The Internet Encyclopedia (Volume 3)

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INTERNETWORKINGMODELS ANDLAYERINGPROTOCOLS 323

Physical

Data Link

Network

Transport

Session

Presentation

Application

Physical

Data Link

Network

Transport

Session

Presentation

Application

A B

Figure 1: The OSI 7-layer model as applied to two networked

hosts, A and B.

therefore the physical layer is known as layer 1 and so

on. These interact with their corresponding layers across

the network on a peer-to-peer basis so that each believes it

has direct contact with its own peer layer. This is shown in

Figure 1 for a single connection. The fact that each layer

conceptually responds to its neighboring layer at the same

level allows network connections to operate without the

upper layers.

The physical layer (layer 1) defines the physical and

electrical characteristics of a network interconnection.

These are implemented in the wiring, the connectors, and

the network interface cards of the hosts. The data link

layer (layer 2) determines how the physical medium is ac-

cessed. HDLC, SLIP, and PPP protocols operate in part

at this level. For the Ethernet MAC addresses are located

at level 2 and this allows a LAN to become aware of its

constituent members by their unique hardware addresses.

The network layer (layer 3) allows systems using the same

protocol to establish, maintain, and terminate connec-

tions. The IP protocol operates at the network layer, as do

local hardware components such as routers. The trans-

port layer (layer 4) ensures data reliability and integrity

(TCP is usually considered to operate at this layer); re-

transmission requests and packet duplication problems

are also resolved. The session layer (layer 5) has respon-

sibility for ensuring sessions are completed without inter-

ference when necessary. This is important to e-commerce

where the transaction cannot be disturbed by load shar-

ing during its processing. The presentation layer (layer 6)

deals with decryption, protocol conversion, and graphics

expansion prior to presenting the information to the ap-

plication layer. Layer 7, the application layer, is the part

the user sees and uses. At this layer are applications such

as FTP and other packages that require network access.

The TCP/IP structure is similar and due to its earlier

commercial support (and support from the DoD in the

USA) is the de facto applied standard. (It should be noted

that the DoD did eventually formally move to the OSI

model.) The structure is shown in Figure 2. See Figure 3

Physical

Application

Transport

Network Interface

Internet

Identical Messages

Identical packet

Identical Datagram

Identical frame

Application

Transport

Network Interface

Internet

Host A Host B

Figure 2: The reference model for TCP/IP. Note the lack of

the session and presentation layers.

for the TCP/IP version of a network connection via a router

(the OSI version is similar).

Each layer provides a recognizable service to the pre-

ceding and succeeding layers, which implies structural

changes to the message stream if it is to be recognizable

by peer layers. At any given layer we require transparency;

e.g., each layer believes it alone is involved in the commu-

nications process with a peer layer elsewhere.

For the data link layer this requires it to know the phys-

ical address of the next destination or source, for the net-

work layer it must know the network address, and for the

user a pseudonym for the file is sufficient. This is achieved

by a process known as encapsulation. See Figure 4. The

original data packet from the TCP has been encapsulated

within a datagram by IP, which in turn is enclosed within

a frame. Depending upon the final protocol used for trans-

mission at the data link and physical layers the structure

may be further modified. The reader is referred to the ap-

propriate chapter for the details.

It is evident that having a common network infrastruc-

ture provided by third parties will lead to further proto-

cols and standards that apply to the physical layer of a

real system. These parties operate their lines at a variety

of speeds that determine the pricing for user access. Since

Physical Net

Application

Transport

Network

Interface

Internet

Identical Messages

Identical packet

Identical Datagram

Identical frame

Application

Transport

Network

Interface

Internet

Host A Host B

Physical Net

Network

Interface

Internet

Gateway

Figure 3: A simple TCP/IP Internet connection. Note the

higher layers are not required.