Physical Interconnection Requirements Habib Youssef, Ph.D Department of Computer Engineering King Fahd University of Petroleum and Minerals Dhahran, Saudi Arabia COMPUTER NETWORKS Communication Requirements Essential issues in a data communication system: l Physical Interface Connectors : Shape, size, no. of pins, serial/parallel. l Protocols Rules of communication at various layers. Codes/formats. Communication Requirements (Cont.) Basic concept behind a protocol is l Handshaking (hardware) l Syntax, semantics, and procedure rules (software) Communication Requirements (Cont.) The protocol allows each party to show the other end that it has something to send, it is ready to accept messages, a message has been received, and the reception has been successful. If any of the communication steps fails, the protocol should indicate this, and each party follows a predefined set of rules too handle the exception. Purpose of Physical Layer Connections l The basic purpose of the OSI Physical Layer is To adapt the digital signals to allow them to be communicated across the physical medium. l Examples include Convert digital signals to tones for communications across a voice grade telephone circuit. Convert digital signals to light (on/off) for communications across a fiber-optic circuit. Purpose of Physical Layer Connections (Cont.) l The communications circuit may need to be Established (initially) Controlled or maintained Released when no longer needed l The Physical Layer may also be responsible for sharing (multiplexing) the communications circuit. Inter- vs. Intracomputer Communications l Data communications characteristics differ from those within a computer system. Bit serial transmission Handling control information (inband control) Higher error rate (need error detectioon and correction) l These issues are discussed on the following slides Inter- vs. Intracomputer Communications(Cont.) Host Internal Bus External Communications Line Serial vs. Parallel Transmission l Internal computer buses transfer many bits in parallel. Data Address Timing & Control Inband Control Bit Serial transmission line Which bits are data, which are address, and which are control ? How is timing (clocking) determined at the receiver? Framing Control l A sequence of bits on the line is called frame l There is a known format of the serial data frame Control InformationData Framing Control (Cont.) l Need to determine the beginning of the frame StartFrame l Known format then provides separation of control and data StartControl InformationData Frame Some Examples of Framing Control l Using the flag pattern of the data link protocols FlagFrameFlag l Using the Ethernet preamble/start pattern Frame 1011 (Null) l The Token Ring start and stop indicators StartEndFrame Error Rates l The physically lines have inherently different error properties. l The average error rate: the fraction of bits delivered with errors; e.g.,one in 10 5 for telephone channels For lengthy transmissions, this error rate is often unsatisfactory It must be improved by higher level protocol mechanisms Error Rates (Cont.) l Some media may have error rates as low as one in May be adequate for many purposes; e.g., digitized images Still typically have higher level protocol recovery mechanisms Switched Voice-Grade Telephone Channels l Direct-dial analog telephone channels Dial-up modem use l Normal voice line Limited to about 3000 Hz bandwidth l The local loop is a two-wire circuit To the central office(exchange) Switched Voice-Grade Telephone Channels (Cont.) Modem Analog Digital Switched telephone network Switched Voice-Grade Telephone Channels (Cont.) PSTN PAD Home PC Leased Voice-Grade Telephone Channels l Leased (dedicated) analog telephone channels Sometimes called conditioned lines l Often used for 19.2-kbit/s transmission l Fixed monthly cost, independent of usage Leased Voice-Grade Telephone Channels (Cont.) 4-wire modem Two one-way analog circuits Router PSTN Modem Analog Communications Channels l Voice-grade telephone channels have a 3kHz bandwidth 300 to 3300 Hz l Data rate depends on BandWidth (BW) The bit/s data rate is usually two to three time the BW For example, 9600 bit/s over 3000 Hz (3 kHz) l Data rate also depends on the signal-to- noise ratio Analog Communications Channels (Cont.) kH bandwidth Pass 100% Frequency, Hz kHz=1000 hertz Digitized Voice Channels l Digitized voice channels can also be used