Lecture 04

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Lecture 04

Making Connection

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Introduction• Connection concept

– Interface concept and standard (Level 1)• EIA-232F• USB

• Data Link Connections (Level 2)

• Terminal-to-mainframe computer connections

• Application examples

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Connection concept

• How computer networks are connected?

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Connection concept

• Recalled: OSI model – level 1– Physical level, which requires peripheral

devices to connect two different computers or devices together

– Termed as “interface”

• Two types of standards• Modes of data flow• Connection to systems

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Connection (cont.)

• Characteristics of interface standards

• Two important interface standards– EIA-232F– USB (Universal Serial Bus)

• Other interfacing standards

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Other interfacing standards

• Other peripheral interfacing standards that provide power, flexibility and ease-of-installation include: – FireWire (low cost device for digital)– SCSI, iSCSC(mainly for permanent storage,

CD/DVD) – InfiniBand, Fibre Channel (high speed connection)

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Data terminating equipment Data communicating equipment

Level 1

Level 2

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Characteristics of Interface Standards

• There are essentially two types of standards– Official standards

• Created by standards-making organizations such as ITU (International Telecommunications Union), IEEE (Institute for Electrical and Electronics Engineers), EIA (Electronic Industries Association), ISO (International Organization for Standardization), and ANSI (American National Standards Institute)

• CSA (Canadian standard), UL (USA for computer hardware)

– De facto standards • Created by other groups that are not official standards but because of

their widespread use, become “almost” standards

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data flow

There are 3 types of data flow:

i) simplex transmission

ii) half-duplex transmission

iii) full duplex transmission

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simplex transmission

• i) simplex transmission– data is transmitted in one direction only– ie no data transmission on opposite direction

is allowed• see Figure 8-2

– Application examples?

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half-duplex transmission

• ii) half-duplex transmission– transmission in either direction on a circuit but

only one direction at a time – eg an inquiry is sent to the computer and then

a response is sent back on the same circuit to the terminal

– Application examples?

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full duplex transmission

• iii) full duplex transmission– data transmission in both directions

simultaneously on the circuit– machine needs to be intelligence at both ends

(why?)

– Application examples?

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connection to the system

• two types of physical connection in the system

– i) Parallel data transmission– ii) Series data transmission

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Parallel data transmission

• i) Parallel data transmission– connected via direct cable that has one wire

for each bit in a character of data code being used by the terminal

» See Figure 8.3

– with multiple wires, all the bits of a characters can be transmitted between the terminals and computer at once

– Disadv: very expensive & no practice over long distance (why?)

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FIGURE 8-3 Parallel and serial transmission.

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• ii) Series data transmission– bits of each character are sent down to a line

one after another– complicated process because machine needs

to know how to decompose and to reconstruct of bits at each respective end

– Adv or Disadv?

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Characteristics of Interface Standards (continued)

• There are four possible components to an interface standard:– Electrical component– Mechanical component– Functional component– Procedural component

Explanations

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Characteristics of Interface Standards (continued)

• Four components – Electrical component – deals with voltages, line capacitance,

and other electrical characteristics

– Mechanical component – deals with items such as the connector or plug description

– Functional component – describes the function of each pin or circuit that is used in a particular interface

– Procedural component – describes how the particular circuits are used to perform an operation

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EIA-232F

• EIA-232F – an older standard originally designed to connect a modem to a computer

• Originally named RS-232 but has gone through many revisions

• The electrical component is defined by another standard: V.28• The mechanical component is often defined by ISO 2110, the

DB-25 connector. The DB-9 connector is now more common than the DB-25.

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Worked as full duplex (why?)Its functions

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EIA-232F (continued)

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EIA-232F (continued)• The functional and procedural components are

defined by the V.24 standard• For example, V.24 defines the function of each

of the pins on the DB-9 connector, as shown on the Table 4.1

• Table 4.2 shows an example of the procedural dialog that can be used to create a connection between two endpoints– Note the level of complexity needed to establish a full-

duplex connection

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EIA-232F (continued)

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EIA-232F (continued)

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Universal Serial Bus (USB)

• a newer standard that is much more powerful than EIA-232F

• The USB interface is a modern standard for interconnecting a wide range of peripheral devices to computers

• Supports plug and play

• Can daisy-chain multiple devices

• USB 2.0 can support 480 Mbps (USB 1.0 is only 12 Mbps); USB 3.0

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Universal Serial Bus (USB) (continued)

• The USB interface defines all four components– The electrical component defines two wires VBUS

and Ground to carry a 5-volt signal, while the D+ and D- wires carry the data and signaling information

– The mechanical component precisely defines the size of four different connectors and uses only four wires (the metal shell counts as one more connector)

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Universal Serial Bus (USB) (continued)

• Four types of USB connectors

• The functional and procedural components are fairly complex but are based on the polled bus

• The computer takes turns asking each peripheral if it has anything to send

• More on polling near the end of this chapter

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Universal Serial Bus (USB) (continued)

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Universal Serial Bus (USB) (continued)

• The functional and procedural components are fairly complex but are based on the polled bus

• The computer takes turns asking each peripheral if it has anything to send

• More on polling near the end of this chapter

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FireWire• Low-cost digital interface (real time connection for PC)

• A FireWire connection lets you send data to and from high-bandwidth digital devices such as digital camcorders, and it's faster than USB

• Capable of supporting transfer speeds of up to 400 Mbps

• Hot pluggable• Supports two types of data connections:

– Asynchronous connection– Isochronous connection

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SCSI and iSCSI• SCSI (Small Computer System Interface)

– A technique for interfacing a computer to high-speed devices such as hard disk drives, tape drives, CDs, and DVDs

– Designed to support devices of a more permanent nature• SCSI is a systems interface

