Lecture 4 Data Transmission Modes and Switching Data Transmission & Switching 1-1
Agenda Introduction to Data Transmission Modes Parallel and Serial Transmission Transmission Media Guided and unguided media Data Switching Techniques Circuit Switching Packet Switching Data Transmission & Switching 1-2
The Heterogeneity in the Internet PC server wireless laptop cellular handheld router access points wired links billions of connected computing devices: hosts = end systems running network apps communication links fiber, copper, radio, satellite transmission rate = bandwidth routers: forward packets (chunks of data) Mobile network Global ISP Home network Regional ISP Institutional network Data Transmission & Switching 1-3
Data Transmission Modes How are data sent via computer networks and related connecting wires/cables? Do we send 1 bit at a time? Do we group bits into larger groups? The transmission of binary data across a link can be accomplished in either parallel or serial mode Data Transmission & Switching 1-4
Data Transmission Modes Data Transmission & Switching 1-5
Parallel Data Transmission Binary data may be organized into groups of n bits each (i.e. n parallel channels) Multiple bits (usually 8 bits or a byte/character) are sent simultaneously on different wires/frequency channels with each clock tick Major advantage: speed Major disadvantage: cost Data Transmission & Switching 1-6
Serial Data Transmission One communication channel rather than n to transmit data between two communicating devices one bit is sent with each clock tick Major advantage: reduced cost Major disadvantage: speed Note: communication within devices is parallel, so there is a need to a converter (some overhead time is needed) Data Transmission & Switching 1-7
Serial Data Transmission Serial transmission occurs in one of three ways Asynchronous groups of bits are sent as independent units with start/stop flags and no data link synchronization Data Transmission & Switching 1-8
Serial Data Transmission Serial transmission occurs in one of three ways Synchronous sending bits one after another without start or stop bits or gaps. It is the responsibility of the receiver to group the bits. Data Transmission & Switching 1-9
Serial Data Transmission Serial transmission occurs in one of three ways Isochronous For real-time data transmission (e.g. audio and video applications) Guaranteeing delays between an entire sets of frames to be accepted (synchronous fails in guaranteeing fixed delay for data arrivals) Allowing a part of the access time of a channel to send isochronous real time data Data Transmission & Switching 1-10
Data Transmission & Switching 1-11
Transmission Media Transmission media are actually located below the physical layer and are directly controlled by the physical layer A transmission medium can be broadly defined as anything that can carry information from a source to a destination free space, metallic cable, or fiber-optic cable Data Transmission & Switching 1-12
Transmission Media Some Challenges in Transmission Media Unreliability due to the poor quality of the wires Noise Contention Long distances Power attenuation Various applications and operating communication frequency bands Wide frequency spectrum Electromagnetic energy and its impacts Reactive power Data Transmission & Switching 1-13
Transmission Media Two broad categories Guided media A signal traveling along any of these media is directed and contained by the physical limits of the medium Unguided media Signals are normally broadcast through free space and thus are available to anyone who has a device capable of receiving them Data Transmission & Switching 1-14
Transmission Media Data Transmission & Switching 1-15
Guided Transmission Media Twisted-Pair Cable (shielded and unshielded) Telephone lines Coaxial Cable Cable TV networks Fiber Optic Cable Backbone networks Data Transmission & Switching 1-16
Unguided Transmission Media Unguided signals can travel from a source to a destination in several ways Ground propagation radio waves travel through the lowest portion of the atmosphere Sky propagation higher-frequency radio waves radiate upward into the ionosphere where they are reflected back to earth Line-of-sight propagation very high-frequency signals are transmitted in straight lines directly from antenna to antenna Data Transmission & Switching 1-17
Data Switching A network is a set of connected devices how to connect them to make one-to-one communication possible? point-to-point connection between each pair of devices between a central device and every other device impractical and wasteful when applied to very large networks!!! A practical solution is network switching Data Transmission & Switching 1-18
Switched Network 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 Data Transmission & Switching 1-19
Taxonomy of Switched Networks Data Transmission & Switching 1-20
Taxonomy of Switched Networks Message Switched Networks Data routed in its entirety from the source node to the destination node, one hope at a time Data Transmission & Switching 1-21
Taxonomy of Switched Networks Message Switched Networks In message switching, each switch stores the whole message and forwards it to the next switch No packetization No resource allocation It is still used in some applications like electronic mail (e-mail) Data Transmission & Switching 1-22
Taxonomy of Switched Networks Circuit Switched Networks made of a set of switches connected by physical links, in which each link is divided into n channels Data Transmission & Switching 1-23
