SNA Overview 435
The following list describes the layers:
·
Transaction services--Provides application services, such as databases and document
interchange.
·
Presentation services--Organizes data for presentation media and coordinates resource
sharing.
·
Data-flow control--Synchronizes data flow, groups data into units, and correlates data
exchange.
·
Transmission control--Controls data exchanges to match processing capabilities
(pacing), tracks session status, controls sequencing, provides error correction, and
encrypts data if security is needed.
·
Path control--Controls routing of data and controls traffic in the network. Transmission
groups, explicit routes, and virtual routes all operate at this layer. Path control is also
responsible for sending messages.
·
Data-link control--Transmits data between nodes--SDLC and LLC2.
·
Physical control--Connects nodes physically and electrically.
When you compare the SNA and OSI models as described in Table 12-2, they might look
almost the same. But there are several differences between TCP/IP and SNA networks:
·
SNA does not have any feature equivalent to OSI's X.25 packet-level acknowledgment.
·
SNA does use the concept of an X.25 permanent or virtual circuit.
·
SNA headers differ from OSI headers.
·
SNA flow control differs from OSI flow control.
·
SNA command structures differ from OSI command structures.
·
TCP/IP networks are an unreliable, connectionless packet delivery system. SNA networks
are connection-oriented and provide reliable data transfer.
·
TCP/IP networks are peer-to-peer networks with no one particular device in control of the
network. Traditional SNA networks are hierarchical, with the mainframe being in control
of the network. The mainframe and the software within it (VTAM) control all the devices
in the network.
SNA Stack
OSI Model
3. Path control layer
3. Network layer
2. Data-link control layer
2. Data link layer
1. Physical control layer
1. Physical layer
Table 12-2
The SNA Model (Continued)
87200333.book Page 435 Wednesday, August 22, 2001 2:53 PM