KEY SNA FEATURES

The key features of SNA include SDLC, the system services control point (SSCP), layering, network-addressable units (NAUs), and sessions. These features are discussed in the following paragraphs.

SOLe

The purpose of protocols is to open, manage, and close the data transmission process between network hardware components. SDLC, SNA's common com-munications protocol, is generally viewed as a single unifying protocol for SNA. In reality, it has several variations that differ in the commands and responses allowed between network hardware components. These essential differences allow SDLC to be used with different communications media (e.g., half-duplex point-to-point channels, multipoint full-duplex channels, looped channels).

SDLC differs from pre-SNA synchronous protocols because it is bit rather than character synchronous. SDLC commands are consequently a series of bit patterns that have no intrinsic meaning as members of a specific character set (e.g., ASCII or EBCDIC). The use of such commands makes SDLC more efficient by reducing transmission overnead. BSC commands, for example, require several 8-bit characters, while SDLC commands require a single 8-bit

Number~--~----~~~---~--~--~--~

of Bits

Figure 6-2. SDLC Frame

Bit Stream

Data Transmitted I 1 I 1 I 0

I

Figure 6-3. Invert-on-Zero Transmission Coding

field. Data transfer uses a single-frame format (see Figure 6-2). Synchronous identification of the sequential bits within each SDLC frame is ensured by the unique flags (a bit sequence of a zero, six ones, and a zero: 01111110) and invert-on-zero encoding of data transmitted in the frame (see Figure 6-3). The unique flag patterns ensure that a data stream will never contain all ones (and thus no state transitions in the bit stream), and synchronization between modem and terminal is maintained.

SDLC is transparent to the code structure of the data being transmitted over the network. Data transparency is ensured by the unique flag pattern and the information sequencing.in the SDLC frame. Once an SDLC device begins transmission of a frame by sending a flag pattern, it monitors the bit stream for a sequence of five ones and inserts a zero after the fifth one. (The receiving device removes these zeros.) Because the flags are unique and occur only at the beginning and end of each frame, the receiving device can tell which informa-tion is in the address, command, error checking, and informainforma-tion fields by the position relative to the flag.

SDLe's most significant shortcoming is that it permits only seven frames to be unacknowledged by the receiver on a communications link. This is sufficient for most mainframe-to-terminal communications. When transmission occurs over satellite channels, however, with 3OO-millisecond one-way transmission delay and speeds of more than 9,600 bits per second, allowing only seven outstanding frames can severely reduce transmission efficiency and thereby increase transmission delays and response time.

SSCP

The system services control point (SSCP) is the SNA network manager for a single SNA domain. SSCP checks physical resources to ensure that they are active whenever their corresponding logical resources are active, coordinates communications between network elements, and retains error performance

data. In complex multidomain SNA networks, each domain has its own SSCP, which communicates with the others, managing the network cooperatively.

Because SSCP is resident in the access method in the mainframe, communica-tions funccommunica-tions have not been distributed to any great degree, and SNA net-works remain highly host dependent.

Layers

SNA groups related services into layers; each layer interacts with its adja-cent layers in the SNA network. Originally, three major layers-the communi-cations system layer, the transmission subsystem layer, and the common network layer-and several sublayers were identified. SNA layers have been further defined to include network-addressable unit services, function manage-ment data services, and data flow control layers within the communications system layer; transmission control within the transmission subsystem layer;

and path control and data link control layers within the common network layer.

The layered approach is advantageous in allowing new features to be added by making changes in a single layer, without affecting hardware and software in other layers. (For example, MSNF was developed using changes to the path control layer and the SSCP.) In reality, of course, the SNA layers are concep-tual entities, and their boundaries are not rigid. For example, parameters for function management data services and data flow control can be passed to the transmission control layer for inclusion in the request header that the latter appends to the data being transmitted. In addition, the telecommunications access-method software contains elements from the communications system layer and the transmission subsystem layer.

NAUs

Network-addressable units (NAUs) represent logical units, physical units, and SSCPs. Data transfers occur between these units, and each NAU has a unique network name and address assigned. The NAU separates the physical network elements from the logical network design of SNA, removing commu-nications functions from application programs and isolating them in the hard-ware and softhard-ware. The problem, however, is that NAUs must be defined several times when an SNA network is constructed: to the communications access method, the network control program, and frequently to the terminal controllers.

Sessions

Data transfer through an SNA network occurs in sessions (a session is a logical and physical path through the network connecting two NAUs, through which large amounts of data can be exchanged). Once a session is activated, its physical path is fixed. If deactivated and then reactivated, however, the new session might use a different path. If two application programs were to use the same terminal, two separate and distinct sessions would be required between

the tenninal and each program, and the sessions could not be active simultane-ously. An application program can have many sessions simultaneously active with separate tenninals under SNA. Session activation and deactivation are controlled by the SSCP.

Sessions, however, present a problem in that they can be disrupted by equipment malfunctions, requiring complete reestablishment before the tele-processing can be completed. In contrast, a datagram approach-in which all information required to control and route the information through the network is in a self-contained packet-would make such malfunctions less disruptive.

(Most current architectures communicate by using sessions.)