Chapter 8 Data Communication

2 Objectives Explain various ways to connect Data Terminal Equipment (DTE) to Data Communication Equipment (DCE). Describe a null-modem cable. Describe ASCII coding and Extended ASCII coding. Explain serial transmission.

3 Objectives (continued) Explain parallel transmission. Explain what a UART does. Explain asynchronous transmission. Explain synchronous transmission. Explain the error-detection technique used by synchronous transmission. Explain the error-correction techniques used by synchronous transmission.

4 Objectives (continued) Explain the term sliding window. Explain ACK and NAK. Explain baud rate and how it relates to bit rate. Explain how a modem converts a data signal into modulations of an analog signal. Explain what a CSU/DSU is used for.

5 Objectives (continued) Explain why a modem must be used to connect a PC to a regular phone line. Explain how a 56-Kbps modem achieves that speed. Explain how the end-to-end speed between two PCs can be 115,000 bps when the speed between the two modems is 28,800 bps. Explain the function of SS7. Explain how caller ID works.

6 Data Communication The primary difference between voice and data lies in the signal used to convey information. The PSTN started life as an analog network designed to handle analog voice signals. When digital technology was integrated into the switching matrix of class 5 SPC switching systems, the codec was moved from the channel unit of the T1 carrier system to the line circuit of the switch.

7 Data Communication Several digital technologies have been developed for voice communication that move the codec from the line circuit to the customer’s premises. Integrated Services Digital Network (ISDN) Asymmetric Digital Subscriber Line (ASDL)

8 Data Communication Most signals are two-state digital signals. T1 carrier systems use alternating +3 V and -3 V signals to represent a 1, and use 0 V to represent a 0 The PC belongs to a category of equipment called data terminal equipment (DTE). The equipment used to connect a DTE to the PSTN is called data communication equipment (DCE).

9 Data Communication The most common connection used to connect DTE to DCE is an EIA-232 interface. The EIA-232 interface cable comes in a 25- conductor and a 9-conductor cable. The transmit lead and receive lead in the cable get their designation from the DTE end of the cable.

10 EIA-232 Interface

11 Data Communication A communication circuit will not work if the transmitter is talking to another transmitter. The two computers must be connected using modems or a null-modem. The word modem is an acronym for modulator/demodulator. When data from a computer is fed into a modem, it modulates (changes) the analog signal being sent to the other modem.

12 RS-232 Null-Modem Cable

Data and the Public Switched Telephone Network Because the PSTN has been evolving from an analog into a digital network, it is possible to integrate data directly into the digital PSTN if the analog line circuit of a SPC switch is bypassed. If a facility is to be used only for data an not for voice, a channel service unit (CSU)/data service unit (DSU) is used to interface the data circuit directly into the PSTN.

14 Voice Circuit through Codec

15 Data Blocked by Codec

16 Modem Signal Passed by Codec

17 CSU/DSU Interface between Customer’s Premises and C.O.

18 Data and the PSTN It is also possible to transmit data directly over the local loop connecting the customer’s premises to the central exchange with a digital subscriber line circuit and ISDN. Cost is $100/month Versus $20/month for an analog line circuit The protocols used to transfer data are not the same for ISDN and a modem.

Data Communication and Coding Data communication is the transmission of data from one location to another. The data transmitted can be numeric or alphabetic. When data is transmitted, some code is used to represent various characters or numbers sent over the system. With the use of computers, a data communication network has become essential.

ASCII Coding The standard binary code developed for transmission of data is the American Standard Code for Information Interchange (ASCII). ASCII is the coding scheme used by a PC to store information. It uses 7 bits, each of which can be either a 1 or a 0.

21 Codes for Selected Decimal Digits

22 ASCII Code Chart

23 Universal Asynchronous Receiver/Transmitter (UART) PCs contain a hardware device between the data bus of the PC and the RS-232 serial port. The UART will send a start bit (a 0, which is +3 V or greater). It will then send the least significant data bit and each higher-order data bit until the most significant bit is sent. It then sends the parity bit. It then sends a stop bit (a 1, which is -3 V or less).

Extended ASCII Coding It was not long before we needed to represent more than 128 different characters. To represent more than 128 different characters, an eight-level coding system was developed called Extended ASCII. The first 128 codes are the same as for regular seven-level ASCII. The codes from 128 to 255 are used to represent additional characters.

25 Extended ASCII Code Chart

26 EBCDIC Extended Binary Coded Decimal Interchange Code An eight-level coding system developed by IBM. Used in its mainframes.

27 Unicode A computing industry standard allowing computers to consistently represent and manipulate text expressed in most of the world's writing systems. Developed in tandem with the Universal Character Set Consists of a repertoire of more than 100,000 characters. The most commonly used encoding is UTF-8 Uses 1 byte for all ASCII characters, which have the same code values as in the standard ASCII encoding. Uses up to 4 bytes for other characters.

Serial vs. Parallel Transmission of Data Data can be transmitted one bit or several bits at a time. Serial transmission sends one bit at a time over the same wire. Parallel transmission sends several bits at a time, each bit is transmitted over its own wire. Most printers that connect to a PC are parallel printers transferring 8 bits at a time. Note that in computers, an 8-bit byte is referred to as a word.

Asynchronous vs. Synchronous Transmission There are two ways to transmit data using serial transmission: Asynchronous Transmission Synchronous Transmission

30 Asynchronous Transmission Each character has its own synchronizing information. The beginning of the character is designated by a bit called the start bit. The start bit is a bit whose value is a 0. A bit that has a value of 0 is called a space bit. The end of a character is designated by a bit known as the stop bit. This bit has a value of 1. A bit that has a value of 1 is known as a mark bit. Asynchronous transmission is also called start-and- stop transmission.

