622 Adapters

Performance Report on ATM adapters, Part No. A5483A, A5513A, and A5515A 09/30/99 9

Table 10 Server and Workstation platform characteristics

Platform CPU ICACHE/DCACHE Memory SPECint95 OS

N4000 PA8500 440 MHz 0.5 MB/1 MB 1 GB

34 11.00 May 99

V2500 PA8500 440 Mhz 0.5MB/1 MB 1 GB NA 11.00 Dec 98

V2250 PA8200 240 Mhz 2 MB/2 MB 1 GB 16.4 11.00 Dec 98

Protocol Overheads and Theoretical Maximum TCP Throughput

The Throughput as seen by a TCP application is less than the OC-3 physical layer throughput of 155

Mbps or OC-12 (622 Mbps). This is because of the various protocol headers that are inserted by the

protocol stack. For example, the protocol headers for Classical IP interface are as follows:

♦ TCP/IP Overhead

This Consists of 20 bytes of TCP header + 20 bytes of IP header + eight bytes of LLC header.

This amounts to 48 bytes

♦ ATM AAL5 Overhead

The Convergence Sublayer appends an 8-byte trailer. It will also add a Pad. The Pad size varies

from 0 –47 bytes, depending on Application Message size.

♦ ATM Layer Overhead

The ATM Layer will append a five byte header to each Segmentation and Reassembly PDU (48

bytes) to form an ATM cell.

♦ Physical Layer Overhead

The Transmission Convergence sublayer creates SONET frames of 27 ATM cells by appending

one overhead ATM cell to 26 data ATM cells.

This leads to the following equation for Theoretical TCP throughput (T)

T = (D / (D+ 56 + p)) * (48/53) * (26/27) * 155.52 Mbps (or 622 Mbps)

Where, D = application message size in bytes.

P = 0 –47 bytes padding added at AAL5 layer.

For Message sizes smaller than 4Kbytes, The TCP/IP and AAL5 layer protocol overhead forms a

major part and theoretical throughput reduces dramatically. For typical message sizes, the

theoretical TCP throughput is as follows:

8 GB for the Multiple adapter tests.