Chapter 2 - The Open Systems Interconnection Specifications

Question 1

Two types of packets are used at the Network layer:

Answer 1

Data Packets and Route-Update Packets

Question 2

Data Packets These are used to transport user data through the internetwork. Protocols used to support data traffic are called routed protocols . Two examples of routed protocols are Internet Protocol (IP) and Internet Protocol version 6 (IPv6), which you’ll learn all about coming up in Chapter 7, “IP Addressing.”

Answer 2

Route-Update Packets These are used to update neighboring routers about the networks connected to all routers within the internetwork. Protocols that send route-update packets are called routing protocols, and some common ones are Routing Information Protocol (RIP), RIPv2, Enhanced Interior Gateway Routing Protocol (EIGRP), and Open Shortest Path First (OSPF). Route-update packets are used to help build and maintain routing tables on each router.

Question 3

hop count is the number of routers a packet passes through en route to a remote network.

Answer 3

routers break up broadcast domains and switches break up collision domains.

Question 4

A router can also be referred to as a Layer 3 switch. These terms are interchangeable.

Answer 4

REMEMBER

Routers, by default, won’t forward any broadcast or multicast packets.

Routers use the logical address in a Network layer header to determine the next-hop router to forward the packet to.

Routers can use access lists, created by an administrator, to control security on the types of packets that are allowed to enter or exit an interface.

Question 5

REMEMBER

Routers can provide Layer 2 bridging functions if needed and can simultaneously route through the same interface.

Layer 3 devices (routers, in this case) provide connections between virtual LANs (VLANs).

Routers can provide quality of service (QoS) for specific types of network traffic.

Answer 5

The IEEE Ethernet Data Link layer has two sublayers:

Media Access Control (MAC)

Logical Link Control (LLC)

Question 6

The OSI model is the primary architectural model for networks. It describes how data and network information are communicated from an application on one computer through the network media to an application on another computer.

Answer 6

a reference model is a conceptual blueprint of how communications should take place. It addresses all the processes required for effective communication and divides these processes into logical groupings called layers.

Question 7

When a communication system is designed in this manner, it’s known as layered architecture.

Answer 7

the central purpose of the OSI, and all networking models, is to allow different vendors’ networks to interoperate smoothly.

Question 8

important advantages we gain by using the OSI layered model:

The OSI model divides network communication processes into smaller and simpler components, thus aiding component development, design, and troubleshooting.

It allows multiple-vendor development through the standardization of network components.

Answer 8

Continued … important advantages we gain by using the OSI layered model:

It encourages industry standardization by defining the specific functions that occur at each layer of the model.

It allows various types of network hardware and software to communicate.

It prevents changes in one layer from affecting other layers, facilitating development and making application programming much easier.

Question 9

The OSI model has seven layers: Application (Layer 7) Presentation (Layer 6) Session (Layer 5) Transport (Layer 4) Network (Layer 3) Data Link (Layer 2) Physical (Layer 1)

Answer 9

The OSI’s seven layers are divided into two groups. The top three layers define the rules of how the applications working within host machines communicate with each other as well as with end users.

Question 10

The bottom four layers define how the actual data is transmitted from end to end. Figure 2.2 shows the top three layers and their functions, and Figure 2.3 shows the four lower layers and their functions.

Answer 10

The Application Layer

Technically, users communicate with the network stack through application processes, interfaces, or APIs that connect the application in use to the operating system of the computer.

Question 11

The Application Layer

The Application layer chooses and determines the availability of communicating partners along with the resources necessary to make their required connections. It coordinates partnering applications and forms a consensus on procedures for controlling data integrity and error recovery.

Answer 11

The Application layer is also responsible for identifying and establishing the availability of the intended communication partner and determining whether sufficient resources for the requested communication exist.

Question 12

The Presentation Layer

The Presentation layer gets its name from its purpose: it presents data to the Application layer and is responsible for data translation and code formatting.

Answer 12

Presentation layer ensures that the data transferred from one system’s Application layer can be read and understood by the Application layer on another system.

Question 13

The OSI has protocol standards that define how standard data should be formatted. Tasks like data compression, decompression, encryption, and decryption are all associated with this layer. Some Presentation layer standards are even involved in multimedia operations.

