Slide 1: 1 Physical Layer Part 2
Transmission Media Slide 2: 2 MULTIPLEXING Whenever the bandwidth of a medium linking two devices is greater than the bandwidth needs of the devices, the link can be shared. Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link. As data and telecommunications use increases, so does traffic. Frequency-Division MultiplexingTime-Division Multiplexing
Wavelength-Division Multiplexing Topics discussed in this section: Slide 3: 3 Dividing a link into channels Slide 4: 4 Categories of multiplexing Slide 5: 5 Frequency-division multiplexing Slide 6: 6 FDM is an analog multiplexing technique that combines analog signals. Frequency Division Multiplexing : Frequency Division Multiplexing Each signal is modulated to a different carrier frequency
Carrier frequencies separated so signals do not overlap (guard bands)
Channel allocated even if no data
each channel occupies a fraction of the bandwidth of the link
a channel is defined by its center frequency, and its bandwidth.
Example: radio and television signal transmission Frequency Division MultiplexingDiagram : Frequency Division MultiplexingDiagram FDM of Three Voiceband Signals : FDM of Three Voiceband Signals Slide 10: 10 FDM process Slide 11: 8 FDM demultiplexing example Slide 12: 12 Wavelength Division Multiplexing : Wavelength Division Multiplexing Multiple beams of light at different frequency
Carried by optical fiber
A form of FDM
Each color of light (wavelength) carries separate data channel
1997 Bell Labs
Each at 10 Gbps
Giving 1 terabit per second (Tbps) WDM Operation : WDM Operation Same general architecture as other FDM
Number of sources generating laser beams at different frequencies
Multiplexer consolidates sources for transmission over single fiber
Optical amplifiers amplify all wavelengths
Typically tens of km apart
Demux separates channels at the destination Slide 15: 15 Wavelength-division multiplexing Slide 16: 16 WDM is an analog multiplexing technique to combine optical signals. Slide 17: 17 Prisms in wavelength-division multiplexing and demultiplexing Slide 18: 18 TDM Slide 19: 19 TDM is a digital multiplexing technique for combining several low-rate channels into one high-rate one. Slide 20: Transmission Media A transmission medium can be broadly defined as anything that can carry information from a source to a destination.
The transmission medium is usually free space, metallic cable or fiber – optic cable. Slide 21: 21 Transmission medium and physical layer Slide 22: 22 Classes of transmission media Slide 23: 23 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. Out of these twisted-pair cable, coaxial cable transport signals in the form of electric signals and fiber-optic cable transport signals in the form of light. Twisted-Pair CableCoaxial CableFiber-Optic Cable Topics discussed in this section: Slide 24: 24 Twisted-pair cable One of the wires is used to carry signals to the receiver, and the other is used only as a ground reference. The receiver uses the difference between the two. Slide 25: 25 UTP and STP cables Slide 26: 26 Categories of unshielded twisted-pair cables Slide 27: 9 UTP connector Slide 28: Coaxial cable Coaxial cable carries signals of higher frequency ranges than those in twisted-pair cable, in part because the two media are constructed quite differently. 10 Slide 29: 29 Categories of coaxial cables Slide 30: 30 BNC connectors (Bayone-Neil-Concelman) BNC Connector: used to connect the end of the cable to a device, such as TV Set BNC T Connector: used in Ethernet networks to branch out to a connection to a computer or other device. BNC Terminator: used at the end of the cable to prevent the reflection of the signal. Slide 31: 13 Coaxial cable performance Slide 32: 32 Bending of light ray 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, the ray changes direction. Slide 33: 33 Optical fiber Slide 34: 16 Propagation modes Slide 35: 35 Modes Slide 36: 36 Fiber types Slide 37: 37 Fiber construction Outer jacket – PVC or Teflon Kevlar – for fabrication of bullet proof vests Plastic buffer: cushioning Slide 38: 38 Fiber-optic cable connectors Slide 39: Fiber-optic cable connectors Subscriber Channel (SC) Connector is used for cable TV. It uses a push/pull locking system
Straight – tip (ST) Connector is used for connecting cable to networking devices. It uses a bayonet locking system and is more reliable than SC.
MT-RJ is a connector that is the same size as RJ45. Slide 40: 40 UNGUIDED MEDIA: WIRELESS Unguided media transport electromagnetic waves without using a physical conductor. This type of communication is often referred to as wireless communication. Radio Waves
Infrared Topics discussed in this section: Slide 41: 41 Electromagnetic spectrum for wireless communication Slide 42: 42 Propagation methods Slide 43: 43 Bands Slide 44: 44 Wireless transmission waves Slide 45: 45 Omnidirectional antenna Slide 46: 46 Radio waves are used for multicast communications, such as radio and television, and paging systems. Slide 47: 47 Unidirectional antennas Slide 48: 48 Microwaves are used for unicast communication such as cellular telephones, satellite networks, and wireless LANs. Slide 49: 49 Infrared signals can be used for short-range communication in a closed area using line-of-sight propagation. Slide 50: 50 CIRCUIT-SWITCHED NETWORKS A circuit-switched network consists of a set of switches connected by physical links. A connection between two stations is a dedicated path made of one or more links. However, each connection uses only one dedicated channel on each link. Each link is normally divided into n channels by using FDM or TDM. Slide 51: 51 A circuit-switched network is made of a set of switches connected by physical links, in which each link is divided into n channels. Slide 52: 52 In circuit switching, the resources need to be reserved during the setup phase; the resources remain dedicated for the entire duration of data transfer until the teardown phase. Slide 53: 53 Delay in a circuit-switched network Slide 54: 54 Switching at the physical layer in the traditional telephone network uses
the circuit-switching approach. Slide 55: 55 DATAGRAM NETWORKS In data communications, we need 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. The size of the packet is determined by the network and the governing protocol. Slide 56: 56 In a packet-switched network, there is no resource reservation;
resources are allocated on demand. Slide 57: 57 A datagram network with four switches (routers) Slide 58: 58 Routing table in a datagram network Slide 59: 59 A switch in a datagram network uses a routing table that is based on the destination address. Slide 60: 60 The destination address in the header of a packet in a datagram network
remains the same during the entire journey of the packet. Slide 61: 61 Delay in a datagram network Slide 62: 62 Switching in the Internet is done by using the datagram approach to packet switching at the network layer.