for digital data l Analog voice signals are digitized Time Samples 8000 samples per second 56 kbit/s or 64 kbit/s Send digitized value of each sample 7 or 8 bits per sample Digitized Voice Channels (Cont.) l Digitized samples are placed in a slot in each frame Frame NFrame N Slot no. 2 Digitized Voice Channels (Cont.) The frames for digitized voice have two different forms : l T1 has 24 slots per frame 24 slots at 56 kbit/s (or 64 kbit/s) A total of Mbit/s l E1 or CEPT 32 slots at 64 kbit/s 2.048 Mbit/s Digital Telephone Channels l Digital (instead of analog) telephone communications channels are also available 56 or 64 kbit/s channels (or a multiple) 1.544 Mbit/s (US, Canada, and Japan) or 2.048Mbit/s (Europe) channels Digital Telephone Channels (Cont.) l Instead of modem, Data Service Unit / Channel Service Unit (DSU/CSU) adapter devices are needed. The DSU adapts the digital signal (transmit and receive voltages and timing) The CSU normalizes voltage levels, provides maintenance capabilities, and protect the public network. Digital Telephone Channels (Cont.) DSU/CSU Inter-central office/exchange links (high data rates) Central office or exchange Reason for Going Digital l Computer data are inherently digital Adapt more easily to digital transmission l Easier to multiplex Time Division Multiplexing (TDM) l Easier to switch l Better error rate Noise is not cumulative, since repeaters can reject most induced noise Repeater Direction of Data Flow l Simplex l Half Duplex l Duplex (or Full duplex) ANALOG AND DIGITAL PHYSICAL INTERFACES COMPUTER NETWORK The RS-232/CCITT V.24 and V.28 Interface l Data processing (DTE) to modem (DCE) interface l The CCITT V.24 Recommendation defines the interchange circuits V.28 defines the electrical characteristics The RS-232/CCITT V.24 and V.28 Interface (Cont.) l In EIA, known as RS-232-C (the third [- C] version of RS-232) More recent version of RS-232-D (now EIA- 232-D) Sometimes TIA-232-D (Telecommunications Industry Association) The RS-232/CCITT V.24 and V.28 Interface (Cont.) l A 25-pin connector/interface ISO 2110 is used Is not part of the RS-232-C standard l Bit serial data (full duplex) l Out of band control lines The RS-232/CCITT V.24 and V.28 Interface (Cont.) The RS-232/CCITT V.24 and V.28 With Null Modems Pin Assignments for V.24/EIA- 232 RS-232/CCITT V.24 & V.28 Related Products l It is often convenient to switch RS- 232/V.24 signals from a computer to one of several devices For example, to different types of printers l Simple multiple switches are available for this purpose l Specialized companies have been developed to handle the interface market with products such as Multiple switches RS-232/V.24 cables Null modems RS-232/V.24 gender changers l Breakout boxes to monitor control signals RS-232/CCITT V.24 & V.28 Related Products (Cont.) Limitations of RS-232/V.28 l An upper data rate of about 20 kbit/s l An upper cable length of about 50 to 100 feet (about 20 to 40 m) l Some products are available to extend these, but a new approach is needed The Evolution of RS-232-C RS-232-C EIA-232-D (1987) Unbalanced circuits RS-530 (1987) Balanced Circuits V.35 Balanced Circuits RS-449 signals RS-422/423 electrical (1977) RS-442 balanced circuits RS-443 unbalanced circuits Synchronous Transmission l Has a known timing relationship between bits and characters l Characters are sent one after the other l The receiver recovers this timing from transitions in the arriving data StartEnd 1 0 Characters V.24/EIA-232 dial-up operation RS-423/CCITT V.10 Single Ended Interchange Circuit Signal return Trans Recvr Error Noise Note: V.10 is the same as X.26 or RS-423-A (unbalanced) RS-422/CCITT V.11 Differential Interchange Circuit Trans Recvr Noise Sensitive to differential signal Termination resistor Noise was rejected Note : V.11 is the same as X.27 or RS-422-A (balanced) CCITT X.21 Interface l Physical-level interface between DTE and DCE l For synchronous operations on public data networks l X.21 uses control transitions and ASCII characters rather than using separate signal lines CCITT X.21 Interface (Cont.) l The X.21 electrical characteristics are CCITT X.27 (balanced; same as V.11 and RS-422) CCITT X.26 (unbalanced; V.10 and RS- 423) (Note: For operation above 9600 bit/s, X.27 is required) l X.21 mechanical characteristics are 15-pin connector per ISO Standard 4903 CCITT X.21 Interface (Cont.) X.21 Switched 64 kbit/s