Chapter Four

1
Chapter Four

• Making Connections
• Data Communications and Computer Networks A
  Business Users Approach, Fourth Edition

2
After reading this chapter, you should be able
to

• List the four components of all interface
  standards
• Discuss the basic operations of the USB and
  EIA-232F interface standards
• Cite the advantages of FireWire, SCSI, iSCSI,
  InfiniBand, and Fibre Channel interface standards
• Outline the characteristics of asynchronous,
  synchronous, and isochronous data link interfaces

3
After reading this chapter, you should be able
to (continued)

• Recognize the difference between half-duplex and
  full-duplex connections
• Identify the operating characteristics of
  terminal-to-mainframe connections and why they
  are unique compared to other types of computer
  connections

4
Introduction

• Connecting peripheral devices to a computer has,
  in the past, been a fairly challenging task
• Newer interfaces have made this task much easier
• Lets examine the interface between a computer
  and a device
• This interface occurs primarily at the physical
  layer

5
Interfacing a Computer to Peripheral Devices

• The connection to a peripheral is often called
  the interface
• The process of providing all the proper
  interconnections between a computer and a
  peripheral is called interfacing

6
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)
• De facto standards
• Created by other groups that are not official
  standards but because of their widespread use,
  become almost standards

7
Characteristics of Interface Standards
(continued)

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

8
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

9
Important Interface Standards

• EIA-232F an older standard originally designed
  to connect a modem to a computer
• USB-2 (Universal Serial Bus-2) a newer standard
  that is more powerful than EIA-232F
• I-1394 (Firewire) another newer standard that
  is much more powerful than EIA 232F but has
  different focus in functionality than USB
• (Ulra)-IDE, (Ultra)SCSI, FC-AL internal PC
  buses

10
EIA-232F

• 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.

11
EIA-232F (continued)
12
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
  next slide

13
EIA-232F (continued)
14
EIA-232F (continued)

• The next slide 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

15
EIA-232F (continued)
16
EIA-232F (continued)

• A half-duplex connection transmits data in both
  directions but in only one direction at a time
• A full-duplex connection transmits data in both
  directions and at the same time
• A simplex connection can transmit data in only
  one direction

17
Universal Serial Bus (USB)
18
Universal Serial Bus (USB) (continued)

• 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)

19
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)

20
Universal Serial Bus (USB) (continued)

• The USB standard uses the NRZI combined with
  4B/5B bit stuffing scheme
• USB networks use a tiered-star topology Hubs up
  to 16 devices but can be chained so no limit on
  the number of devices.
• 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

21
Universal Serial Bus (USB) functions, endpoints,
pipes

• USB device can be used as a host, peripheral
  device, hub, or host controller. Each use has
  associated functions. These can be seen as USB
  devices which provide a capability (or function)
  such as a Printer, FAX, scanner, etc.
• Endpoints are sources/sinks of data. As the bus
  is host centric, endpoints occur at the end of
  the communications channel at the USB function.
• Client software transfers data through pipes. A
  pipe is a logical connection between the host and
  an endpoint or (endpoints).

22
Universal Serial Bus (USB) addressing

23
Universal Serial Bus (USB) transactions

• Each USB transaction consists of
• Token Packet header that defines what it expects
  to follow,
• (Optional) Data Packet contains the data
  (payload)
• Status Packet used to acknowledge transactions
  and to provide a means of error correction

24
Universal Serial Bus (USB) a scenario for
control function

• Setup Stage is where the request is sent.
• The setup token that contains the address and
  endpoint number is sent first.
• The data packet is sent next. It contains a setup
  packet which details the type of request.
• The last packet is a handshake used for
  acknowledging successful receipt or to indicate
  an error. If the function successfully receives
  the setup data (CRC, PID, etc. are OK) it
  responds with ACK, otherwise it ignores the data
  and doesnt send a handshake packet.