– Need SCSI adapter

• iSCSI (Internet SCSI) – A technique for interfacing disk storage to a computer via the Internet

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InfiniBand and Fibre Channel• InfiniBand – a serial connection or bus that can carry

multiple channels of data at the same time– Can support data transfer speeds of 2.5 billion bits (2.5

gigabits) per second and address thousands of devices, using both copper wire and fiber-optic cables

– A network of high-speed links and switches

• Fibre Channel – also a serial, high-speed network that connects a computer to multiple input/output devices– Supports data transfer rates up to billions of bits per second,

but can support the interconnection of up to 126 devices only

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Data Link Connections

– Take place at level 2– technique used to transmit data on a comm

line– two methods could be used to transmit data:

• i) Asynchronous transmission (Asych) • ii) Synchronous transmission (Synch)• Iii) Isochronous Connections (Isoch)

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Asynchronous transmission

i) Asynchronous transmission (Asych)

Pattern of data presentation:– data transmission is sent preceded by an

extra bit, called a start bit, and followed by one more extra bit called stop bit (start/stop transmission)

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Asynchronous transmission

– Eg:

– 1 1 0 0 0 0 0 1 0

represent a A character Stop bit

start bit or representation of or space bit

mark bit a code system

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Alternative presentation

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Asynchronous Connections (continued)

More example

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Asynchronous Connections (continued)

Send the word “ H E L L O”(to p37)

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Asynchronous transmission

– Penalty term is used to measure the efficiency of a code system

– known as transmission efficiency

Eg.

Code No. of bits start/stop total bite Eff.

Baudot 5 2 7 5/7=71.5

ASCII 7 2 9 7/9=77

EBCDIC 8 2 10 8/10=80(to p41)

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Asynchronous transmission

– Asyn has a function of character synchronization, which allows when a start bit is sensed, the receiver knows that the next n bits on the line make up a characters

– Without Char Syn, receiver cannot rocog the first bit of charc, and thus character could not be interpreted.

– Adv. Equipment cost is low– Disadv. Slow speed, less than 300bps

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Synchronous transmission

• ii) Synchronous transmission (Synch)– design for line speed that cannot handle by

Asyn– its function is that bit Synch is maintained by

clock circuitry in the transmitter and in the receiver

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Synchronous transmission

– that is timing generated by the transmitter’s clock is sent along with data so that the receiver can keep its clock synchronized with that of the transmitter throughout a long transmission

– data is usually sent in a block oriented, contains special synch charactersynch character with a unique bit pattern

– similar the Asych, synch char performs a function similar to that of start bit

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Synchronous transmission

– It has 1 to 4 synchronizing “characters” for each “block” of data; whereas Asych has 2 bits for each character

– Semantic view

– efficiency (to p46)

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Synchronous Connections (continued)

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Synchronous transmission

– Example: consider a character consists of

0100101

Asynch: 250 char x (7 data + 2 start/stop) = 2250

Synch: (250 + 4 synch char) x 7 bits) = 1778

Thus, Synch is 21% more efficient than Aysnch

– Note: Mostly, host computers adopt Synch transmission. (to p35)

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Isochronous Connections

• A third type of connection defined at the data link layer used to support real-time applications

• Data must be delivered at just the right speed (real-time) – not too fast and not too slow

• Typically an isochronous connection must allocate resources on both ends to maintain real-time

• USB and Firewire can both support isochronous• (provide data transmission in a regular period of time)

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Terminal-to-Mainframe Computer Connections

• Two main ways for connections:– Point-to-point connection – a direct, unshared

connection between a terminal and a mainframe computer

– Multipoint connection – a shared connection between multiple terminals and a mainframe computer

• The mainframe is the primary and the terminals are the secondaries (how do you draw them semantically?)

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Terminal-to-Mainframe Computer Connections

(continued)

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Terminal-to-Mainframe Computer Connections (continued)

• To allow a terminal to transmit data to a mainframe, the mainframe must poll the terminal

• Two basic forms of polling: roll-call polling and hub polling

1. In roll-call polling, the mainframe polls each terminal in a round-robin fashion

2. In hub polling, the mainframe polls the first terminal, and this terminal passes the poll onto the next terminal (how it works, semantically?)

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Terminal-to-Mainframe Computer Connections (continued)

Roll-call pulling

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Making Computer Connections In Action

• The back panel of a personal computer has many different types of connectors, or connections:– RS-232 connectors– USB connectors– Parallel printer connectors– Serial port connectors

Layout (to p53)

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Making Computer Connections In Action (continued)

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Making Computer Connections In Action (continued)

• 1 and 2 – DIN connectors for keyboard and mouse

• 3 – USB connectors• 4 and 6 – DB-9 connectors• 5 – Parallel port connector (Centronics) • 7, 8, and 9 – audio connectors• Will Bluetooth replace these someday?

Solution!

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Making Computer Connections In Action (continued)

• A company wants to transfer files that are typically 700K chars in size

• If an asynchronous connection is used, each character will have a start bit, a stop bit, and maybe a parity bit

• 700,000 chars * 11 bits/char (8 bits data + start + stop + parity) = 7,700,000 bits

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Making Computer Connections In Action (continued)

• If a synchronous connection is used, assume maximum payload size – 1500 bytes

• To transfer a 700K char file requires 467 1500-character (byte) frames

• Each frame will also contain 1-byte header, 1-byte address, 1-byte control, and 2-byte checksum, thus 5 bytes overhead

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Making Computer Connections In Action (continued)

• 1500 bytes payload + 5 byte overhead = 1505 byte frames

• 467 frames * 1505 bytes/frame = 716,380 bytes, or 5,731,040 bits

• Significantly less data using synchronous connection