Flexible time division multiplexed bus using sonet formatting

Details Flexible time division multiplexed bus using sonet formatting Flexible time division multiplexed bus using sonet formatting

1999-07-30

2002-03-12

Kizou, Hassan (Department: 2662)

Multiplex communications

Pathfinding or routing

Through a circuit switch

C370S369000, C370S375000, C370S423000, C370S442000, C370S538000

Reexamination Certificate

active

06356550

ABSTRACT:

The present invention relates generally to a bus in a communication system, and more particularly to a time division multiplexed bus.
BACKGROUND OF THE INVENTION
Communications networks provide communications paths for voice and data using different protocols. In North America and other locations, a set of transmission signals referred to as the North American time division multiplexing hierarchy is used. This hierarchy includes DS1, DS2 and DS3 communications signals which are well-defined according to the following standards: ANSI T1.107, ANSI T1.403, Bellcore TR-TSY-000007, AT&T TR62411 and AT&T TR54016.
A DS1 signal has a transmission rate of 1.544 Mbps per second (Mbps). A DS2 signal includes four DS1 signals plus some overhead bits and has a transmission rate of 6.312 Mbps. A DS3 signal includes twenty-eight DS1 signals and has a transmission rate of 44.736 Mbps.
In other geographical areas, such as Europe, the time division multiplexing hierarchy described above is not used, but a European hierarchy with different transmission rates from that of the DS1, DS2 and DS3 is used. For example, an E1 signal with a rate of 2.048 Mbps that carries thirty channels is the lowest level of the multiplexing hierarchy, rather than a DS1 signal with a rate of 1.544 Mbps that carries twenty-four channels.
A standard called the synchronous optical network (SONET) protocol provides a common interface for transporting the different signaling hierarchies over an optical fiber. The SONET protocol can transport signals from both the North American and European hierarchies. The SONET standard defines a hierarchy of digital data rates, and is published in International Telecommunications Union (ITU) Recommendations G.707, G.708 and G.709. The SONET standards also include ANSI T1.105, ANSI T1.105.01, ANSI T1.105.02, ANSI T1.105.03, ANSI T1.105.03a, ANSI T1.105.03b, ANSI T1.105.04, ANSI T1.105.05, ANSI T1.105.06, ANSI T1.105.07, ANSI T1.105.07a and ANSI T1.105.09.
Each level of the SONET hierarchy is referred to as a synchronous transport signal (STS) level. The lowest level, STS-1, has a transmission rate of 51.84 Mbps. The STS-1 level can transport a single DS-3 signal or many lower rate signals, such as DS1 and DS2. Higher transmission rates are supported in the SONET hierarchy by combining multiple STS-1 signals into an STS-N signal. The SONET hierarchy ranges from the STS-1 level which has a transmission rate of 51.84 Mbps and a payload rate of 50.112 Mbps to an STS-48 level which has a transmission rate of 2,488.2 Mbps and a payload rate of 2,405.376 Mbps.
In
FIG. 1
, in a prior art system, a communications path is provided between an optical fiber
20
and facility lines
22
using a SONET transceiver
24
and SONET mappers
26
. The SONET transceiver
24
provides the optical to electrical interface between the fiber
20
and the SONET mappers
26
. In the SONET transceiver
24
, a SONET transmitter
32
is electrically connected to an add bus
34
from the SONET mappers
26
and a SONET receiver
35
is electrically connected to a drop bus
38
of the SONET mappers
26
.
The SONET mappers
26
transmit and receive facility signals on the facility lines
22
. In one embodiment, the facility signals are electrical signals having a predetermined transmission rate. To transmit facility signals, the SONET mappers
26
map the facility signals to a SONET signal having a predefined format. The predefined format includes timeslots that are associated with each facility signal. The SONET mappers
26
also receive a SONET signal and map the SONET signal to the facility signals, using the predefined format, for transmission over the facility lines
22
.
The add bus
34
and the drop bus
38
from each SONET mapper
26