25
Universal Serial Bus (USB) a scenario for
control function (continued)

• Data Stage is where IN or OUT data transfers
  happen (one or multiple determined by amount of
  data requested by SETUP packet).
• IN The host is ready to receive control data, it
  issues an IN Token. Reply can be a DATA packet
  containing the control data, a STALL packet
  indicating an error at the endpoint or a NAK
  packet indicating that the endpoint is OK, but
  temporary has no data to send.
• OUT The host needs to send a control data
  packet, it issues an OUT token followed by a data
  packet containing the control data. If the
  function's endpoint buffer got the data OK the
  endpoint issues an ACK, if endpoint is busy then
  NAK, if error then STALL.

26
Universal Serial Bus (USB) a scenario for
control function (continued)

• Status Stage reports the status of the overall
  request. Status reporting is always performed by
  the function.
• IN If the host sent IN token during the data
  stage, the host acknowledges the successful
  receipt of this data by sending an OUT token
  followed by a zero length data packet. An ACK
  indicates the function has completed the command.
  If an error occurred during the processing of
  this command, the function issues a STALL. If the
  function is still processing the command, it
  returns a NAK indicating to the host to repeat
  the status stage later.
• OUT If the host sent OUT token during the data
  stage, the function acknowledges the receipt of
  data by sending a zero length packet in response
  to an IN token issued by the host. If an error
  occurred during processing, the function issues a
  STALL. If it is still busy processing data, it
  issues a NAK asking the host to retry the status
  stage later.

27
Universal Serial Bus (USB) (continued)

• Supports three types of data transfers
• Interrupt (asynchronous) transfers
• Guaranteed Latency
• Stream Pipe - Unidirectional
• Error detection and next period retry.
• Isochronous transfers
• Guaranteed access to USB bandwidth.
• Bounded latency.
• Stream Pipe - Unidirectional
• Error detection via CRC, but no retry or
  guarantee of delivery.
• Full high speed modes only.

28
Universal Serial Bus (USB) (continued)

• Bulk transfers
• Used to transfer large bursty data.
• Error detection via CRC, with guarantee of
  delivery.
• No guarantee of bandwidth or minimum latency.
• Stream Pipe - Unidirectional
• Full high speed modes only.

29
FireWire

• Low-cost digital interface (non-polled,
  peer-to-peer device network) 63 peripherials.
• Capable of supporting transfer speeds of up to
  400 Mbps
• Hot pluggable, daisy-chainable.
• Supports two types of data connections
• Asynchronous connection
• Isochronous connection
• Uses Serial Bus Protocol 2
• (for more see FireWire Concepts)

30
FireWire functions

• Identification When a device is plugged in, it
  immediately broadcasts its unique identification
  number to other devices on the network and
  becomes part of that network.
• Asynchronous mode transfer The address of both
  the sender and the receiver is transmitted
  followed by the actual packet data. Once the
  receiver accepts the packet, a packet
  acknowledgment is returned to the original
  sender. To improve throughput, the sender may
  continue transmission until 64 transactions are
  outstanding.

31
FireWire functions (continued)

• Isochronous mode transfer The sender requests an
  isochronous channel by introducing a request
  token into a time slot that must not be disrupted
  by peers. Channel IDs are transmitted followed by
  the packet data. The receiver monitors the
  incoming data's channel ID and accepts only data
  with the specified ID, 64 isochronous channels
  may be defined.

32
SCSI and iSCSI

• SCSI (Small Computer System Interface)- a
  protocol of 64 commands. Initially, parallel bus
  (8 bits)- uses Low Voltage Differential
  signaling
• 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 (for more
  information see what is SCSI?)
• iSCSI (Internet SCSI)
• A technique for interfacing disk storage to a
  computer via the Internet

33
InfiniBand and Fibre Channel

• InfiniBand a serial connection or switched bus
  connection 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 (for
  more see High Performance Computing- SW fabric
  and What is InfiniBand?)
• Fibre Channel- Arbitrated Loop (FC-AL) also a
  serial, high-speed network that connects a
  computer to multiple input/output devices.
  Designed for RAID at a speed 400 MBYTES/s.
• Supports data transfer rates up to billions of
  bits per second, but can support the
  interconnection of up to 126 devices only (for
  more see Arbitrated loop and What is FC-AL?)

