Network Models

Chapter 2
Network
Models

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Chapter 2: Outline

2.1 Protocol Layering
2.2 TCP/IP Protocol Suite
2.3 OSI Model

1.#

1

Chapter 2: Objective
The first section introduces the concept of protocol layering using two scenarios. The section also discusses the two principles upon which the protocol layering is based. The first principle dictates that each layer needs to have two opposite tasks. The second principle dictates that the corresponding layers should be identical. The section ends with a brief discussion of logical connection between two identical layers in protocol layering. Throughout the book, we need to distinguish between logical and physical connections. Chapter 2: Objective (continued)
The second section discusses the five layers of the TCP/IP protocol suite. We show how packets in each of the five layers (physical, data-link, network, transport, and application) are named. We also mention the addressing mechanism used in each layer. Each layer of the TCP/IP protocol suite is a subject of a part of the book. In other words, each layer is discussed in several chapters; this section is just an introduction and preparation.
The third section gives a brief discussion of the OSI model.
This model was never implemented in practice, but a brief discussion of the model and its comparison with the TCP/IP protocol suite may be useful to better understand the TCP/IP protocol suite. In this section we also give a brief reason for the OSI model’s lack of success.

1.#

2

2-1 PROTOCOL LAYERING
A word we hear all the time when we talk about the Internet is protocol. A protocol defines the rules that both the sender and receiver and all intermediate devices need to follow to be able to communicate effectively.
When communication is simple, we may need only one simple protocol; when the communication is complex, we need a protocol at each layer, or protocol layering.
2.5

2.1.1 Scenarios
Let us develop two simple scenarios to better understand the need for protocol layering.
In the first scenario, communication is so simple that it can occur in only one layer.
In the second, the communication between Maria and Ann takes place in three layers.

2.6

1.#

3

Figure 2.1: A single-layer protocol

2.7

Figure 2.2: A three-layer protocol

Postal carrier facility

2.8

1.#

4

2.1.2 Principles of Protocol Layering
Let us discuss two principles of protocol layering.
The first principle dictates that if we want bidirectional communication, we need to make each layer so that it is able to perform two opposite tasks, one in each direction.
The second principle that we need to follow in protocol layering is that the two objects under each layer at both sites should be identical.

2.10

1.#

5

2.1.3 Logical Connections
After following the above two principles, we can think about logical connection between each layer as shown in Figure 2.3. This means that we have layer-to-layer communication. Maria and Ann can think that there is a logical (imaginary) connection at each layer through which they can send the object created from that layer. We will see that the concept of logical connection will help us better understand the task of layering we encounter in data communication and networking.

2.11

Figure 2.3: Logical connection between peer layers

2.12

1.#

6

2-2 TCP/IP PROTOCOL SUITE
A word we hear all the time when we talk about the Internet is protocol. A protocol defines the rules that both the sender and receiver and all intermediate devices need to follow to be able to communicate effectively.
When communication is simple, we may need only one simple protocol; when the communication is complex, we need a protocol at each layer, or protocol layering.
2.13

Figure 2.4: Layers in the TCP/IP protocol suite

2.14

1.#

7

2.2.1 Layered Architecture
To show how the layers in the TCP/IP protocol suite are involved in communication between two hosts, we assume that we want to use the suite in a small internet made up of three LANs (links), each with a link-layer switch. We also assume that the links are connected by one router, as shown in Figure 2.5.

2.15

Figure 2.5: Communication through an internet

2.16

1.#

8

2.2.2 Layers in the TCP/IP Protocol Suite
After the above introduction, we briefly discuss the functions and duties of layers in the TCP/IP protocol suite. Each layer is discussed in detail in the next five parts of the book. To better understand the duties of each layer, we need to think about the logical connections between layers. Figure 2.6 shows logical connections in our simple internet.

2.17

Figure 2.6: Logical connections between layers in TCP/IP

Logical connections

2.18

1.#

9

Figure 2.7: Identical objects in the TCP/IP protocol suite

Identical objects (messages)
Identical objects (segment or user datagram)

Identical objects (datagram)

Identical objects (datagram)

Identical objects (frame)

Identical objects (frame)

Identical objects (bits)

Identical objects (bits)

2.19

2.2.3 Description of Each Layer
After understanding the concept of logical communication, we are ready to briefly discuss the duty of each layer. Our discussion in this chapter will be very brief, but we come back to the duty of each layer in next five parts of the book.

2.20

1.#

10

Physical Layer

The physical layer is responsible for movements of individual bits from one hop (node) to the next.