DSUBridge 4 CCITT X.21 Interface (Cont.) CCITT X.21 bis l As an interim (perhaps longer term) provision, we have X.21 bis l X.21 bis utilizes RS-232 for use with X.25 l Particularly used in countries where X.21 has not yet become available CCITT X.21 bis (Cont.) l RS-232 signals are used to represent X.21 events To initiate the call l Some X.21 features are not supported Call progress signals ISDN Interface COMPUTER NETWORK Synchronous / Asynchronous Transmission Asynchronous Timing l Asynchronous means no predefined timing between characters l The sending and receiving ends provide their own clocking l The timing of asynchronous characters is T Character Start bit Next Character Start bit Asynchronous Timing (Cont.) l The receiver does not know when the next unit of data is coming The term async frequently is used this way X.25 PAD Async Clocking at the Sending End l The sending device determines when to transmit the start bit The start bit indicates the beginning of a character The bits of the character follow with a well- defined timing (LSB first) A party (error-check) bit is generated and sent There is at least one stop bit There is an arbitrary time before the next character is sent Clocking at the Sending End (Cont.) l Each character is framed with these control bits Memory Serial I/O hardware Character Start bit P Stop bit Hardware generated I/O = input/output Synchronous Transmission l Has a known timing relationship between bits and characters l Characters are sent one after the other l The receiver recovers this timing from transitions in the arriving data StartEnd 1 0 Characters Modulation l We will explore methods used to transmit digital data across analog channels. l A primary example of analog channels is the telephone companys voice-grade circuit. l There is one primary reason to use modems To be compatible with the voice-grade channel Modulation (Cont.) l The process of converting digital data into analog form is called modulation. Analog Digital l Generally, we get about 2 to3 bit/s per Hz of bandwidth of the analog channel (more or less based on complexity) Data Communications Interfacing Transmission line interface device Digital data transmitter/ receiver Transmission line interface device Digital data transmitter/ receiver Bit-serial transmission line (or bit-serial interface to network Data terminal equipment (DTE) Data circuit- terminating equipment (DCE) Generic interface to transmission medium Data Communications Interfacing (Cont.) Network EIA 232/ V.24 interface Modem External Modem Connections CCITT Modems Typical Modern Modem Capabilities l Many modern modems can operate in a number of modes, which are negotiated when the connection is established. V.32 operation at 9600 bit/s Or V.32 bis at bit/s Or V.42 bis at 2400 bit/s Typical Modern Modem Capabilities (Cont.) l Modems can automatically dial the telephone number V.25 bis sync/async autodial Or the non-CCITT Hayes AT command set (discussed later) l Modems can perform operations previously done by software V.42 error correction (discussed later) V.42 bis error compression (discussed later) Typical Modern Modem Capabilities (Cont.) l Modems can fall back to a lesser data rate if needed for communications, and some can later fall forward when possible l Leased-line modems can automatically dial a backup line as needed. The Hayes AT Command Set l The Hayes AT command set is an industry standard Controls modem operation Initiates dial sequence Hangs up Runs diagnostics Selects data compression feature Etc. l For more than 50 such modem commands The Hayes AT Command Set (Cont.) l The AT commands start with an escape sequence and AT(tention) l An example AT command is to dial a number +++ATDT When D is for dial, T is for tone, and is the telephone number CCITT V.42 and V.42 bis Modern Capabilities l The CCITT V.42 recommendation provides a reliable data transfer capability (error correction) There are actually two forms (CCITT couldnt agree on only one) The preferred approach s Link-Access Procedure for Modems (LAPM) MNP 4 is also included (see next slide) CCITT V.42 and V.42 bis Modern Capabilities (Cont.) l The CCITT recommendation V.42 bis builds on V.42 V.42 bis is a data compression standard Uses an automatic adaptation algorithm that handles different degrees of randomness in the data V.42 bis achieves a data compression factor of up to 4X Microcom Network Protocol (MNP) l