34
Interface speeds

• standard parallel port 115kBYTES/s
  (.115MBYTES/s)
• Original USB 12Mbits/s (1.5MBYTES/s)
• ECP/EPP parallel port 3MBYTES/s
• IDE 3.3-16.7MBYTES/s
• SCSI-1 5MBYTES/s
• SCSI-2 (Fast SCSI, Fast Narrow SCSI) 10MBYTES/s
• Fast Wide SCSI (Wide SCSI) 20MBYTES/s
• Ultra SCSI (SCSI-3, Fast-20, Ultra Narrow)
  20MBYTES/s
• UltraIDE 33MBYTES/s

35
Interface speeds (continued)

• Wide Ultra SCSI (Fast Wide 20) 40MBYTES/s
• Ultra2 SCSI 40MBYTES/s
• IEEE-1394 100-400Mbits/s (12.5--50MBYTES/s)
• Hi-Speed USB 480Mbits/s (60MBYTES/s)
• Wide Ultra2 SCSI 80MBYTES/s
• Ultra3 SCSI 80MBYTES/s
• Wide Ultra3 SCSI 160MBYTES/s
• FC-AL Fiber Channel 100-400MBYTES/s
• Infini-BAND 2.5 Gbits/s (312MBYTES/s)

36
Asynchronous Connections

• A type of connection defined at the data link
  layer
• To transmit data from sender to receiver, an
  asynchronous connection creates a one-character
  package called a frame
• Added to the front of the frame is a start bit,
  while a stop bit is added to the end of the frame
• An optional parity bit can be added which can be
  used to detect errors

37
Asynchronous Connections (continued)
38
Asynchronous Connections (continued)
39
Asynchronous Connections (continued)

• The term asynchronous is misleading here because
  you must always maintain synchronization between
  the incoming data stream and the receiver
• Asynchronous connections maintain synchronization
  by using small frames with a leading start bit

40
Synchronous Connections

• A second type of connection defined at the data
  link layer
• A synchronous connection creates a large frame
  that consists of header and trailer flags,
  control information, optional address
  information, error detection code, and data
• A synchronous connection is more elaborate but
  transfers data in a more efficient manner

41
Synchronous Connections (continued)
42
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

43
Terminal-to-Mainframe Computer 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

44
Terminal-to-Mainframe Computer Connections
(continued)
45
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
• In roll-call polling, the mainframe polls each
  terminal in a round-robin fashion
• In hub polling, the mainframe polls the first
  terminal, and this terminal passes the poll onto
  the next terminal

46
Terminal-to-Mainframe Computer Connections
(continued)
47
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

48
Making Computer Connections In Action (continued)
49
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?

50
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

51
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

52
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

53
Summary

• Connection between a computer and a peripheral is
  often called the interface
• Process of providing all the proper
  interconnections between a computer and a
  peripheral is called interfacing
• The interface between computer and peripheral is
  composed of one to four components electrical,
  mechanical, functional, and procedural
• A DTE is a data terminating device
• Computer
• A DCE is a data circuit-terminating device
• Modem

54
Summary (continued)

• Two interface standards worthy of additional
  study Universal Serial Bus, and EIA-232F
• EIA-232F was one of the first highly popular
  standards
• Universal Serial Bus is currently the most
  popular interface standard
• Half-duplex systems can transmit data in both
  directions, but in only one direction at a time
• Full-duplex systems can transmit data in both
  directions at the same time
• Other peripheral interfacing standards that
  provide power, flexibility, and
  ease-of-installation include FireWire, SCSI,
  iSCSI, InfiniBand, and Fibre Channel

55
Summary (continued)

• While much of an interface standard resides at
  the physical layer, a data link connection is
  also required when data is transmitted between
  two points on a network
• Three common data link connections include
  asynchronous connections, synchronous
  connections, and isochronous connections
• Asynchronous connections use single-character
  frames and start and stop bits to establish the
  beginning and ending points of the frame
• Synchronous connections use multiple-character
  frames, sometimes consisting of thousands of
  characters
• Isochronous connections provide real-time
  connections between computers and peripherals and
  require a fairly involved dialog to support the
  connection

56
Summary (continued)

• A point-to-point connection is one between a
  computer terminal and a mainframe computer that
  is dedicated to one terminal
• A multipoint connection is a shared connection
  between more than one computer terminal and a
  mainframe computer