Lesson 1: Introduction to Protocols
This lesson offers an introduction to protocols and their function in a
networking environment. It explains the roles of protocols in network communications
and describes how different protocols work at different OSI levels.
The Function of Protocols
Protocols are rules and procedures for communicating. The term "protocol"
is used in a variety of contexts. For example, diplomats from one country
adhere to rules of protocol designed to help them interact smoothly with
diplomats from other countries. Rules of protocol apply in the same way
in the computer environment. When several computers are networked, the
rules and technical procedures governing their communication and interaction
are called protocols.
Keep three points in mind when you think about protocols in a network
There are many protocols. While each protocol facilitates basic
communications, each has different purposes and accomplishes different
tasks. Each protocol has its own advantages and restrictions.
Some protocols work only at particular OSI layers. The layer at
which a protocol works describes its function. For example, a protocol
that works at the physical layer ensures that the data packet passes through
the network interface card (NIC) and out onto the network cable.
Protocols can also work together in a protocol stack, or suite.
Just as a network incorporates functions at every layer of the OSI reference
model, different protocols also work together at different levels in a
single protocol stack. The levels in the protocol stack "map," or correspond,
to the layers of the OSI reference model. For instance, the TCP/IP protocol's
application layer maps to the OSI reference model's presentation layer.
Taken together, the protocols describe the entire stack's functions and
How Protocols Work
The entire technical operation by which data is transmitted over the network
has to be broken down into discrete, systematic steps. At each step, certain
actions take place that cannot take place at any other step. Each step
includes its own rules and procedures, or protocol.
The protocol steps must be carried out in a consistent order that is
the same on every computer in the network. In the sending computer, these
steps must be executed from the top down. In the receiving computer, these
steps must be carried out from the bottom up.
The Sending Computer
Protocols at the sending computer:
The Receiving Computer
Break the data into smaller sections, called packets, that the protocol
Add addressing information to the packets so that the destination computer
on the network can determine that the data belongs to it.
Prepare the data for transmission through the NIC and out onto the network
Protocols at the receiving computer carry out the same series of steps
in reverse order. They:
Both sending and receiving computers need to perform each step in the same
way so that the data will have the same structure when it is received as
it did when it was sent.
Take the data packets off the cable.
Bring the data packets into the computer through the NIC.
Strip the data packets of all the transmitting information that was added
by the sending computer.
Copy the data from the packets to a buffer for reassembly.
Pass the reassembled data to the application in a usable form.
For example, two different protocols might each break data into packets
and add on various sequencing, timing, and error-checking information,
but each will do it differently. Therefore, a computer using one of these
protocols will not be able to communicate successfully with a computer
that is using the other protocol.
Until the mid-1980s, most local area networks (LANs) were isolated. A LAN
served a single department or company and was rarely connected to any larger
environments. As LAN technology matured, however, and the data communication
needs of businesses expanded, LANs evolved, becoming components in larger
data communication networks in which LANs talked to each other.
Data that is sent from one LAN to another along any of several available
paths is said to be
routed. The protocols that support multipath
LAN-to-LAN communications are known as routable protocols. Because
routable protocols can be used to tie several LANs together and create
new wide-area environments, they are becoming increasingly important.
Protocols in a Layered Architecture
In a network, several protocols have to work together. By working together,
they ensure that the data is properly prepared, transferred to the right
destination, received, and acted upon.
The work of the various protocols must be coordinated so that no conflicts
or incomplete operations take place. The results of this coordination effort
are known as layering.
A protocol stack is a combination of protocols. Each layer of the stack
specifies a different protocol for handling a function or subsystem of
the communication process. Each layer has its own set of rules. In Chapter
5, "Introducing Network Standards," we discussed the OSI reference
model. Figure 6.1 shows the OSI reference model and the rules associated
with each layer. The protocols define the rules for each layer in the OSI
Figure 6.1 The OSI reference model showing the
layers of protocols
The lower layers in the OSI reference model specify how manufacturers
can make their equipment connect to equipment from other manufacturers,
for example, by using NICs from several manufacturers on the same LAN.
As long as they operate with the same protocols, they are able to send
and receive data from each other. The upper layers specify rules for conducting
communications sessions (the time during which two computers maintain a
connection) and the interpretation of applications. The higher they are
in the stack, the more sophisticated the tasks and their associated protocols
The Binding Process
The binding process—the process by which protocols become connected
to each other and the NIC—allows a great deal of flexibility in setting
up a network. Protocols and NICs can be mixed and matched on an as-needed
basis. For example, two protocol stacks, such as Internetwork Packet Exchange
and Sequenced Packet Exchange (IPX/SPX), discussed in Lesson 3: NetWare
Protocols, and Transmission Control Protocol/Internet Protocol (TCP/IP),
discussed in Lesson 2: TCP/IP, can be bound to one NIC. If there is more
than one NIC in the computer, one protocol stack can be bound to either
or both NICs.
