Nov 16, 2013 ~ Network Services

Introducing the OSI Model The OSI model was designed to promote interoperability by creating a guideline for network data transmission between computers and components that have different hardware vendors, software, operating systems, and protocols. For example, look at...

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Network+™ "Quoting one of e-books" Chapter 2 : The OSI Model: Introducing the OSI Model The OSI Model’s Lower Layers ...

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Answers to Review Questions 1. B, C. Computers participating in a peer-to-peer network can be either client or server or both. Additionally, the peer-to-peer model has some practical limitations, including the number of computers involved. Answer A is incorrect because the administration is not centralized. Answer D is incorrect because...

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Review Questions 1. Which of the following are characteristic of a peer-to-peer network? (Choose all that apply.) A. It has centralized security and administration. B. A computer can be both a client and a server. C. A limited number of computers are involved. D. It does not require a hub. 2. Which cabling standard can send data at up to 10,000Mbps? A....

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Exam Essentials Know how to identify different network topologies. A single cable with computers attached to it is a bus. A central hub with cables radiating out to computers is a star. A crisscross, redundant connection to all computers is a mesh. An outer loop connecting all computers is a ring. Know the operational characteristics of...

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Introducing the OSI Model
The OSI model was designed to promote interoperability by creating a guideline for network data transmission between computers and components that have different hardware vendors,
software, operating systems, and protocols. For example, look at the simple process of transferring a file. From a user’s perspective, a single operation has been performed to transfer the file. In reality, however, many different procedures had to take place behind the scenes to accomplish this seemingly simple task. Network data transmission (like the file transfer) is performed through the use of a protocol suite, also known as a protocol stack, especially when
installed in a given device.

A protocol is most easily defined as a set of rules used to determine how devices communicate with each other. It is similar to language. If one person speaks English and another speaks English, they can communicate. But if one person speaks only Spanish and the other speaks only English, they won’t be able to communicate. A protocol suite is a set of similar protocols that work together to make sure communications happen properly.

The OSI model is used to describe what tasks a protocol suite performs as you explore how data moves from the user interface of a transmitter down to its physical network access, across
a network, and then up the layers of the receiving device to its user interface. Keep in mind that not all protocols map directly to the guideline provided for us through the OSI model, but there
are enough similarities so that you can use the OSI model to examine how these protocols function. This is one of the OSI model’s greatest advantages. It is at once very specific in the separation of functionality within a device (specifying more layers than most other models) and very generic in how it explains what happens at each layer. With this duality, networking engineers
and administrators are able to make both broad comparisons and precise distinctions between the functionality and interoperability of different protocol stacks. There are a myriad of protocol suites in use today, including IPX/SPX, NetBIOS, and TCP/IP, with the first two being quite a bit less prolific in today’s market than the last one. Each performs a specific function. Many of these functions that are provided through the use of a protocol stack and its components are

Standard functions performed by other components in other protocol stacks, thus paving the way for devices and software that can enable the interoperation between differing stacks.
The most commonly referenced protocol model, the OSI model, was developed in 1977 by the International Organization for Standardization (commonly referred to as ISO) to provide
“common ground” when describing any network protocol.

Note:

ISO is not an acronym for the International Organization for Standardizationbut is instead derived from the Greek word isos, which means “equal,” andwas adopted by the organization. For more information, go to www.iso.ch

As you can see in Figure 2.1, the OSI model consists of seven layers. Each layer performs a specific function and then passes on the result to another layer. When a sending station has data
to send, it formats a network request and then passes that request to the network protocol at the top layer, the Application layer. The protocol that runs at the Application layer performs an operation on the request and then passes it to the next (lower) layer. Each protocol at each layer below the Application layer performs its own calculations and appends its own information to the data sent from the layer above it. At the receiving station, the process happens in reverse. Figure 2.2 illustrates this basic process.

