CompTIA Network+ Certification Study Guide part 28 ppsx

In essence, it is the gateway between the transport layer and the session layer in the OSI model, providing a common interface developers can use to access both transport and session lay

the tDI boundary Layer

The TDI provides a portal into the transport protocols for kernel mode

components such as servers and redirectors In essence, it is the gateway between the transport layer and the session layer in the OSI model, providing

a common interface developers can use to access both transport and session layer functionality

the API boundary Layer

The API is the interface through which developers can access network infra-structure services such as various application layer protocols Dynamic

Host Configuration Protocol (DHCP), DNS, and Windows Internet Name Service (WINS) all work at this level and connect to the lower layers through

APIs There are also Windows Sockets (WinSock), NetBIOS, telephony, and messaging APIs used to assist in carrying out lower-level network

functions

Understanding Component Layers

Within each layer are component layers that provide very specific functionality

the NDIs Wrapper

The NDIS wrapper is a library of common NDIS functions that can be used both by the MAC protocols beneath it and by TCP/IP above it The NDIS

wrapper is implemented by a file called Ndis.sys, which is software code

that surrounds all NDIS device drivers It provides a common interface for device and protocol drivers The NDIS wrapper is used to reduce platform dependencies during development of network interface devices

Network transport Protocols

Network Transport Protocols all applications or clients to send and receive data over the network Other network transport protocols include IPX/SPX, Asynchronous Transfer Mode (ATM), NetBEUI, Infrared Data Association (IrDA), AppleTalk, and SNA These protocols are used on a variety of non-Microsoft operating systems including Novell, Apple, and IBM

File system Drivers

The file system drivers are the Redirector and the Server service When

there is a request to open a shared file, the I/O Manager sends a request to the Redirector, which selects the appropriate transport layer protocol via the TDI layer When there is a request to access a local file, the Server service responds to requests from the remote Redirector and provides

access to the requested file Named pipes, mailslots, Server service, and

Redirector are file system drivers that work at both the presentation and

session layers of the OSI model

Applications and user Mode services

Applications must interface with the lower layer protocols and must interact

in some manner with the user These services are implemented in a number

of ways, but there are four commonly used APIs implemented at this point

that provide access to lower transport protocols

The WinSock API allows Windows-based applications to communicate

with the lower layers Winsock is a protocol-independent networking API

that provides standardized access to datagram and session services over

TCP/IP, IPX/SPX, AppleTalk, and others

Telephony integrates computers with telephone technology and utilizes

the Telephony API (TAPI) to provide a standardized interface to networking

protocols for various telephony applications The NetBIOS API has been

used for developing client/server applications and is supported in Windows

2003 for backward compatibility The Messaging API (MAPI) is an industry

standard that assists applications in interfacing with messaging services via

a single interface Microsoft Exchange uses MAPI

So in sum, although you won’t be asked questions on the Network exam

that are directly related to this information, not having it at all leaves massive

gaps in your networking information Understanding these concepts helps to

give you key terminology you may encounter on the exam so you will

under-stand what it means when you see it, which will help to differentiate wrong

answers and so on Now, let’s learn about the DoD model and map it to the OSI

model to understand the similarities and differences among these two models

ThE dod NETworKING ModEl

In the mid-1960s, computer systems were huge mainframes that were all

owned and maintained by large companies, universities, and governmental

agencies Users, especially in the academic, scientific, and governmental

arenas, often needed to share data with other users The problem was that

mainframe computers all ran different proprietary software, and operating

systems could not easily communicate with one another To share data,

programmers had to write code that would allow one mainframe to

communicate with another specific mainframe

This cumbersome one-to-one process was prohibitive, both in terms of

the time and cost required to develop unique, proprietary solutions, and

in terms of the limitations those solutions often imposed After an interface was written, that main-frame still could communicate only with its specified counterpart If either mainframe’s operating system changed, the interface might be broken and programmers would have

to be called back in to reestablish the communication system between the two mainframes

The U.S Department of Defense

Advanced Research Projects Agency

(DARPA) tackled this problem with

an experiment designed to demon-strate a way to share computer data across a wide area This experiment

was called Advanced Research

Proj-ects Agency Network (ARPANet) and

it became the foundation for what we know today as the Internet It also resulted in the development of the TCP/IP protocols in the late 1960s TCP/IP is one of the few computer technologies from the 1960s that is still in prominent use today, a testament to the superb design of the TCP/IP suite Although it has undergone some modifications over time, TCP/IP is still the protocol suite of choice for almost all large networks and for global connectivity to the Internet, which relies on TCP/IP