The Microcom Network Protocol (MNP) is a set of communications protocols for enhancing modem communications Some are industry standards Others are proprietary to Microcom l Three protocols are identified by terms such as MNP 4, MNP class 4, or MNP level 4 Microcom Network Protocol (MNP) (Cont.) l MNP 4 is a reliable public-domain delivery protocol MNP 4 is built into hundreds of thousands of modems MNP 4 is part of the CCITT V.42 recommendation XMODEM File Transfer Protocol (1978) l XMODEM was the first file transfer protocol for use with PCs XMODEM actually predates PCs and DOS l XMODEM is available from many bulletin boards l Transfers are limited in many ways Transfers data in small (128-byte) blocks Operates as a simple stop and wait ACK/NAK protocol Inefficient use of links in excess of 1200 bit/s XMODEM File Transfer Protocol (Cont.) l There are many variations : YMODEM, ZMODEM, etc. Larger block sizes Better error detection DOS = disk operating system ACK = acknowledgement NAK = negative acknowledgement XMODEM File Transfer Protocol (Cont.) l The operating mode is negotiated at connection establishment Kermit (1981) l Kermit is available on many bulletin boards l Kermit was developed at Columbia University Well documented Intended for use between different computers Mainframes, minis, PCs Kermit (Cont.) l All transmitted bytes are printable ASCII (except ASCII SOH start) 7-bit code Avoids problems with control characters, for example, which might affect PAD operation. Remote-Control Software l The idea is that the remote PC takes over control of the office PC Remote keyboard and screen mirrors the other PC operations For access to your office PC from a remote PC; e.g. a laptop Or, to assist a remote user without having to go to that location Remote-Control Software (Cont.) l Remote-control software is required in both PCs A typical configuration is shown in our example internetwork PSTN Remotely controlled Roving laptop Terminal Emulation l A terminal-emulation program allows your PC to appear to be a terminal hat a remote host knows how talk to It may appear to be a scroll-mode terminal (e.g., VT100) It may appear to be a page-mode terminal (e.g., an IBM 3270) Terminal Emulation (Cont.) l Terminal emulation is a common approach To log in at a host or server To log in at any other device to access services For network management To read and write network management objects (variables) Fax Modem Facts l Some modems provide facsimile (fax) as well as data capabilities l Two commonly used recommendations for fax transmission V.29at 9600bit/s V.17 at bit/s Fax Modem Facts (Cont.) l Flow is unidirectional l Support software is required Class 1: Minimal processing on the fax board Class 2: More on-board processing, less required by the PC COMPUTER NETWORK MULTIPLEXING Multiplexing l It costs about the same amount of money to install and maintain a high bandwidth cable as a low bandwidth wire between two stations Need for multiplexing techniques to share a single communication channel between multiple stations. l Two classes of multiplexing schemes : Frequency Division Multiplexing (FDM) The frequency spectrum is divided among the logical channel, with each station having exclusive possession of its frequency band. Filters limit the usable bandwidth per channel. Multiplexing (Cont.) Time Division Multiplexing The stations take turns, each one periodically getting the entire bandwidth for a short interval of time. Multiplexing of Communications Links MUX Modem MUX CPU Remote terminals Time Division Multiplexing l Each user gets the channels full capacity for a period of time l Each user gets a time slot in each frame Start User NUser1User2User3 Start User1 One Frame l One character of user data is sent in each slot l If a user has nothing to send, the slot contains null Statistical Time Division Multiplexing (STDM) l Few users fill every slot assigned to them l This results in wasted slots l A better approach is statistical TDM l It operates as follows A user character is tagged with the port number Statistical Time Division Multiplexing (Cont.) Port no. Character Data fieldControl field (5)(8) Frame of tagged characters For example Statistical Time Division Multiplexing (Cont.) l Statistical multiplexing can be generalized to produce packet switching More control information Multiple characters of data Typical Statistical Multiplexer (STAT MUX) Example