are well-known Telecom bus interfaces. Mapping is performed according to ANSI standard T1.105 and Bellcore standard GR-253-CORE for T1.5 and International Telecommunications Union (ITU) G.709 for a synchronous mapping structure. Both the add bus
34
and the drop bus
38
of the SONET mappers
26
have an eight bit wide data path and use additional timing signals. Alternately, the add bus
34
and the drop bus
38
of the SONET mappers
26
have an nine bit wide data path that includes a parity bit and use additional timing signals.
As shown in
FIG. 2A
, the basic SONET building block is an STS-1 frame
40
which has a header
42
and a SONET payload envelope (SPE)
44
. The frame
40
has 810 bytes (octets) and, is transmitted once every 125 &mgr;sec. The frame
40
is typically viewed as a matrix having nine rows and ninety columns. Transmission is one row at a time from left to right and top to bottom. The header
42
includes overhead octets, and the SPE
44
carries data. The SPE
44
has eighty-seven columns of which forty-eight carry data and the remainder are overhead.
The SONET specification defines synchronous formats for the SPE for transmission rates below the STS-1 level. The STS-1 SPE
44
is subdivided into virtual tributaries in which each virtual tributary (VT) is associated with a signal having a predefined transmission rate.
FIGS. 2B
,
2
C,
2
D and
2
E show each type of VT and the number of rows and columns associated with that VT. As shown in
FIG. 2B
, to transport DS1 signal, VT 1 uses nine rows and three columns of the SPE. Table 1 below summarizes the virtual tributaries, their bit rate and size.
TABLE 1
Summary of Virtual Tributary
VT Type
Bit Rate
Size of VT
VT 1
1.728 Mbps
9 rows, 3 columns
VT 2
2.304 Mbps
9 rows, 4 columns
VT 3
3.456 Mbps
9 rows, 6 columns
VT 6
6.912 Mbps
9 rows, 12 columns
A VT 1 has sufficient bandwidth to transmit a DS1 signal, while a VT 6 has sufficient bandwidth to transmit a DS2 signal. A VT 3 is not the same as a DS3 and has a much lower bit rate than the DS3. To transmit a DS3 signal, the entire STS-1 SPE is dedicated to the one DS3.
FIG. 2F
shows four VT 1's, labeled A, B, C and D. Each VT 1 has the capacity to transmit a DS1 signal. The VT 1's are interleaved among themselves for transmission.
FIG. 2G
shows three VT 2's, labeled X, Y and Z. The VT 2's are interleaved among themselves for transmission.
FIG. 2H
shows two VT 3's, labeled M and N. The VT 3's are interleaved among themselves for transmission.
FIG. 2I
shows a VT 6, labeled O.
In
FIGS. 2J and 2K
, an exemplary format for an SPE
44
that transmits the four VT 1's, the three VT 2's, the two VT 3's and the one VT 6 is shown. A complex interleaving pattern associates each VT with particular columns or timeslots. The VT label associated with a timeslot is indicated in each column and the timeslot number of the SPE
44
is shown below each column. Some of the columns that do not have an associated label contain pointer values and are used to compensate for timing variations. The interleaving patterns are defined in the ANSI T1.105 specification.
Many communication systems, such as switches and private branch exchanges, use time-division multiplexed buses. In computer telephony integration, commonly used internal system buses include the Multi-Vendor Interface Protocol (MVIP) and H.100A buses. The MVIP and H.100A buses are targeted towards connections that use integral multiples of sixty-four kilobits per second (Kbps). MVIP and H.100A also impose a maximum transmission speed of 16 Mbps. Typical transmission speeds encountered in communications networks are 1.544 Mbps (DS1), 44.736 Mbps (DS3) and 51.84 Mbps (SONET STS-1). Of the three aforementioned rates, DS3 and STS-1 exceed the maximum transmission speed of MVIP and H.100A by a factor of over two, and only 51.84 Mbps is a multiple of sixty-four Kbps.
Therefore, a time-division multiplexed bus for use internally in a communications system that supports transmission rates that are not integral rates of 64 Kbps is needed. The bus should also be capable of handling transmission speeds exceeding 16 Mbps, including 44.736 Mbps and 51.84 Mbps.
Another disadvantage of the MVIP and H.100 buses is that the MVIP and H.100 buses have an absolute clocking constra

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