Answer 13

The Session Layer

The Session layer is responsible for setting up, managing, and then tearing down sessions between Presentation layer entities.

Question 14

Session Layer

This layer also provides dialog control between devices, or nodes. It coordinates communication between systems and serves to organize their communication by offering three different modes: simplex, half duplex, and full duplex.

Answer 14

To sum up, the Session layer basically keeps applications’ data separate from other applications’ data. For a good example, the Session layer allows multiple web browser sessions on your desktop at the same time.

Question 15

The Transport layer segments and reassembles data into a data stream. Services located in the Transport layer handle data from upper-layer applications and unite it onto the same data stream. They provide end-to-end data transport services and can establish a logical connection between the sending host and destination host on an internetwork.

Answer 15

The Transport layer is responsible for providing the mechanisms for multiplexing upper-layer applications, establishing virtual connections, and tearing down virtual circuits. It also hides the many and sundry details of any network-dependent information from the higher layers, facilitating data transfer.

Question 16

Connection-Oriented Communication Before a transmitting host starts to send segments down the model, the sender’s TCP process contacts the destination’s TCP process to establish a connection. The resulting creation is known as a virtual circuit. This type of communication is called connection-oriented.

Answer 16

During this initial handshake, the two TCP processes also agree on the amount of information that will be sent in either direction before the respective recipient’s TCP sends back an acknowledgment. With everything agreed on in advance, the path is paved for reliable communication to take place.

Question 17

The two operating systems communicate by sending messages over the network confirming that the transfer is approved and that both sides are ready for it to take place. After all of this required synchronization occurs, a connection is fully established and the data transfer begins. This virtual circuit setup is called overhead.

Answer 17

To summarize, a service is considered connection-oriented if it has the following characteristics: A virtual circuit is set up (such as a three-way handshake). It uses sequencing. It uses acknowledgments. It uses flow control.

Question 18

Windows are used to control the amount of outstanding, unacknowledged data segments.

Answer 18

The quantity of data segments (measured in bytes) that the transmitting machine is allowedto send without receiving an acknowledgment is represented by something called a window.

Question 19

It’s important to understand that the size of the window controls how much information is transferred from one end to the other. Although some protocols quantify information by observing the number of packets, TCP/IP measures it by counting the number of bytes.

Answer 19

When you’ve configured a window size of 1, the sending machine waits for an acknowledgment for each data segment it transmits before transmitting another. If you’ve configured a window size of 3, the sending machine is allowed to transmit three data segments before an acknowledgment is received.

Question 20

If a receiving host fails to receive all the segments that it should acknowledge, the host can improve the communication session by decreasing the window size.

Answer 20

positive acknowledgment with retransmission—a technique that requires a receiving machine to communicate with the transmitting source by sending an acknowledgment message back to the sender when it receives data.

Question 21

The Network layer manages logical device addressing, tracks the location of devices on the network, and determines the best way to move data. This means that the Network layer must transport traffic between devices that aren’t locally attached. Routers are Layer 3 devices that are specified at the Network layer and provide the routing services within an internetwork.

Answer 21

Data Packets These are used to transport user data through the internetwork. Protocols used to support data traffic are called routed protocols. Two examples of routed protocols are Internet Protocol (IP) and Internet Protocol version 6 (IPv6), which you’ll learn all about coming up in Chapter 7, “IP Addressing.”

Question 22

routers break up broadcast domains and switches break up collision domains.

Answer 22

Here are some key points about routers that you really should commit to memory: Routers, by default, won’t forward any broadcast or multicast packets. Routers use the logical address in a Network layer header to determine the next-hop router to forward the packet to. Routers can use access lists, created by an administrator, to control security on the types of packets that are allowed to enter or exit an interface. Routers can provide Layer 2 bridging functions if needed and can simultaneously route through the same interface. Layer 3 devices (routers, in this case) provide connections between virtual LANs (VLANs).

Question 23

Routers can provide quality of service (QoS) for specific types of network traffic.

Answer 23

The Data Link layer provides the physical transmission of the data and handles error notification, network topology, and flow control. This means the Data Link layer ensures that messages are delivered to the proper device on a LAN using hardware (MAC) addresses and translates messages from the Network layer into bits for the Physical layer to transmit.

