Browse Prior Art Database

Fiber Distributed Data Network Jitter Compensation

IP.com Disclosure Number: IPCOM000106322D
Original Publication Date: 1993-Oct-01
Included in the Prior Art Database: 2005-Mar-20
Document File: 2 page(s) / 119K

Publishing Venue

IBM

Related People

Tracol, A: AUTHOR [+2]

Abstract

The FDDI - Fiber Distributed Data Network - is a recent LAN standard which has been designed for improving the LANs in terms of thruput (100 Mbps media), reliability and availability (dual fiber ring architecture) and fairness (timed token protocol).

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This is the abbreviated version, containing approximately 52% of the total text.

Fiber Distributed Data Network Jitter Compensation

      The FDDI - Fiber Distributed Data Network - is a recent LAN
standard which has been designed for improving the LANs in terms of
thruput (100 Mbps media), reliability and availability (dual fiber
ring architecture) and fairness (timed token protocol).

      The FDDI standard has not been initially designed as a
'Multimedia' LAN, but rather as an improvement of previous packet
traffic LANs (e.g, Ethernet and Token Ring).  The plain Multimedia
LAN is supposed to be implemented through the FDDI-2 standard.
FDDI-2 is a significant evolu tion of FDDI based on a TDM access of
the dual ring, with dynamic share of bandwidth between packet traffic
(FDDI-like) and isochronous traffic carried in multiple B channels (N
x 64 kbps).

      The problem addressed by the present publication is to support
multimedia traffic on an FDDI LAN, and not rely on an FDDI-2 LAN.

The key motivations for avoiding FDDI-2 solution are:

FDDI-2 is not yet a stable standard.

FDDI-2 is not perceived as a future widely adopted LAN.

ATM is a serious competitor.

When implemented, FDDI-2 is anyway a costly solution when compared to
FDDI.

      The particular problem to solve in this objective is related to
the jitter parameter.  While there is no real concern about the
available throughput (100 Mbps to share between a reasonable number
of multimedia stations leaves room for significant services), there
is a basic concern with the latency which should be as constant as
possible so as to mini mize the size of the play-out buffers - and
then minimize the end-to-end transport delay.  Indeed, because FDDI
is a packet based LAN, the data transfers are bursty by nature.
Then, the receive adapter must store the incoming traffic in a buffer
('play-out buffer'), and read data at con stant rate from the
play-out buffer to deliver it as a constant bit rate (CBR) stream to
the appropriate device, e.g., speaker.  The read opera tion can begin
only when the play-out buffer has received a certain amount of packet
data ('threshold'), so that variation of arrival rate (jitter) can be
damped by the buffer whose filling level will oscillate around its
threshold.

      It is then clear that any minimization of the burstiness of the
incoming flow will allow the use of a smaller play-out buffer and the
setting of a lower threshold, which in turn will lead to a smaller
end- to-end transport delay.

      Similarly, on the transmit side, there is also an advantage to
provide a non-bursty traffic on the ring, in order to minimize the
size of the packet buffer located between the traffic source, e.g.,
video codec and the FDDI ring interface.

      The TRT (Token Rotation Time) variations are caused by the
dynamicity of the traffic on the ring.  If each station offers a
constant load, the TRTs are all equal and constant.  When the offered
load varies in a station, the token rotati...