The DARPA architecture, known as the DARPA model or the Department

of Defense (DoD) model, defines four layers starting at the network cable (or interface) and working its way up This model can be seen in Figure 6.10 Each layer is designed with a specific function and together they provide the foundation for Internetworking Different protocols within the TCP/IP suite work at different layers, as you’ll discover when we examine the individual components of the TCP/IP suite

layer 1: Network Interface

The Network Interface layer of the DoD model corresponds to the lowest

level of the TCP/IP protocol architecture and correlates to Layers 1 and 2

in the OSI model Figure 6.11 shows the mapping of layers from the OSI

FIGurE 6.10 The DoD Networking Model.

model to DoD model The Network Interface layer provides most of the

capabilities provided for in the physical and data link layers of the OSI

model

Let’s begin with a brief overview of the hardware involved in the

network at this level We have the network medium, which is typically

coaxial, fiber optic, or twisted-pair cabling (although wireless networking

is increasing in popularity – see Chapter 5); and we have the NIC that has

both a physical MAC address and a logical IP address (we’ll discuss the

MAC and IP addresses a bit later) The NIC has logic (a circuit board and

chips) built into it that gives it basic functionality It uses a driver, which

is a small software program that interfaces between the hardware and the

operating system, to provide additional functionality

The specifications related to how the network technology is

imple-mented are defined by IEEE (called the Eye-triple E by industry members)

The IEEE helps define common standards for use in a variety of technical

fields, including computing Although it may seem like humorous trivia, it’s

FIGurE 6.11 Mapping the OSI Model with DoD Model.

Physical

Network Interface

Host-to-Host Internet

Process/

Application

Data Link

Network

Transport

Session

Presentation

Application

absolutely true that the standard known as the 802 standard was named so

because the initial committee meeting was in 1980, in February (the second month) This standard defines specifications for the lower-level networking technologies; that is, those at the physical layer (NIC, connectors, and cables) and at the data link layer (access methods)

As you’ll see, the standards vary, depending on the network technology (Ethernet, Token Ring, ATM, and Frame Relay) Because TCP/IP works independently of network technology, it can be used with each of these types

of networks and can be used to send information between two dissimilar networks as well For more information on the IEEE, you can visit the IEEE

at www.ieee.org

The standards set by the 802 committee pertaining to networking are

as follows:

■

■ 802.1: Internetworking standards that deal with the management

of LANs and metropolitan area networks (MANs), including bridges and the spanning tree algorithm used by bridges to prevent looping

■

■ 802.2: LLC and the division of OSI Layer 2 into two sublayers, LLC

and MAC

■

■ 802.3: CSMA/CD, the MAC method used on Ethernet networks

and frame formats for Ethernet

■

■ 802.4: Token Bus networks that use 75 ohm coax or fiber-optic

cabling and the token-passing access method

■

■ 802.5: Token Ring, the technology developed by IBM that uses a

physical star and logical ring topology with twisted-pair cabling (shielded or unshielded) and the token-passing access method

■

■ 802.6: MANs, networks of a size and scope that falls between that

of the LAN and the WAN

Exam warning

For the Network exam, it’s imperative that you understand the IEEE 802 model and its specific standards Although there are many standard committees, you should definitely focus on the newer ones affecting today’s current technologies (or areas of technology), such as Ethernet, wireless, and security Most significantly, Ethernet is defined in 802.3, Token Ring in 802.5, and wireless networking in 802.11.