As you can see from the diagram, it is possible to have communication between two devices with vastly differing personalities (operating systems), as long as the protocols they are running for network access and communication are compatible (both TCP/IP, for example). Here we have a DOS-compatible PC and a Macintosh talking together over what could be a common
network medium, like Ethernet over copper. The term peer communication comes from the fact that, through the use of headers, equivalent protocols in each stack appear to talk directly with one another. Because the header that one protocol creates means something only to that protocol, and because this control information is encapsulated deeper with each successive lowerlayer
protocol, only the compatible protocol on the receiving device, or perhaps an intermediate device like a router, will be able to access the control information found in the header created by the corresponding process on the transmitting device.

FIGURE 2 . 1 The Open Systems Interconnect (OSI) model

FIGURE 2 . 2 How data travels through the layers

of the OSI model

As an example, if TCP on the DOS device creates a TCP header, then this header will be passed transparently by all intermediate devices to the Macintosh device, which will be the only device capable of de-encapsulating the incoming frame far enough to access the TCP header, as well as the only device along the way running TCP because intermediate devices such as routers,

switches, and hubs are involved with only the bottom three layers for through traffic and TCP is a layer 4 protocol. Even if this were not true, it is the peer communication between the IP protocol

of the DOS device and the IP protocol of every device in between, including the Macintosh device, that tells each one that this IP packet traversing the network is destined for the Macintosh.

This alone would be enough to discourage any intermediate device from trying to process the encapsulated TCP header.

Peer communication can be seen in operation at every layer of any protocol or reference model from the way that two devices communicating on a shared segment have to use a common

cabling protocol with agreed-upon pin configurations and encoding methods at the Physical layer all the way up to the Application layer, where one device will be able to send a message—written, graphical, or otherwise—to another device and rightly expect that message to come across as intended. These two devices with Application layer peer communication could have considerably more degrees of separation from one another than two devices with

peer communication at the Physical layer, allowing any number of devices along the way to produce a path for this message to make it from one end to the other. This example also illustrates the importance of having different protocols in each protocol stack because each layer provides a different scope or diameter of communication; lower-layer protocols require that communicating

devices be neighbors, whereas higher-layer protocols don’t require any such adjacency. This difference in the scope of the layers allows highly advanced protocol suites to be developed and implemented as stacks.

Note:

You can use mnemonic devices to help you remember the order of the OSI
model layers: APSTNDP (from top to bottom). The most popular mnemonic for
this arrangement is All People Seem To Need Data Processing. A reverse mnemonic
(from Physical to Application, bottom to top) is Please Do Not Throw
Sausage Pizza Away. (Good advice, don’t you think?)

The OSI model is mainly a reference model as opposed to a mainstream protocol suite. Although the ISO has created protocols that operate at each of the higher layers of its model,
very few entities have standardizaed on the OSI protocol suite, due mainly to the overbearing popularity of the TCP/IP protocol suite. Let’s take a brief look at the layers of the OSI model and the basic protocol functions they describe:

The Application Layer The Application layer, the top layer of the OSI model, does not refer to applications such as word processors, but rather to a set of tools that an application can use to
accomplish a task such as a word processor application requesting a file transfer. This layer is responsible for defining how interactions occur between network services (applications) and the
network. Services that function at the Application layer include, but are not limited to, file, print, and messaging services. The Application layer may also support error recovery.

As an example, a web browsing application may appear, at first glance, to exist at the Application layer because it is indeed an application and it is involved, most often, with network communication. While the browser software is an application, it is not a protocol because the web services it connects to do not have to operate exactly the same way that the browser operates.
The fact that one application is a server application and the other is a client application speaks to their differences and need for underlying compatible protocols. The Application layer protocol
in common between the two applications is most likely HTTP, which allows the server to deliver an HTML file to the client for display in its browser window. The same browser could
speak FTP to an FTP server. HTTP and FTP are the Application layer protocols here, not the browser software. These protocols give support to the applications that call upon them and offer an entryway into the networking process.