The binding order determines the sequence in which the operating
system runs the protocol. When multiple protocols are bound to a single
NIC, the binding order is the sequence in which the protocols will be utilized
to attempt a successful connection. Typically, the binding process is initiated
when either the operating system or the protocol is installed or initialized.
For example, if TCP/IP is the first protocol to be bound, the network operating
system will attempt a network connection via TCP/IP before attempting to
use another protocol. If this network connection fails, the computer will
attempt to make a connection by using the next protocol in the binding
The binding process consists of more than just binding the protocol
stack to the NIC. Protocol stacks need to be bound or associated with the
components above and below them so that data can proceed smoothly through
the stack during execution. For example, TCP/IP may be bound to the Network
Basic Input/Output System (NetBIOS) session layer above as well as to the
NIC driver below it. The NIC driver is also bound to the NIC.
The computer industry has designated several kinds of stacks as standard
protocol models. Hardware and software manufacturers can develop their
products to meet any one or a combination of these protocols. The most
important models include:
Protocols exist at each layer of these stacks, performing the tasks specified
by that layer. However, the communication tasks that networks need to perform
are grouped into one of three protocol types. Each type is comprised of
one or more layers of the OSI. As shown in Figure 6.2, these three protocol
types map roughly to layers of the OSI reference model (application, transport,
The ISO/OSI protocol suite.
The IBM Systems Network Architecture (SNA).
The Internet protocol suite, TCP/IP.
Many protocols were written long before the OSI reference model
came into common use. Thus, it is not uncommon to find protocol stacks
that do not map directly to the OSI model.
Figure 6.2 Communication tasks within the OSI
Application protocols work at the uppermost layer of the OSI reference
model. They provide application-to-application interaction and data exchange.
Popular application protocols are shown in Table 6.1.
Table 6.1 Popular Application Protocols
|APPC (Advanced Program-to-Program Communication)
||IBM's peer-to-peer SNA protocol, mostly used on AS/400 computers.
APPC is defined as an applica- tion protocol, because it works in the presentation
layer of the OSI reference model. However, it is also considered a transport
protocol because APPC uses the LU 6.2 protocol that works in both the transport
and session layers of the OSI reference model.
|FTAM (File Transfer Access and Management)
||An OSI file access protocol.
||A CCITT protocol for international e-mail transmissions.
||A CCITT protocol for file and directory services across
|SMTP (Simple Mail Transfer Protocol)
||An Internet protocol for transferring e-mail.
|FTP (File Transfer Protocol)
||An Internet file transfer protocol.
|SNMP (Simple Network Management Protocol)
||An Internet protocol for monitoring networks and network
||An Internet protocol for logging on to remote hosts and
processing data locally.
|Microsoft SMBs (Server Message Blocks) and client shells
||A client/server, request response protocol.
|NCP (Novell NetWare Core Protocol) and Novell client
shells or redirectors
||A set of service protocols.
|AppleTalk and AppleShare
||Apple's networking protocol suite.
|AFP (AppleTalk filing Protocol)
||Apple's protocol for remote file access.
|DAP (Data Access Protocol)
||A DECnet file access protocol.
Transport protocols facilitate communication sessions between computers
and ensure that data is able to move reliably between computers.
Popular transport protocols are shown in Table 6.2.
Table 6.2 Popular Transport Protocols
||The TCP/IP protocol for guaranteed delivery of sequenced
||Part of Novell's IPX/SPX protocol suite for sequenced data.
||The Microsoft implementation of the IPX/SPX protocol.
|NetBEUI (NetBIOS extended user interface)
||Establishes communication sessions between computers (NetBIOS)
and provides the underlying data transport services (NetBEUI).
|ATP (AppleTalk Transaction Protocol) and NBP (Name Binding
||Apple's communication-session and data-transport protocols.
Network protocols provide what are called "link services." These protocols
handle addressing and routing information, error checking, and retransmission
requests. Network protocols also define rules for communicating in a
particular networking environment such as Ethernet or Token Ring. Popular
network protocols are shown in Table 6.3.
