SNAX/XF and SNAX/APN Configuration and Management Manual
Configuring for Token-Ring Support
SNAX/XF and SNAX/APN Configuration and Management Manual—425836-006
9-8
Data Rates and Data Throughput
a NonStop system have the priority setting of zero. There is no current method that
allows an individual application to change this priority level.
The early token release feature, incorporated in the IEEE 802.5 standard, enhances
the original token-ring architecture by allowing multiple frames on a ring at the same
time. The 16-Mbps implementation allows a station to transmit a token immediately
after sending a frame, without waiting for the frame to return and be stripped from the
network. Ring efficiency can be enhanced by allowing stations to place more data on
the network. The HP implementation supports early token release.
Data Rates and Data Throughput
There are two basic speeds for the network: 4 Mbps or approximately 500,000 bytes
per second, and 16 Mbps or approximately 2,000,000 bytes per second. This does not
include overhead such as management traffic, so user application throughput will be
much lower than these rated figures. Stations operating at different speeds must not be
attached to the same Token-Ring.
At 4 Mbps, most frames take much longer to transmit onto the ring than they do to
travel around the ring. For a typical ring of 50 stations, the round-trip time might be 50
microseconds. A 1000-byte frame (8000 bits) takes 2000 microseconds to transmit at 4
Mbps, so the transmitting station will be removing the beginning of the frame that has
already made the trip round the ring long before the station completes sending out the
rest of the frame. At 16 Mbps, and with a larger number of stations or smaller frames,
this is less likely, and so the early release token can be used to maximize traffic
performance on the ring.
A 4-Mbps Token-Ring can support frame sizes up to 4,500 bytes, and a 16-Mbps
Token-Ring supports frame sizes up to 18,000 bytes. This large frame size means that
Token-Ring is extremely well suited for bulk data transfer (see the first usage scenario
at the beginning of this section). By comparison, Ethernet at 10 Mbps can support only
1,500 bytes maximum frame size. A 16,000-byte message carried in a single 16-Mbps
Token-Ring frame would require 11 frames to be transmitted on Ethernet.
The IEEE Specifications
The IEEE 802 committee has created a set of LAN standards and protocols for the
physical and data link layers of the OSI model (see Figure 9-6
on page 9-10). The data
link layer has been subdivided into two sublayers: the medium access control (MAC)
layer and the logical link control (LLC) layer. There are three MAC technologies, with
associated physical media below them:
802.3—a bus topology using a carrier sense multiple access/collision detect
(CSMA/CD) access method
802.4—a bus topology using a token-passing access method
802.5—a ring topology using a token-passing access method