To prove that these functions can be separated from the application itself, open your favorite web browser and surf to your favorite website. On the File menu, save the web page to your Desktop or other favorite local resource. Next, unplug your network connection or shut down your wireless access. You could even pull out your NIC card, for that matter. Open a new instance of your web browser and open the file you saved. Notice that the HTML file displays, even without the support of HTTP and an active network connection. If the application does not need to enter the network process to get its job done, then no Application layer protocol’s services will be required. Said another way, if an Application layer protocol is used at one end, then a corresponding Application layer protocol must be used at the other end, and because your web browser can be used independently of a network connection, it is not an Application layer process, as is HTTP, for example.

The Presentation Layer The Presentation layer is responsible for the formatting and code conversion of data being passed up to the Application layer. In this layer, character sets are converted
(e.g., from ASCII to Unicode or EBCDIC) and data is encrypted. Data may also be compressed in this layer. Of course, anything that is done to the data on the transmitting device must be undone on the receiving device.

Note:

that character-set conversion is not a result of the transmitting device’s having done anything to the data and is only performed on the receiving device, in response to the Presentation layer’s
recognizing that incoming data is not based on the same character set that its own upper-layer processes require. On the other hand, compression and encryption services must be supported by both end devices in the conversation, one to add these features, the other to remove them.

It is the Presentation layer that is responsible for recognizing file types in an incoming data stream and performing any massaging to the data to make a file presentable to the Application
protocol. Think of this function as providing a common syntax for data and using this syntax to convert to and from the application data. The Multipurpose Internet Mail Extensions
(MIME) system denotes the file type of incoming data, helping the Presentation layer know what to do with the incoming stream. File types like MIDI, MPEG, JPEG, and GIF are considered to be Presentation layer entities.

The Session Layer   The Session layer defines how two computers establish, synchronize, maintain, and end a session. Practical functions such as security authentication, connection ID establishment, data transfer, acknowledgments, and connection release take place here. This list is not all-inclusive. Any communications that require milestones—or, put another way, require an answer to “Have you got that data I sent?”—are performed here. Typically these milestones are called
checkpoints. Once a checkpoint has been crossed, any data not received needs retransmission only from the last good checkpoint. Adjusting checkpoints to account for very reliable or unreliable connections can greatly improve the actual throughput of data transmission.

The Transport Layer   The Transport layer is responsible for checking that the data was delivered error-free. It is also used to divide a message that is too long into smaller segments and, in
the reverse, take a series of short messages and combine them into one longer segment. These smaller or combined segments must later be correctly reassembled. This is accomplished through segment sequencing (usually by appending a number to each of the segments).
This layer also handles logical address/name resolution. Additionally, this layer can send an acknowledgment that it got the data packet. Frequently you will see this referred to as an ACK,
which is short for acknowledgment.  This layer is responsible for the majority of error and flow control in network communications.

The major difference in the sessions that the Session layer deals with and the connections that a connection-oriented Transport layer protocol (such as TCP) will create lies with the size or scope of the communication. The Session layer is responsible for the ordered bidirectional communication of entire messages, in the form of a dialog, while a connection-oriented Transport layer protocol is only responsible for the ordered transmission of segments of these messages. Session layer functionality will have to be called upon to salvage a session that is broken before a normal end can occur, while Transport layer functionality is fine to reestablish lost segments or broken virtual circuits while the session itself is still established.

The Network Layer   The Network layer is responsible for logical addressing and translating logical addresses into physical addresses. A little-known function of the Network layer is prioritizing
data. Not all data is of equal importance. Nobody is hurt if an e-mail message is delayed a fraction of a second. Delaying audio or video data a fraction of a second could be disastrous to the message. This prioritization is known as quality of service (QoS).

In addition, the Network layer controls congestion, routes data from source to destination, and builds and tears down packets. Most routing protocols perform their function on packets native to this layer.