31 Synchronous Transmission This type of transmission sends data as a block of characters at a time (a typical block contains 128, 256, 512, or 1024 characters). A header is placed at the start of the data sent. A trailer is placed at the end of the data. Synchronous transmission is typically used on a dedicated circuit connecting two high- speed computers.

Asynchronous Transmission Error Checking The simplest error-checking method is parity checking. ASCII uses 7 bits for coding allowing the 8 th bit to be used for parity. There are five types of parity: Odd – total number of 1s is odd. Even – total number of 1s is even. Mark – parity bit is always a 1. Space – parity bit is always a 0. None – used for Extended ASCII

Synchronous Error Detection and Correction Cyclic Redundancy Checking (CRC) Automatic Retransmission Request (ARQ)

34 Cyclic Redundancy Checking Parity checking provides an end-to-end check (PC to PC) on each byte transmitted between two PCs, but no error checking is performed. Modems use a synchronous error-detection technique called cyclic redundancy checking (CRC). The sender calculates a CRC remainder (typically 16- or 32-bits). The receiver recalculates the CRC based on the received data and compares its CRC with that of the sender.

35 Automatic Retransmission Request Error conditions are handled by requesting that the originating modem retransmit the data. Discrete ARQ Continuous ARQ

36 Discrete ARQ Also called stop-and-wait ARQ because after sending a block of data, the transmitting end will wait for a signal from the receiving end before sending another block of data. The signal from the receiving end will be either a positive acknowledgment (ACK) that the data was good, or a negative acknowledgment (NAK) that it was corrupted. On receipt of an ACK, the next block of data is sent. If a NAK is received, the last block of data is retransmitted.

37 Continuous ARQ With the use of discrete ARQ, the transmitting modem spends a lot of time waiting for the acknowledgment message. Continuous ARQ eliminates the need for the transmitting modem to wait for an ACK or NAK on each block of data sent. The transmitting modem keeps sending data (up to a specified maximum number of blocks) until an ACK or NAK is received. If a NAK is received there are two ways the transmitter can respond. Go-Back-N Selective Repeat (Selective ARQ)

Flow Control Modems are continually storing and retrieving data from their memory. Some type of flow control is necessary to prevent the memory buffers from overflowing and still keep enough data in memory to allow for retransmission. There are two basic types of flow control: Software flow control Hardware flow control

39 Software Flow Control Although both types of flow control use software programs to control the flow of data across a connection, the type based on what activates or terminates flow control as software operation. Activation and deactivation can be done through specific messages sent between the two devices. The messages are special characters. We refer to this as X-on/X-off software flow control.

40 Hardware Flow Control Hardware flow control uses a physical lead or wire between devices to activate flow control. Between a PC and a modem, two wires are used to provide a physical path for signals to the flow control software. Request-to-Send (RTS) Clear-to-Send (CTS) When the PC has data to send, it sends an RTS signal to the modem. When a modem has approached a full-buffer condition, it will remove the CTS signal.

Serial Ports and UARTs The Universal Asynchronous Receiver/ Transmitter (UART) is a parallel-to-serial and serial-to-parallel converter. The UART between the parallel data bus and the EIA-232 connector converts parallel transmitted data from the computer into serial transmission. It also converts the received serial data from the EIA-232 interface into parallel transmission for the computer.

42 Hardware Flow Control RTS and CTS

43 PC-to-Modem Speed and Modem-to-Modem Speed

Baud Rate and Modulation of the Carrier Modems communicate with each other via audio frequency waves modulated by digital input from a PC. The audio signal between two modems is called the carrier signal. The modulation to the audio signal can be accomplished by changing the amplitude, frequency, or phase of the audio signal. How fast these signal changes occur is called the baud rate. Baud refers to the number of changes occurring in a signal.

45 V.32bis Constellation Pattern

46 V.34 Quarter-Superconstellation with 240 Signal Points

Connections to the Internet

Packet Switching Networks In a circuit switched network, we establish a circuit path through the PSTN for a call. The public data network (PDN) uses a packet switched network (PSN).

49 Connectionless Networks In the PSN a call does not have to remain on the same path through the network. The Packet Assembler/Disassembler (PAD) assembles data from customers into packets. The PAD sends the data packet to a Data Switching Exchange (DSE). The DSE checks the address of the intended receiver and determines which route to take through the PDN. This type of switching is called a connectionless network because a connection is not maintained for the duration of the call.

50 Connection-Oriented Networks It is also possible to establish connection- oriented packet circuits. Switched virtual circuits and permanent virtual circuits can be established in the PDN to achieve the equivalent of a circuit switched facility. A virtual circuit allows all the packets to use the same route.

Signaling System 7 Network Signaling System 7 (SS7) network is the packet data network used by the PSTN. All toll switches and many class 5 local switching centers connect to the SS7 network. SS7 can only be used by computer-controlled switching systems. The SS7 network is necessary for ISDN and Custom Local Area Signaling Services (CLASS).

Caller Identification A service provided by CLASS (others include selective call forwarding and selective call rejection). When a customer with caller ID receives a call: The CPU of the receiving switch will receive the originating number over the SS7 network. The CPU will then cause this number to be sent over the local loop to the receiving telephone.

Summary Data communication is the transmission of characters, numbers, graphics, and symbols using digital signals. The transmission between a PC an a modem is asynchronous transmission, where a start bit precedes each character and a stop bit follows each character. Modems contain sophisticated error-detection and error-correction techniques.

54 Summary (continued) Baud rate is defined as the number of times a signal changes. The PDN is a network of data switches that allows the public to transmit data in the form of packets.