Communications Server

Network Administration Guide


Appendix A. Planning for Communications Server

This appendix provides the information you need to know for planning your Communications Server environment.

Compatible Hardware

This section describes the IBM hardware that supports Communications Server. These descriptions include information on:

Communication Adapters

For a list of communication adapters supported by Communications Server, see the following URL on the Internet:


Communications Server supports the following types of modems:

X.25 users will require either a synchronous modem and CSU/DSU units that support the V.24bis command set or a modem compatible with the Hayes AutoSync protocol. Refer to your network supplier for the recommended connection type and modem.

For a list of modems supported by Communications Server, see the following URL on the Internet:

Session-Level Encryption Adapters

To use session-level encryption, you must have the Programmed Cryptographic Facility program and one of the following adapters:

Note:If you use an adapter other than the IBM 4758, you will need to use cryptographic API verbs instead. Refer to the Communications Server Programming Guide and Reference for more information.

Emulator Software

For a list of emulator software supported by Communications Server, see the following URL on the Internet:

Data Compression Considerations

Prior to using SNA data compression several configuration steps have to be worked out on the involved systems. SNA data compression is supported on:

When compression is done, each RU is compressed independently and a compression header is added to the RU. The TH and RH are not compressed. No compression is done when the RU is smaller than 20 bytes, or greater than 64K bytes, or, in the case of RLE only, if the size of the compressed data, including the compression header, is greater than the original data.

Before you start to configure the appropriate features of Communications Server for data compression, ensure that your partner system is able and prepared to support data compression.

Planning the Host/VTAM Connection

Before you can request data compression on a session to an S/370 or S/390* host with 3270 emulation or LU 6.2 type communications, VTAM Version 3 Release 4.1 (VTAM V3R4.1) must be installed and configured to negotiate such requests.

Your host VTAM system programmer should supply you with the proper VTAM definitions required for data compression. Here is a summary list of the definitions required. Examples of these are given following the list.

This is the VTAM startup options parameter indicating the maximum compression level allowed for any session connected to this host. The syntax of this parameter is:

where n is the level of compression allowed for a request, ranging from 0 to 4. The recommended value is 4, which allows all levels of data compression to be used.

This is the VTAM MODEENT table entry in the LOGMODE definition that will be used by your LU. The syntax is:

where value stands for SYSTEM, REQUESTED or PROHIBITED. SYSTEM should be coded for all supported LU types.

This is the VTAM application (APPL) definition parameter that sets the requested compression level for outbound Request/Response Unit (RU) data. The syntax is:

where "n" is the requested data compression level for the PLU -> SLU direction of data flow, also called outbound data. For all LU type sessions supported by Communications Server, the value of "n" can be equal to 0, 1, or 2, representing the data compression level 0, level 1, and level 2. However, for LU types 0, 1, 2, and 3, Communications Server always reserves compression resources at level 2.

This is the VTAM application (APPL) definition parameter that sets the requested compression level for inbound Request/Response Unit (RU) data. The syntax is:

where "m" is the requested data compression level for the SLU -> PLU data flow, also called inbound data. The value for m depends on the LU type used for the session:

Note:VTAM always represents the primary LU (PLU) and Communications Server represents the secondary LU (SLU) for LU types 0, 1, 2 and 3.

Following are coded examples, which were extracted from the original VTAM listings, pointing out the usage and placement of the parameters discussed above. Pay special attention to the highlighted lines.

Figure 37. VTAM Startup Parameter. Compression level up to 4 is allowed in this VTAM.

SSCPID=20,HOSTSA=20,XNETALS=YES,                                       X
GWSSCP=YES,                                                            X
NODELST=NODES1,                                                        X
CMPVTAM=4,                                                             X
CONFIG=K0,SUPP=NOSUP,                                                  X
SSCPDYN=YES,SSCPORD=PRIORITY,                                          X
ASYDE=TERM,                                                            X
NOTRACE,TYPE=VTAM,IOINT=0,                                             X
NOTRACE,TYPE=SMS,ID=VTAMBUF,                                           X
PPOLOG=YES,                                                            X
NODETYPE=NN,                                                           X
CPCP=YES,                                                              X
CSALIMIT=0,                                                            X
NOTNSTAT,DYNLU=YES,                                                    X
IOBUF=(3500,256,3,,1,58),                                              X
LPBUF=(1100,,2,,1,4),                                                  X
LFBUF=(100,,,,1,1),                                                    X
CRPLBUF=(2400,,,,1,4),                                                 X

Figure 38. VTAM Logmode Table. The MODEENT macro configured with data compression requested.