■ 802.7: Broadband transmissions that use frequency-division

multiplexing (FDM), including CATV

■

■ 802.8: Fiber optics networks, including FDDI using the

token-passing access method

■

■ 802.9: Integrated services (voice and data) over Integrated Services

Digital Network (ISDN)

■

■ 802.10: Virtual Private Networking (VPN) to create a secure

connection to a private network over the public Internet

■

■ 802.11: Wireless networking technologies, including the most

common 802.11b, faster 802.11a, and newer 802.11g and 802.11n

wireless communications methods

■

■ 802.12: The 100VG AnyLAN technology developed by Hewlett

Packard, which uses the demand priority access method

■

■ 802.15: Wireless personal area networks

■

■ 802.16: Broadband Wireless MANs

■

■ 802.17: Resilient Packet Rings

■

■ 802.18: Radio Regulatory Technical Advisory Group

■

■ 802.19: Coexistence Technical Advisory Group

■

■ 802.20: Mobile Broadband Wireless Access (MBWA)

Note

The missing numbers in the 802.xx series may be unused or disbanded prior to reaching

the standard The 802 committee pertaining to network standards works continuously in

bringing newer, faster, more efficient, and more secured protocols To know the latest,

please visit www.ieee802.org or www.ieee.org.

Note

Although some of this material may have been covered earlier, knowing it is imperative

to passing the test, and repetition builds your ability to recall information when needed

The 802 standards need to be committed to memory, as you will definitely need to know

them come exam time.

Media access control refers to the method used to allocate the use of the medium among the computers and devices on the network The MAC method performs

a function similar to the chairperson of a meeting, whose responsibility it is to recognize each speaker in turn and keep everyone from talking at once

In networking, access control is important only when many devices share a common medium, such as a coaxial cable or twisted-pair cable, and

then it is very important Various schemes have been devised to control

access to the media by the connected devices If no methods were in place, all devices would send data whenever it suited them On a small network, this might not be a problem, but if there are more than a few devices, it quickly causes congestion, collisions, and errors because everybody’s talking

at once Therefore, as the size of the typical network grew, it was important

to develop standard methods to control access to the shared media so that communication would proceed in an orderly and predictable manner The access control method lays out rules defining how access is allocated, just as Robert’s Rules of Order govern how meetings proceed (to see Robert’s Rules

of Order, visit www.constitution.org/rror/rror 00.htm)

MAC is performed by MAC layer protocols Although there are many different MAC protocols for a wide variety of media used by many different communications technologies (cellular, cable TV, satellite, etc.), we’re going

to concentrate on those that are most common in computing today These include as follows:

CSMA/CD

■

■ CSMA/CA

■

■ Token passing

■

■

Network Interface Hardware/Software

The network interface is established through the NIC Each type of NIC uses a different type of connector to connect to the physical medium The connector types are delineated in the IEEE 802 specifications Each network technology is delineated in its own section of the 802 specification, as described previously Again, most significantly, Ethernet is defined in 802.3, Token Ring in 802.5, and wireless networking in 802.11

The NIC uses both hardware and software in connecting the device to the network media The TCP/IP Network Interface layer defines protocols used by the NIC to receive, assemble, address, and transmit For example, most Ethernet networks in use today employ an Ethernet NIC, which,

among other things, uses CSMA/CD to control media access The most

common type of Ethernet NIC uses a Category 5 or greater unshielded

twisted-pair cable (typically referred to as UTP CAT5, CAT5e, or CAT6)

with specified pin connections In some cases (although not very common

anymore by today’s standards), Ethernet is still deployed occasionally over

thin (diameter, 1/4 inch) or thick (diameter, 1/2 inch) coaxial cable

Ethernet can also be deployed over fiber-optic cable Regardless of the

cable type, Ethernet networks use the same contention-based access control

method

UTP cabling connects to the NIC via an RJ-45 modular plug and jack

(similar to a large phone jack), and thin coax (Thinnet) connects via a BNC

connector (Bayonet Neill Concelman, after its twist-on style and the two

men who invented it) shaped like a T Thick coax (Thicknet) is connected

via a vampire tap (a metal pin that penetrates the cable) to an external

transceiver, which in turn connects to the NIC Other types of Ethernet

NICs have the transceiver built onto the NIC itself Some NICs (seen in

older PC deployments), called combo cards, have connectors for more than

one type of cable

The Ethernet NIC is also responsible for receiving/sending and

assembling/disassembling data to and from the network connection The

Network Interface layer in the DoD model encompasses the functions of

the OSI model’s physical and data link control layers and controls media

access and the assembly/disassembly of data at the lowest level of the

hierarchy

layer 2: Internet

The next layer in the DARPA model is the Internet layer, which maps to the

network layer of the OSI model The Internet layer, so-called because of the

addressing scheme that makes communications possible across a network

Test day Tip

It’s common to see new technologies being learned, standardized, and implemented

at a very rapid speed, but it’s also common to be replacing older technologies with said

new ones Therefore, it’s common to see historical information on the Network exam,

historical in that it covers technologies that are not commonly installed anymore but

are definitely commonly removed, migrated, upgraded, or replaced Prevalent in older

renditions of the exam was the need to know about things that were very common to

older networking topologies such as coaxial cabling, 10Base5, 10Base2 technologies,