The physical layer coordinates the functions required to carry a bit stream over a physical medium. It deals with the mechanical and electrical specifications of the interface and transmission medium. It also defines the procedures and functions that physical devices and interfaces have to perform for transmission to occur.
The physical layer is also concerned with the following: o Physical characteristics of interfaces and medium. The physical layer defines the characteristics of the interface between the devices and the transmission medium. It also defines the type of transmission medium. o Representation of bits. The physical layer data consists of a stream of bits (sequence of 0s or 1s) with no interpretation. To be transmitted, bits must be encoded into signals--electrical or optical. The physical layer defines the type of encoding (how 0s and 1s are changed to signals).

1.#

11

o Data rate. The transmission rate-the number of bits sent each second-is also defined by the physical layer. In other words, the physical layer defines the duration of a bit, which is how long it lasts. o Synchronization of bits. The sender and receiver not only must use the same bit rate but also must be synchronized at the bit level. In other words, the sender and the receiver clocks must be synchronized. o Line configuration. The physical layer is concerned with the connection of devices to the media. In a point-to-point configuration, two devices are connected through a dedicated link. In a multipoint configuration, a link is shared among several devices.

o Physical topology. The physical topology defines how devices are connected to make a network. Devices can be connected by using a mesh topology (every device is connected to every other device), a star topology (devices are connected through a central device), a ring topology (each device is connected to the next, forming a ring), a bus topology (every device is on a common link), or a hybrid topology (this is a combination of two or more topologies). o Transmission mode. The physical layer also defines the direction of transmission between two devices: simplex, halfduplex, or full-duplex. In simplex mode, only one device can send; the other can only receive. The simplex mode is a one-way communication. In the half-duplex mode, two devices can send and receive, but not at the same time. In a full-duplex (or simply duplex) mode, two devices can send and receive at the same time. 1.#

12

Data Link Layer

The data link layer is responsible for moving frames from one hop
(node) to the next.

The data link layer transforms the physical layer, a raw transmission facility, to a reliable link. It makes the physical layer appear error-free to the upper layer (network layer).
Other responsibilities of the data link layer include the following:
Framing. The data link layer divides the stream of bits received from the network layer into manageable data units called frames.
Physical addressing. If frames are to be distributed to different systems on the network, the data link layer adds a header to the frame to define the sender and/or receiver of the frame. If the frame is intended for a system outside the sender's network, the receiver address is the address of the device that connects the network to the next one.
Flow control. If the rate at which the data are absorbed by the receiver is less than the rate at which data are produced in the sender, the data link layer imposes a flow control mechanism to avoid overwhelming the receiver.

1.#

13

Error control. The data link layer adds reliability to the physical layer by adding mechanisms to detect and retransmit damaged or lost frames. It also uses a mechanism to recognize duplicate frames. Error control is normally achieved through a trailer added to the end of the frame.
Access control. When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any given time.

Network Layer

The network layer is responsible for the delivery of individual packets from the source host to the destination host.

1.#

14

The network layer is responsible for the source-to-destination delivery of a packet, possibly across multiple networks (links).
Whereas the data link layer oversees the delivery of the packet between two systems on the same network (links), the network layer ensures that each packet gets from its point of origin to its final destination.
If two systems are connected to the same link, there is usually no need for a network layer. However, if the two systems are attached to different networks (links) with connecting devices between the networks (links), there is often a need for the network layer to accomplish source-to-destination delivery.

Other responsibilities of the network layer include the following: o Logical addressing. The physical addressing implemented by the data link layer handles the addressing problem locally. If a packet passes the network boundary, we need another addressing system to help distinguish the source and destination systems. The network layer adds a header to the packet coming from the upper layer that, among other things, includes the logical addresses of the sender and receiver. We discuss logical addresses later in this chapter. o Routing. When independent networks or links are connected to create intemetworks (network of networks) or a large network, the connecting devices (called routers or switches) route or switch the packets to their final destination. One of the functions of the network layer is to provide this mechanism.

1.#

15

Transport Layer

The transport layer is responsible for the delivery of a message from one process to another.

The transport layer is responsible for process-toprocess delivery of the entire message. A process is an application program running on a host. Whereas the network layer oversees source-to-destination delivery of individual packets, it does not recognize any relationship between those packets. It treats each one independently, as though each piece belonged to a separate message, whether or not it does. The transport layer, on the other hand, ensures that the whole message arrives intact and in order, overseeing both error control and flow control at the source-todestination level.