Table 6.3 Popular Network Protocols
||The TCP/IP protocol for packet-forwarding routing.
||NetWare's protocol for packet forwarding and routing.
||The Microsoft implementation of the IPX/SPX protocol.
||A transport protocol that provides data-transport services
for NetBIOS sessions and applications.
|DDP (Datagram Delivery Protocol)
||An AppleTalk data-transport protocol.
The OSI reference model is used to define which protocols should be used
at each layer. Figure 6.3 shows the OSI reference model and how several
popular network manufacturers apply their protocols. Products from different
manufacturers that subscribe to this model can communicate with each other.
Figure 6.3 Manufacturer compatibility
The ISO, the Institute of Electrical and Electronic Engineers (IEEE),
ANSI (American National Standards Institute), CCITT (Comité Consultatif
Internationale de Télégraphie et Téléphonie),
now called the ITU (International Telecommunications Union), and other
standards bodies have developed protocols that map to some of the layers
in the OSI reference model.
The IEEE protocols at the physical layer are:
As described in Chapter 5, Lesson
1: Open Systems Interconnection (OSI) Reference Model, the data-link
layer is divided into two sublayers (see Figure 6.4).
802.3 (Ethernet) This is a logical bus network that can transmit
data at 10 Mbps. Data is transmitted on the network to every computer.
Only computers meant to receive the data acknowledge the transmission.
The carrier-sense multiple access with collision detection (CSMA/CD) protocol
regulates network traffic by allowing a transmission only when the network
is clear and no other computer is transmitting.
802.4 (token passing) This is a bus layout that uses a token-passing
scheme. Each computer receives all the data, but only the computers that
are addressed respond. A token that travels the network determines which
computer is able to broadcast.
802.5 (Token Ring) This is a logical ring network that transmits
at either 4 Mbps or 16 Mbps. Although this is called a ring, it more resembles
a star with each computer branching off a hub. The ring is actually inside
the hub. A token traveling around the ring determines which computer can
The IEEE has further defined these protocols to facilitate communications
activity at the Media Access Control (MAC) sublayer.
Figure 6.4 MAC driver or NIC driver
A MAC driver is located at the Media Access Control sublayer; this device
driver is also known as the NIC driver. It provides low-level access to
network adapters by providing data-transmission support and some basic
adapter management functions.
A MAC protocol determines which computer can use the network cable when
several computers try to use it simultaneously. CSMA/CD, the 802.3 protocol,
allows computers to transmit data when no other computer is transmitting.
If two hosts transmit simultaneously, a collision occurs. The protocol
detects the collision and halts all transmission until the wire is clear.
Then, each computer can begin to transmit again after waiting a random
period of time.
Implementing and Removing Protocols
Protocols are implemented and removed in much the same way that drivers
are added and removed. Essential protocols are installed automatically
at the same time the initial operating system is installed on the computer.
To install protocols such as NWLink after the initial installation, the
network operating system usually includes a utility that leads the administrator
through the process. For example, a network operating system setup program
might provide a series of graphical windows that lead the administrator
through the process of:
Installing a new protocol.
Changing the order in which the installed protocols have been linked.
Removing a protocol.
Exercise 6.1 (a): Matching the OSI Model Rules to Layers
This exercise is designed to help you reinforce your understanding of network
protocol stacks. The following table contains two columns. In the left
column are listed the seven layers of the OSI reference model. In the right
column, enter the rule that applies to the layer on the left.
OSI Reference Model Rules
Exercise 6.1 (b): Matching the OSI Model Layers with Communication Tasks
Because many protocols were written before the OSI reference model was
developed, some protocol stacks developed earlier don't match the OSI reference
model; in those stacks, tasks are often grouped together.
Communication tasks can be classified into three groups. In this part
of the exercise, the seven layers of the OSI reference model are again
listed in the left column. In the right column, write in the name of one
of the three groups in the following list. Your task is to identify which
of these three groups maps to each of the OSI layers in the left column.
The three groups are:
Matching OSI Reference Model with Communication Tasks
Application-level network service users.
The following points summarize the main elements of this lesson:
Protocols in a networking environment define the rules and procedures for
To send data over a network successfully requires a series of separate
steps that must be carried out in a prescribed order.
The sending and receiving computers use protocols to:
Break data into packets.
Add addressing information to the packets.
Prepare the packets for transmission.
Take the packets off the cable.
Copy the data from the packets for reassembly.
Pass the reassembled data to the computer.
Several stacks are used as standard protocols; the most prominent standard
protocols are based on the OSI reference model layers.
Protocols are implemented and removed in the same manner as drivers.