Bus networking topologies, and so on It still holds true today that you should know

about these technologies for the exam, so do not overlook studying for them.

of networks, or internetwork, is responsible for packaging, addressing, and routing the data When this layer was originally conceived, the Internet as

we know it today did not exist The concept behind this layer was to define a framework for two computers to connect to one another to share data This laid the foundation for widespread internetworking, which led to what we now know as the Internet

Before data can be sent out over the network interface, they must have a standard format, size, and addressing scheme The Network Interface layer

is responsible only for taking the data it is given and translating them into signals on a physical medium The Internet layer defines packet structure (what each bit of a data segment means), addressing, and routing

layer 3: host-to-host

Layer 3 in the DARPA model is the Host-to-Host Transport layer, some-times called the transport layer since this layer maps to the transport layer

(Layer 4) in the OSI model As the name implies, this layer is responsible for transporting the data It sets up communications between the application layer and the lower layers The Internet layer is responsible for formatting, addressing, and routing the data, and the Host-to-Host Transport layer is responsible for setting up the connection between hosts so that formatted data can be sent

Because this layer establishes a connection, it can also take on some

of the responsibilities of the session layer of the OSI model In TCP/IP, the two core protocols used at the Host-to-Host Transport layer are TCP

and the UDP TCP is a more complex protocol that provides reliable data

transport, the application sending the data receives acknowledgement that the data was received UDP is a much simpler protocol that does not provide acknowledgement messages Although this makes UDP data transport less reliable, it is a very useful protocol in certain applications where fast, simple communication is required

layer 4: Application

The application layer of the DARPA model operates at the session,

presentation, and application layers of the OSI model One of the main reasons why the DoD model is still used when referencing TCP/IP is because the TCP/IP protocol suite’s protocols (such as FTP, Telnet, and so on) do not map perfectly into the OSI model; they have overlap, and this is why you will see three OSI model layers under one layer in the DoD model The DoD model’s application layer enables applications to communicate with

one another and it provides access to the services of the other underlying

layers (Network Interface, Internet, and Host-to-Host Transport) There are

wide varieties of application layer protocols, and more are continually being

developed because they can rely on all the services beneath them If you

think of how your computer software is configured, you will realize that you

use many different applications that rely upon the services of the underlying

operating system Each application does not have to provide duplicate

services, such as a routine for accessing your disk drive; that is provided

by the operating system and the application utilizes that functionality

This is how the application layer of the networking model works as well

It relies upon the underlying services In this way, developers do not have

to write code continually to provide the underlying functionality but can

simply access that functionality by adhering to agreed-upon standards and

specifications We’ll look at a number of application layer protocols when

we look at TCP/IP in detail

We’ve discussed the four layers of the DARPA or DoD model of

internetworking Throughout this discussion, we’ve mentioned the OSI

model Now, let’s take a look at the application layer protocols

NETworKING proToColS

We briefly mentioned some of the application layer protocols in our discussion

of the OSI application layer In the following sections, we describe some of

these in more detail We won’t cover every single application layer protocol

in use today (we couldn’t, without turning this book into an encyclopedia

set), but we will cover some of the protocols and services that you’re not only

likely to work with on the job as a network technician but that you’re also

likely to encounter on the Network certification exam

NetBIoS over TCp

NetBIOS over TCP (NetBT) is a legacy protocol and naming service that

has been largely supplanted by the use of DNS, discussed later However,

in organizations running operating systems or applications that cannot use

DNS for name services, NetBT must still be enabled

NetBT is an application layer set of protocols that provides name, session,

and datagram services for NetBIOS applications NetBIOS was originally

developed for IBM by Systek Corporation to extend the capabilities of the

BIOS to include the ability to work across a network It is a software interface

and a naming convention, not a protocol (although you will see it referred