1.#

16

Other responsibilities of the transport layer include the following: o Service-point addressing. Computers often run several programs at the same time. For this reason, source-to-destination delivery means delivery not only from one computer to the next but also from a specific process (running program) on one computer to a specific process (running program) on the other. The transport layer header must therefore include a type of address called a service-point address (or port address). The network layer gets each packet to the correct computer; the transport layer gets the entire message to the correct process on that computer. o Segmentation and reassembly. A message is divided into transmittable segments, with each segment containing a sequence number. These numbers enable the transport layer to reassemble the message correctly upon arriving at the destination and to identify and replace packets that were lost in transmission.

o Connection control. The transport layer can be either connectionless or connection-oriented. A connectionless transport layer treats each segment as an independent packet and delivers it to the transport layer at the destination machine. A connectionoriented transport layer makes a connection with the transport layer at the destination machine first before delivering the packets.
After all the data are transferred, the connection is terminated. o Flow control. Like the data link layer, the transport layer is responsible for flow control. However, flow control at this layer is performed end to end rather than across a single link. o Error control. Like the data link layer, the transport layer is responsible for error control. However, error control at this layer is performed process-to-process rather than across a single link. The sending transport layer makes sure that the entire message arrives at the receiving transport layer without error (damage, loss, or duplication). Error correction is usually achieved through retransmission. 1.#

17

2.2.4 Encapsulation and Decapsulation
One of the important concepts in protocol layering in the Internet is encapsulation/ decapsulation.
Figure 2.8 shows this concept for the small internet in Figure 2.5.

2.35

Figure 2.8: Encapsulation / Decapsulation

2.36

1.#

18

2.2.5 Addressing
It is worth mentioning another concept related to protocol layering in the Internet, addressing. As we discussed before, we have logical communication between pairs of layers in this model. Any communication that involves two parties needs two addresses: source address and destination address.
Although it looks as if we need five pairs of addresses, one pair per layer, we normally have only four because the physical layer does not need addresses; the unit of data exchange at the physical layer is a bit, which definitely cannot have an address. 2.37

Figure 2.9: Addressing in the TCP/IP protocol suite

2.38

1.#

19

2.2.6 Multiplexing and Demultiplexing
Since the TCP/IP protocol suite uses several protocols at some layers, we can say that we have multiplexing at the source and demultiplexing at the destination. Multiplexing in this case means that a protocol at a layer can encapsulate a packet from several next-higher layer protocols (one at a time); demultiplexing means that a protocol can decapsulate and deliver a packet to several nexthigher layer protocols (one at a time). Figure 2.10 shows the concept of multiplexing and demultiplexing at the three upper layers.
2.39

Figure 2.10: Multiplexing and demultiplexing

2.40

1.#

20

2-3 OSI MODEL
A word we hear all the time when we talk about the Internet is protocol. A protocol defines the rules that both the sender and receiver and all intermediate devices need to follow to be able to communicate effectively.
When communication is simple, we may need only one simple protocol; when the communication is complex, we need a protocol at each layer, or protocol layering.
2.41

OSI by ISO
International Organization for Standardization
Open Systems Interconnection
ISO is the organization
OSI is the model

1.#

21

Figure 2.11: The OSI model

2.43

Figure 3-14

Summary of Layer Functions

WCB/McGraw-Hill

1.#

 The McGraw-Hill Companies, Inc., 1998

22

2.3.1 OSI versus TCP/IP
When we compare the two models, we find that two layers, session and presentation, are missing from the TCP/IP protocol suite. These two layers were not added to the TCP/IP protocol suite after the publication of the OSI model. The application layer in the suite is usually considered to be the combination of three layers in the OSI model, as shown in Figure 2.12.

2.45

Figure 2.12: TCP/IP and OSI model

2.46

1.#

23

2.3.2 Lack of OSI Model’s Success
The OSI model appeared after the TCP/IP protocol suite. Most experts were at first excited and thought that the TCP/IP protocol would be fully replaced by the OSI model. This did not happen for several reasons, but we describe only three, which are agreed upon by all experts in the field.
1. OSI was completed when TCP/IP was fully in place. 2. Some layers (protocols) were never fully defined.
3. When OSI was implemented by an organization in a different application, it didn’t show a high enough level of performance.
2.47

TCP/IP is a five-layer hierarchical protocol suite developed before the OSI model.
The TCP/IP application layer is equivalent to the combined session, presentation, and application layers of the OSI model.
Four levels of addresses are used in an internet following the
TCP/IP protocols: physical (link) addresses, logical (IP) addresses, port addresses, and specific addresses.
The physical address, also known as the link address, is the address of a node as defined by its LAN or WAN.
The IP address uniquely defines a host on the Internet.
The port address identifies a process on a host.
A specific address is a user-friendly address.

1.#

24