The Data Link Layer The Data Link layer takes raw data from the Physical layer and gives it a logical structure, known as a frame. In the opposite direction of flow, the Data Link layer hands frames down to the Physical layer for bit-level encoding onto the networking medium. Frames include information about where the data is meant to go, which device on the local link sent the data, and the overall validity of the bytes sent. In legacy technologies used over lessdependable links, such as X.25 and LLC2 used in an SNA environment, after a data frame is sent, the Data Link layer waits for a positive ACK. If one is not received or if the frame is damaged, another frame is sent. These days, such acknowledgment and retransmission is left to higher layers to perform.

The Data Link layer also controls functions of logical network topologies and physical addressing as well as data transmission synchronization and connection.

The Physical Layer   The Physical layer is responsible for controlling the functional interface, such as transmission technique, encoding scheme, cable specifications, pin layout, and connector type.

Answers to Review Questions

1. B, C. Computers participating in a peer-to-peer network can be either client or server or both. Additionally, the peer-to-peer model has some practical limitations, including the number of computers involved. Answer A is incorrect because the administration is not centralized. Answer D is incorrect because the use of hubs is not related to the implementation of peer-to-peer or client/ server networks.

2. D. The 10G in the 10GBase-SR designation can be thought of as standing for 10Gbps or 10,000Mbps, whichever helps you remember.

3. C, D. MT-RJ and LC are both forms of SFF fiber connectors. FC and SC are larger and do not permit the port density afforded by the other two.

4. A. Looking at the Link LED first could save you the frustration of waiting for the Activity LED to light up, which may never happen as long as there are issues with network connectivity (indicated by a dark Link LED).

5. B. The major advantage to mesh networks is their increased reliability. There are multiple redundant connections between all nodes in the network. Answer A is incorrect because the cable costs are much, much more than other networks. Answer C is simply a distracter; mesh is not required by fire codes. Answer D is incorrect because most token-based networks could not operate in a mesh environment.

6. B, D. The star topology has the advantage of simplifying the troubleshooting process because, when a device fails, you should check that device and its NIC, the network cable connected to
that NIC, and the port on the concentrating device (hub, MAU, switch, etc.) to which the other end of the network cable is connected. Somewhat related to this point, it also has the advantage
of localizing problems to the single device or cable segment. While it is true that the central concentrating device is a single point of failure, this is one of the disadvantages of star topologies.

7. A, B. The two early ring speeds of Token Ring were 4Mbps and 16Mbps

8. A. Of the physical topologies, the mesh has the most physical connections per device. This complete interconnection is what creates the mesh. The mesh is not used in the majority of LAN
implementations, mostly with WAN links. The most common physical LAN topology is the star topology. Answer C describes a physical ring topology, and D describes a logical bus topology.

9. B. There are only two main types of FireWire connectors, the 4 pin and the 6 pin. The 6-pin connector has two extra pins that provide power from the computer to the device.

10. B. Answer B is the only correct answer because plenum-rated cable meets fire codes for installation in suspended ceilings, raised floors, and any other open area through which ventilation-system air is returned. Plenum cable actually has a higher cost than PVC. Additionally, because the conductors are also made of copper, it doesn’t conduct data any faster than PVC-coated cable.

11. B, D. Both the SX and LX standards of gigabit Ethernet are based on fiber-optic cable, not copper. However, while 1000Base-LX permits the use of single-mode fiber over distances of 2000m,
it does not use the FC connector. Both it and the SX standard allow the use of SC or LC connectors on multimode fiber over a distance of 550m.

12. B. If your PC does not have native connectivity, say via TCP/IP, with a mainframe, none of the devices will assist you in communicating with the mainframe. The only help a router would be is if the mainframe were not on the same LAN segment, but it would have to speak the same protocol as your laptop or a gateway would still be necessary. Of all the answers, gateway is decidedly
the best.

13. D. 100Base-FX can be ruled out immediately because the
F indicates a fiber-optic media dependency, while more subtly, 1000Base-CX can be quickly eliminated due to its media dependency on STP, not UTP. 1000Base-T is incorrect because, although it calls for the use of Category 5e UTP, it requires all four pairs for both transmit and receive use.