Question 24

The Data Link layer formats the message into pieces, each called a data frame, and adds a customized header containing the destination and source hardware addresses.

Answer 24

This added information forms a sort of capsule that surrounds the original message in much the same way that engines, navigational devices, and other tools were attached to the lunar modules of the Apollo project.

Question 25

The IEEE Ethernet Data Link layer has two sublayers: Media Access Control (MAC) Defines how packets are placed on the media. Contention media access is “first come, first served” access, where everyone shares the same bandwidth—hence the name.

Logical Link Control (LLC) Responsible for identifying Network layer protocols and then encapsulating them, an LLC header tells the Data Link layer what to do with a packet once a frame is received.

Answer 25

802. 1 LAN/MAN Management (and Media Access Control Bridges)
803. 2 Logical Link Control
804. 3 CSMA/CD (Ethernet)
805. 4 Token Passing Bus
806. 5 Token Passing Ring
807. 6 Distributed Queue Dual Bus (DQDB) Metropolitan Area Network (MAN)
808. 7 Broadband Local Area Networks
809. 8 Fiber-Optic LANs and MANs
810. 9 Isochronous LANs
811. 10 LAN/MAN Security
812. 11 Wireless LAN

Question 26

802. 12 Demand Priority Access Method 802.15 Wireless Personal Area Network 802.16 Wireless Metropolitan Area Network (also called WiMAX)
803. 17 Resilient Packet Ring

Answer 26

From this list in the sidebar, “Project 802”, you just need to remember that 802.3 calls out anything having to do with Ethernet and 802.11 is anything wireless.

Question 27

The Physical Layer Finally, we’re hitting bottom. Well, not in a bad way—we’ve now arrived at the Physical layer, which does two important things: it sends bits and receives bits. Bits come only in values of 1 or 0—a Morse code with numerical values. The Physical layer communicates directly with the various types of actual communication media.

Answer 27

The Physical layer specifies the electrical, mechanical, procedural, and functional requirements for activating, maintaining, and deactivating a physical link between end systems. This layer is also where you identify the interface between the data terminal equipment (DTE) and the data communication equipment (DCE).

Question 28

(Some older phone company employees still call DCE data circuit-terminating equipment.) The DCE is usually located at the customer, whereas the DTE is the attached device. The services available to the DTE are most often accessed via the DCE device, which is a modem or channel service unit/data service unit (CSU/DSU).

Answer 28

the Physical layer specifies the layout of the transmission media, otherwise known as its topology.

Question 29

When a host transmits data across a network to another device, the data goes through encapsulation: It’s wrapped with protocol information at each layer of the OSI model. Each layer communicates only with its peer layer on the receiving device.

Answer 29

To communicate and exchange information, each layer uses Protocol Data Units (PDUs). These hold the control information attached to the data at each layer of the model. They’re usually attached to the header in front of the data field but can also be in the trailer, or end, of it.

Question 30

At a transmitting device, the data-encapsulation method works like this:

1. User information is converted to data for transmission on the network.
2. Data is converted to segments, and a reliable connection is set up between the transmitting and receiving hosts.

Answer 30

3. Segments are converted to packets or datagrams, and a logical address is placed in the header so each packet can be routed through an internetwork. A packet carries a segment of data.
4. Packets or datagrams are converted to frames for transmission on the local network. Hardware (Ethernet) addresses are used to uniquely identify hosts on a local network segment. Frames carry packets.
5. Frames are converted to bits, and a digital encoding and clocking scheme is used.

Question 31

The purpose of digital modulation is to transfer a digital bit stream over an analog bandpass channel. (A good example would be data transmitting over the public switched telephone network, where a bandpass filter limits the frequency range to 300–3400Hz, or over a limited radio frequency band.)

Answer 31

Analog and digital modulation use something called frequency-division multiplexing (FDM), where several low-pass information signals are transferred simultaneously over the same shared physical network, using separate passband channels (several different frequencies).

Question 32

Time-division multiplexing (TDM) is a method of transmitting and receiving many independent signals over a common signal path by means of synchronized network devices at each end of the transmission line so that each signal appears on the line only a fraction of time in an alternating pattern. The receiving end demultiplexes the signal back to its original form.

Answer 32

End