The two computers use a protocol to communicate with the same layer 

on another computer. The protocol defined by each layer uses a header 

that is transmitted between the computers, to communicate what each 

computer wants to do.

On a single computer, one layer provides a service to a higher layer. 

The software or hardware that implements the higher layer requests 

that the next lower layer perform the needed function.

Layer 7 provides an interface between the communications software and any 

applications that need to communicate outside the computer on which the 

application resides. It also defines processes for user authentication.

This layer's main purpose is to define and negotiate data formats, such as 

ASCII text, EBCDIC text, binary, BCD, and JPEG. Encryption also is 

defined by OSI as a presentation layer service.

The session layer defines how to start, control, and end conversations (called 

sessions). This includes the control and management of multiple bidirectional 

messages so that the application can be notified if only some of a series of 

messages are completed. This allows the presentation layer to have a 

seamless view of an incoming stream of data.

Layer 4 protocols provide a large number of services, as described in Chapter 

6 of this book. Although OSI Layers 5 through 7 focus on issues related to the 

application, Layer 4 focuses on issues related to data delivery to another 

computer—for instance, error recovery and flow control.

The network layer defines three main features: logical addressing, routing 

(forwarding), and path determination. Routing defines how devices (typically 

routers) forward packets to their final destination. Logical addressing defines 

how each device can have an address that can be used by the routing process. 

Path determination refers to the work done by routing protocols to learn all 

possible routes, and choose the best route.

The data link layer defines the rules that determine when a device can send 

data over a particular medium. Data link protocols also define the format of a 

header and trailer that allows devices attached to the medium to send and 

receive data successfully.

This layer typically refers to standards from other organizations. These 

standards deal with the physical characteristics of the transmission medium, 

including connectors, pins, use of pins, electrical currents, encoding, light 

modulation, and the rules for how to activate and deactivate the use of the 

physical medium.