14. D. In broadband communications (television communications, for example), the communications medium is divided into discrete channels. Each channel can carry its own signal. In baseband
communications, the transmission takes up the whole communications channel. Full duplex communications give a sender and receiver the ability to each send and receive signals
simultaneously. Sideband is a distracter.

15. C. Although all the devices listed can be purchased with the variety of interfaces necessary to satisfy the objective, an external transceiver’s sole purpose is to change the type of media a NIC or
device interface connects to (provided there is a transceiver port available on the NIC or that you purchase a transceiver with th e appropriate interconnections).

16. C. Although you can wire any combination of pairs in an RJ-45 connector, you should wire all four pairs in a Category 5 UTP into an RJ-45 connector to support those network technologies
that may need all four pairs (such as 1000Base-T), even if you aren’t currently using them. Additionally, this habit supports all currently available technologies. So, in case you decide to change
from Token Ring to Ethernet, if you create a straight-through wired channel, there is no reason to rewire any cables because the popular wire pairing standards will cover both of these technologies
and many more.

17. C. The maximum segment length for 10GBase-SR is 300 meters.

18. C. The Channel Service Unit/Data Service Unit (CSU/DSU) translates LAN signals into signals that are used on T1 lines. Some people incorrectly call it a “T1 modem.” It’s not a modem
because it doesn’t translate data into analog and back. Every signal stays in the digital format.

19. B. Although a router possesses certain access control capabilities, a firewall’s hardened configuration set makes it a superior choice for establishing a secure entryway into a LAN, blocking malicious traffic with pinpoint accuracy while allowing trusted traffic access to the internal resources. A proxy server may include some firewall capabilities, but not to the level of a standalone firewall. Besides, in the end, the proxy component that provides this functionality is a firewall, making it the best answer here. HTTPS is not a function of NIC cards, nor would hardening
a NIC card help guard an entire LAN against outside threats.

20. B. 100Base-TX works fine with two pairs of at least Category 5 UTP, but due to the fact that 1000Base-T uses all four pairs simultaneously (a reason D is incorrect), a minimum of Category
5e UTP is required for proper functioning. Both of these standards are specified over twistedpair copper (the T in their name signifies this), not fiber optics. Both standards are limited to the
same 100m segment length.

Review Questions
1. Which of the following are characteristic of a peer-to-peer network? (Choose all that apply.)
A. It has centralized security and administration.
B. A computer can be both a client and a server.
C. A limited number of computers are involved.
D. It does not require a hub.

2. Which cabling standard can send data at up to 10,000Mbps?
A. 10Base-T
B. 100Base-TX
C. 1000Base-TX
D. 10GBase-SR

3. Which of the following are not small form factor fiber connectors? (Choose all that apply.)
A. MT-RJ
B. LC
C. FC
D. SC

4. Which LED on your NIC might save you the most frustration if you start your troubleshooting efforts by monitoring its illumination?
A. Link
B. Activity
C. Collision
D. 10/100

5. Which of the following is a characteristic of a mesh network?
A. It controls cable costs.
B. It offers improved reliability.
C. It is required by fire code.
D. It needs a token to operate.

6. Which of the following are advantages of a star-wired topology? (Choose all that apply.)
A. The star topology uses the least amount of cable.
B. A cable cut between a lone device and its concentrating device affects only the lone device.
C. There is a single point of failure in the central concentrating device.
D. Troubleshooting is simplified compared to the other topologies.

7. Besides the newer high-speed Token Ring, what are the other two standard ring speeds for the Token Ring technology? (Choose all that apply.)
A. 4Mbps
B. 16Mbps
C. 100Mbps
D. 4Gbps

8. Which of the following is a characteristic of a physical mesh topology?
A. It has the most physical connections per device.
B. It is the most common physical LAN topology.
C. Each device has only an inbound port and an outbound port.
D. When one device transmits, all other devices hear the transmission.

