
IBM32NPCXX1EPABBE66
IBM Processor for Network Resources
Preliminary
System Environment
Page 4 of 706
pnr261_1intro.fm.06
August 14, 2000
1.7 System Environment
The dataflow context of an ATM subsystem is shown in Figure 2. The purpose of the communications sub-
system of any digital device is to allow the application to share data and to arbitrate the flow of control with
other devices.
Data, in the form of application objects or control structures, are divided into communication frames at the
communication stack interface. The stack may further partition the frames to fit reliability, efficiency, latency,
and protocol requirements.
In most cases, the communication stack encapsulates the data frame with protocol headers and/or trailers.
These header blocks are often located in memory in areas apart from the data frames. A device driver is often
given the task of moving this scattered memory to the actual transmission device. Scatter DMA is often used
to make this operation efficient.
In the case of the IBM Processor for Network Resources, the data can be DMAed into virtually contiguous
buffers connected to and controlled by the PNR. It is also possible to write the frame headers directly from the
processor to PNR memory. The fully assembled frame is enqueued for transmission over a particular logical
channel. (See more on the richness of logical channels in ATM and the PNR in
section 7. Application Notes:
Data Structures
on page 673).
The logical channels with pending work are serviced by the ATM Segmentation Layer which breaks the
enqueued data into 48-byte chunks (depending on the ATM Adaptation Layer (AAL)) and prefixes it with a
five-byte header (yielding a 53-byte ATM cell) in preparation for transmission.
A Transmission Convergence (TC) sublayer appropriate for the Physical Layer (PHY) and Physical Media
Dependent (PMD) connection is then exercised, making ATM cells suitable for transmission.
The receiving process is the reverse of the transmission process, except that the scheduling performed dur-
ing transmission is replaced by an identification-demultiplexing step during the reception of cells.
Figure 2: System Context of an ATM Subsystem
Object Data
Object Data
Application
Data Frames
Frame Headers
and Data
Packet Memory
Local
Channels
LAN Emulation
TCP/IP Mapping
Communication Stack
Device Driver
ATM
Cells
Segmentation
Scheduling
Physical Layer (TC)
PMD / Transceiver
Control Memory