently of all others. - Packets in this approach are referred to as datagrams. - Datagram switching is normally done at the network layer. - Figure 44 shows how the datagram approach is used to deliver four packets from station A to station X. - The switches in a datagram network are traditionally referred to as routers. - The datagram networks are sometimes referred to as connectionless networks. 41  Packet Switching (Continue) - The term connectionless here means that the switch (packet switch) does not keep information about the connection state. - There are no setup or teardown phases. Figure 44. A datagram network with four switches (routers) 42  Packet Switching (Continue) Routing Table - A switch in a datagram network uses a routing table that is based on the destination address. Figure 45. Routing table in a datagram network Destination Address - The destination address in the header of a packet in a datagram network remains the same during the entire journey of the packet. 43  Packet Switching (Continue) Delay - There may be greater delay in a datagram network than in a virtual- circuit network. - Although there are no setup and teardown phases, each packet may experience a wait at a switch before it is forwarded. - In addition, since not all packets in a message necessarily travel through the same switches, the delay is not uniform for the packets of a message. - Fig.46 gives an example of delay in a datagram network for one packet. Figure 46. Delay in a Datagram Network 44  Packet Switching (Continue) - The packet travels through two switches. - There are three transmission times (3T), three propagation delays (slopes 3τ of the lines), and two waiting times (w1 + w2). - The processing time is ignored in each switch. - The total delay is Total delay = 3T + 3τ + W1+ W2 Virtual-Circuit Networks - A virtual circuit (VC) is a means of transporting data over a packet switched computer network in such a way that it appears as though there is a dedicated physical layer link between the source and destination end systems of this data. 45  Packet Switching (Continue) - First packet goes and reserves resources for the subsequent packets which as a result follow the same path for the whole connection time. - A virtual-circuit network is normally implemented in the data- link layer. Figure 47. Virtual-Circuit Network Addressing - In a virtual-circuit network, two types of addressing are involved: global and local (virtual-circuit identifier). 46  Packet Switching (Continue) Global Addressing - A source or a destination needs to have a global address—an address that can be unique in the scope of the network or internationally if the network is part of an international network. Virtual-Circuit Identifier - The identifier that is actually used for data transfer is called the virtual- circuit identifier (VCI) or the label. Figure 48. Virtual-Circuit Identifier 47  Packet Switching (Continue) Three Phases - As in a circuit-switched network, a source and destination need to go through three phases in a virtual-circuit network: setup, data transfer, and teardown. Data-Transfer Phase - To transfer a frame from a source to its destination, all switches need to have a table entry for this virtual circuit. - Figure 49 shows a frame arriving at port 1 with a VCI of 14. - When the frame arrives, the switch looks in its table to find port 1 and a VCI of 14. 48  Packet Switching (Continue) - When it is found, the switch knows to change the VCI to 22 and send out the frame from port 3. - Figure 50 shows how a frame from source A reaches destination B and how its VCI changes during the trip. - Each switch changes the VCI and routes the frame. Figure 49. Switch and tables in a virtual-circuit network 49  Packet Switching (Continue) Figure 50. Source-to-destination data transfer in a virtual-circuit network 50  Packet Switching (Continue) Setup Phase - In the setup phase, a switch creates an entry for a virtual circuit. For example, suppose source A needs to create a virtual circuit to B. - Two steps are required: the setup request and the acknowledgment. Setup Request - A setup request frame is sent from the source to the destination. - Figure 51 shows the process. Figure 51. Setup request in a virtual-circuit network 51  Packet Switching (Continue) Acknowledgment - A special frame, called the acknowledgment frame, completes the entries in the switching tables. - Figure 52 shows the process. Figure 52. Setup Acknowledgment in a Virtual-Circuit Network 52  Packet Switching (Continue) Teardown Phase - In this phase, source A, after sending all frames to B, sends a special frame called a teardown request. - Destination B responds with a teardown confirmation frame. - All switches delete the corresponding entry from their tables. Delay in Virtual-Circuit Networks - In a virtual-circuit network, there is a one-time delay for setup and a one-time delay for teardown. - If resources are allocated during the setup phase, there is no wait time for individual packets. - Figure 53 shows the delay for a packet traveling through two switches in a virtual-circuit network. 53  Packet Switching (Continue) - The packet is traveling through two switches (routers). - There are three transmission times (3T ), three propagation times (3τ), data transfer depicted by the sloping lines, a setup delay, and a teardown delay. - The processing time is ignored in each switch. - The total delay time is Total delay = 3T + 3τ + setup delay + teardown delay - Switching at the data-link layer in a switched WAN is normally implemented by using virtual-circuit techniques. 54  Packet Switching (Continue) Figure 53. Delay in a Virtual-Circuit Network 55  Next Week Lecture ❑ Introduction to Data-Link Layer 56 Thank You 57