9. Which of the following FireWire connectors are for devices that need to be powered from the computer?
A. 4 pin
B. 6 pin
C. 8 pin
D. 10 pin

10. Plenum-rated cable has which of the following characteristics?
A. It has a lower cost than PVC.
B. It meets fire codes for installation in suspended ceilings.
C. It transmits data faster.
D. All the above.

11. Which of the two following cable, connector, length triples meet the specifications for 1000Base-SX and 1000Base-LX?
A. UTP, RJ-45, 100m
B. MMF, LC, 550m
C. SMF, FC, 2000m
D. MMF, SC, 550m

12. Which of the following devices would help your laptop communicate with a mainframe on the same LAN segment?
A. Transceiver
B. Gateway
C. Switch
D. Router

13. Which Ethernet standard is designed to use only two pairs of wires in a UTP cable?
A. 1000Base-CX
B. 100Base-FX
C. 1000Base-T
D. 100Base-TX

14. A transmission technology that divides that transmission medium into discrete channels so that multiple signals can share the same cable is known as ___________________.
A. duplex communications
B. baseband communications
C. sideband communications
D. broadband communications

15. If you need to change the type of media a NIC is able to connect to, which device would you use?
A. Bridge
B. Hub
C. Transceiver
D. All the above

16. An RJ-45 connector should be wired with ____________________ pairs when used on a Category
5e UTP cable.
A. 1
B. 2
C. 4
D. 8

17. 10GBase-SR can be extended to ___________________ meters per segment.
A. 100
B. 200
C. 300
D. 1000

18. Which network component is used in conjunction with a router to provide access to a T1 circuit?
A. Gateway
B. T1 modem
C. CSU/DSU
D. Switch

19. If you need to protect internal LAN resources from an external threat, which device can help most?
A. Router
B. Firewall
C. Proxy server
D. HTTPS-compatible NIC card

20. Which of the following is a difference between the 100Base-TX and 1000Base-T Ethernet specifications?
A. 1000Base-T is 10 times faster because it uses fiber optics.
B. 100Base-TX requires a minimum of Category 5 UTP, while 1000Base-T must be run over no less than Category 5e UTP.
C. Because it’s slower and more stable, 100Base-TX can be run over longer distances.
D. Although both technologies require the same number of pairs for transmitting and receiving, 1000Base-T uses them differently

Exam Essentials
Know how to identify different network topologies. A single cable with computers attached to it is a bus. A central hub with cables radiating out to computers is a star. A crisscross, redundant
connection to all computers is a mesh. An outer loop connecting all computers is a ring. Know the operational characteristics of various cable standards. You should know the signaling rate (in Mbps), signaling method (baseband or broadband), media type (copper or fiber), and the other specifics for the various cable standards such as 10Base-T, 10Base-FL, 100Base- TX, 1000Base-T, and 10GBase-SR.

Be able to recognize different media connectors and describe their uses. You should know that RJ- 11 is used to connect a phone jack to a telephone; RJ-45 is used for 10Base-T, 100Base-TX, and
1000Base-T twisted-pair Ethernet connections; BNC is used for 10Base2 Ethernet connections; AUI, a DB-15 connector/PC game connector, is used for 10Base5 connections from vampire tap to NIC; ST, the most popular fiber-optic connector, is a barrel connector with a locking ring; and SC, another common fiber connector, is a square-ended connector with a latching mechanism. You should also understand the different types of IEEE 1394 (FireWire) connectors and their different uses. Understand the different media types and their uses. You should know the different types of commonly used network media (copper cabling and fiber-optic media) and the different applications of each. You must know the differences between Category 3, 5, 5e, and 6 UTP and what
the category ratings mean. You should also know the operational characteristics of the different types of fiber-optic cable (single mode and multimode). Be able to explain the basic purpose and function of many different network devices. You should understand how each network device—including hubs, switches, routers, bridges, firewalls, and wireless access points—functions.

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