* LOGMODE FOR COMPRESSION TEST - BASED ON D4C32XX3                     *
*                                                                      *
*              3274 MODEL 1C (REMOTE SNA)                      @OY02946*
*              PRIMARY SCREEN 24 X 80 (1920)                           *
*                                                                      *
D4C3COMP MODEENT LOGMODE=D4C3COMP,                                     *
                 FMPROF=X'03',                                         *
                 TSPROF=X'03',                                         *
                 PRIPROT=X'B1',                                        *
                 SECPROT=X'90',                                        *
                 COMPROT=X'3080',                                      *
                 RUSIZES=X'87F8',                                      *
                 PSERVIC=X'028000000000000000000300',                  *
                 APPNCOS=#CONNECT,                                     *

Figure 39. VTAM Application Statement. It controls the maximum supported levels for this session.

* VTAM APPL STATEMENTS FOR CICS/MVS* 3.3                            *
RAKAC001 APPL ACBNAME=RAKAC001,                                    *
              MODETAB=MTAPPC,                                      *
              EAS=20,                                              *
              SONSCIP=YES,                                         *
              AUTH=(ACQ,VPACE,PASS),                               *
              PARSESS=YES,                                         *
              VPACING=5,                                           *
              CMPAPPLI=1                                *

Figure 40. VTAM PU and LU Definitions. The LU RACC1102 uses the logmode D4C3COMP, which is in the AMODETAB modetable.

WTCC1102   PU   ADDR=13,

Planning for AS/400 Data Compression

When connected to an AS/400 system, you need OS/400 V2R3 to be able to use data compression. All that is really required is to ensure that you have a MODE definition that specifies compression support. Three parameters are required on the "Create Mode Description" profile obtained through either the CRTMODD command or by going through the menus. The three parameters are:

This parameter controls the use of data compression. Valid parameters are shown below:

Use the system-wide network value for this parameter.

No data compression allowed.

If the remote system requests data compression, the local system will allow the session to be established.

The local system requests data compression.

The use of data compression is required. If one of the systems is not able to perform data compression at the requested level, the session will not be established.

Whenever data compression is allowed and requested, both systems negotiate the level of data compression to be used on that session according to settings of the INDTACPR and OUTDTACPR parameters.

This value represents the maximum level of compression used for inbound data.

This value represents the maximum level of compression used for outbound data.

For LU 6.2 sessions, data compression levels 0, 1, and 2 are supported in both directions, that is, for both outbound and inbound data flow.

For information about enabling Communications Server for data compression, refer to Quick Beginnings.

Performance Considerations

The performance of computer communications in general, and of Communications Server in particular, is subject to a large number of variables. The intent of the following information is to introduce some of the factors that influence the performance of communications in a Communications Server environment.

Communications performance is affected by the speed of the communications line. This speed is usually expressed as the number of bits per second (bps) that can be transmitted over the line. Typically, the higher the bps, the greater the communications performance that can be achieved on the line.

In addition to the bit rate, several other line-related factors can affect communications performance. For instance, some lines can only carry data in one direction at a time. To change the direction of communications (who can send), the station raises a request to send (RTS) and waits for clear to send (CTS) to be issued. This requires additional time for this line turnaround (which is modem dependent). Another factor is the transmission quality of the communications line. When a line error occurs, most protocols will detect this and resend the data. A line that has fewer transmission errors will give greater throughput.

Greater throughput can also be achieved if the protocols operate in a duplex data transmission mode, where the protocol can receive and transmit simultaneously. LAN and X.25 are duplex protocols. SDLC can operate in a duplex or half-duplex mode.

To use duplex for SDLC, you should use an appropriate adapter (the MPA adapter cannot support duplex). The remote station that can also support duplex.

As line speed increases, the bit-rate capacity of the line becomes less of a constraint in performance. Instead, the constraint becomes related to delays inside the devices that are using the communication line (modems, control units, or computer internal constraints). At very high speeds, these delays can impact communication performance more than the bit rate capacity of the line.

Analyzing these situations takes some effort. For example, longer than expected communication delays can be caused by any of the following:

In a high-speed communication environment, such as a LAN, the bit rate of the communication line is less significant because the communication capacity is largely determined by the speed of the communication devices. Better communication performance can often be achieved by increasing the processing power of the workstation, host, or other network components. This does not mean that the bit rate of a LAN network is never a constraint; rather, it might require several computers running at their communication capacity before the performance of the LAN significantly degrades.

To summarize, in many cases the performance of Communications Server is largely determined by factors that are related to the communication network being used. Providing communications performance at a reasonable cost involves both an understanding of the network as a whole and the analysis of its individual components.

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