Taxonomy of Switched Networks Circuit Switched Networks A connection between two stations is a dedicated path made of one or more links Each connection uses only one dedicated channel on each link Each link is normally divided into n channels by using multiplexing techniques, such as FDM or TDM (making shared medium) Data Transmission & Switching 1-24
Circuit Switched Networks FDM Example: 4 users frequency TDM time frequency time Data Transmission & Switching 1-25
Taxonomy of Switched Networks Circuit Switched Networks Circuit switching takes place at the physical layer Before starting communication, the stations must make a reservation for the resources to be used during the communication (resource reservation) The data are a continuous flow sent by the source station and received by the destination station (continuous data flow) There is no addressing involved during data transfer (no addressing) Data Transmission & Switching 1-26
Example (circuit switched networks) Design and sketch a circuit-switched network that connects computers in two remote offices of a private company. The offices are connected using a T-1 line leased from a communication service provider. There are two 4 X 8 (4 inputs and 8 outputs) switches in this network. For each switch, four output ports are folded into the input ports to allow communication between computers in the same office. Four other output ports allow communication between the two offices. Data Transmission & Switching 1-27
Example (circuit switched networks) Solution Data Transmission & Switching 1-28
Structure of Circuit Switches (Example) Time Division Switch (TDS) Time Slot Interchange (TSI) Data Transmission & Switching 1-29
Circuit Switched Networks Operation Phases Setup Phase A dedicated circuit needs to be established Data Transfer Phase After the establishment of the dedicated circuit, 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. Data Transmission & Switching 1-30
Delay in a Circuit-Switched Network Setup the channel The total delay = T transmission + T propagation + T Setup + T teardown Data Transmission & Switching 1-31
Taxonomy of Switched Networks Packet Switched Networks: Datagram Networks The data message is divided into packets of fixed or variable size The size of the packet is determined by the network and the governing protocol There is no resource pre-allocation for a packet No reserved bandwidth on the links No scheduled processing time for each packet Data Transmission & Switching 1-32
Taxonomy of Switched Networks Packet Switched Networks: Datagram Networks Each packet (called datagram) is treated independently of all others (stop and forward) Packets belong to the same message may travel different paths to reach the destination Data Transmission & Switching 1-33
Delay in a Datagram Network For n links, The total delay = n T transmission + n T propagation + (n 1) T waiting Data Transmission & Switching 1-34
Switching in the Internet Switching in the Internet is done by using the datagram approach to packet switching at the network layer Data Transmission & Switching 1-35
Taxonomy of Switched Networks Packet Switched Networks: Virtual Circuit Switched Networks A virtual-circuit network is a cross between a circuitswitched network and a datagram network There are setup and teardown phases Resources can be allocated during the setup phase Data are packetized and each packet carries an address in the header (common with datagram) All packets follow the same path established during the connection (common with circuit switching) Normally implemented in the data link layer (shared medium) Circuit-switched network is implemented in the physical layer Datagram network in the network layer 1-36
Delay in a Virtual Circuit Switched Network For n links, The total delay = n T transmission + n T propagation + T Setup + T teardown Data Transmission & Switching 1-37
Addressing in Virtual Circuit Switched Networks Global Addressing A source or a destination needs to have a global address (i.e. an address that can be unique) Local Addressing Virtual-Circuit Identifier (VCI) A small number that has only switch scope It is used by a frame between two switches When a frame arrives at a switch, it has a VCI; when it leaves, it has a different VCI Data Transmission & Switching 1-38
Comparison between circuit and packet switching Data Transmission & Switching 1-39
Exercise (a) Use circuit switching and packet switching networks techniques to allocate bandwidth for 4 network users (b) Study the special case for a packet switching network when you have 20 users and there will be 4 active users Each user generates 250kbps when active Probability that user is active 25% of time Hint: calculate the probability that there are 4 or less simultaneously active users 4 users router Link 1 Mbps Data Transmission & Switching 1-40
Lecture-related Questions Why does a circuit-switched network need end-to-end addressing during the setup and teardown phases? Why are no addresses needed during the data transfer phase for this type of network? What is the role of the address field in a packet traveling through a datagram network? Data Transmission & Switching 1-41
Lecture Summary Covered material Introduction to Data Transmission Modes Parallel and Serial Transmission Transmission Media Guided and unguided media Data Switching Techniques Circuit Switching Packet Switching Material to be covered next lecture Introduction to the Data Link Layer Error Detection and Correction Flow and Error Control Data Transmission & Switching 1-42