Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via time channels
Reexamination Certificate
2000-03-10
2004-08-17
Pham, Chi (Department: 2663)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via time channels
C370S465000
Reexamination Certificate
active
06778561
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and/or architecture for hybrid data transportation generally and, more particularly, to sending a mix of different data types over a fiber optic network running SONET/SDH framing.
BACKGROUND OF THE INVENTION
Long distance and metropolitan area network (MAN) communications rely on short-haul and long haul fiber optic networks to transport data and telephony traffic. One conventional way to transmit data in fiber networks is through a Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH) protocol. In a SONET/SDH network, data travels in fixed size envelopes that repeat every 125 microseconds. With this synchronous fixed-length framing, every byte (e.g., 8 bits of data) inside a SONET/SDH frame represents a 64 Kbps (64000 bits/sec) channel. The 64 Kbps channel has the same rate as supported by current telephone channels (also called DS
0
channels).
SONET was designed to efficiently carry telephony Plesiochronous Digital Hierarchy (PDH) channels such as T1/T3. This was easily achieved by dividing the payload area in fixed slots called virtual tributaries (VT). These virtual tributaries are then grouped together to form higher-rate channels. These fixed slots are efficient for carrying fixed-bandwidth telephony channels because any one or more channels can be added or removed from a bundle without processing an entire payload of channels. Because SONET frames repeat at fixed intervals, these virtual tributaries have fixed locations and time intervals, and it is easy to extract T1/T3 or fractions of T1 without processing the entire SONET payload.
With growing volume in data traffic, however, SONET/SDH networks must now carry a significantly large number of data packets—such as ATM (Asynchronous Transfer Mode—53 bytes each) and IP (Internet Protocol—variable-size packets) in addition to traditional T1/T3 channels. The synchronous framing structure of SONET/SDH that is quite efficient for carrying T1/T3 channels is not able to carry both fixed-bandwidth and variable-bandwidth channels in an optimum way.
SONET/SDH has an inefficient utilization of fiber bandwidth for data packets. For data transport, some of the virtual tributaries that are created for transporting fixed-bandwidth T1 traffic while others are used for transporting packet data packets such as ATM and IP. Since an individual virtual tributary has a limited bandwidth, extra mechanisms have to be used for sending data packets of higher bandwidth using virtual tributaries.
In one technique, a 10 Mbps data packet channel, for example, is inverse-multiplexed into smaller bandwidth streams and then sent on many virtual tributaries. At the other end, these streams are integrated to reconstruct the full 10 Mbps channel. In another method, many of the virtual tributaries are concatenated using hardware to create a higher-bandwidth virtual tributary for transmitting the high-bandwidth data packet.
SONET/SDH lacks of support for data mixing. A SONET fiber link carrying frames containing ATM cells cannot carry POS, because ATM cells frequently carry QoS-sensitive data such as CES (Circuit Emulation Service) or multimedia traffic. Introduction of SONET frames containing POS will cause significant delays (e.g., 125 &mgr;S for each POS frame inserted in the link).
In each of these methods, a unique Path Signal Label (PSL) value in the POH (Path Over-Head) field of SONET frame identifies the type of data transmission inside the payload. The payload area is also referred to as SPE (Synchronous Payload Envelope). Because a PSL value identifies contents of entire SONET payload envelope, only one type of transmission can be supported at a time in a SONET frame.
One method for data transmission is to use the entire SONET SPE for data packets. The SONET payload area is filled with IP packets using Packet-over-SONET (POS) packets. POS packets are packets by 0×7E (Hexadecimal) at both ends of a packet, with a framing using PPP (Point-to-Point Protocol). Many packets can be put inside a single SONET SPE. This method can only support variable-length packet protocol such as IP. A SONET fiber containing these packets cannot transport T1/T3 channels or real-time streams using ATM cells. The reason for this limitation is that each SONET SPE containing IP packets, for example, introduces a delay of 125 microseconds. Such a delay is not acceptable for T1/T3 circuits or real-time streams using ATM cells.
Another method for data transmission is to use the entire SONET SPE for ATM cells. In this case, a SONET SPE is filled with ATM Cells. ATM cells are delimited by their fixed length, and are tracked by doing a hunt for their header checksum byte. Services such as T1, Frame Relay, Ethernet, etc. are transported over ATM using standard protocols. This requires complex implementations in hardware and incorporation of ATM service interworking at each service boundary.
Another method for data transmission is to use the virtual tributaries (VT) for data packets and ATM cells. In this method, a SONET SPE is partitioned in many fixed-bandwidth slots called virtual tributaries (VT). For data transport, some of these virtual tributaries may contain T1/T3 type of fixed-bandwidth traffic while others are used for transporting packet data packets such as ATM and IP.
Since an individual virtual tributary has a limited bandwidth, extra mechanisms have to be used for sending data packets of higher bandwidth using virtual tributaries. In one technique, a 10 Mbps data packet channel, for example, is inverse-multiplexed into smaller bandwidth streams and then sent on many virtual tributaries. At the other end, these streams are integrated to reconstruct the full 10 Mbps channel. In another method, many of the virtual tributaries are concatenated using hardware to create a higher-bandwidth virtual tributary for transmitting the high-bandwidth data packet.
Each of these methods uses a fixed-bandwidth channel or a set of channels for transmitting network data packets. In each method, bandwidth capacity of the fiber is poorly utilized since network data packets are bursty in nature and average bandwidth utilization is quite low.
Referring to
FIG. 1
, examples of various data types are shown. A set of time-division-multiplexed (TDM) packets
12
a
-
12
n
, a set of ATM packets
14
a
-
14
n
and a set of POS packets
16
a
-
16
n
are shown in connection with a SONET fiber line
18
. In a SONET network, only one type of data can be transferred at a time. The data is identified by a unique PSL (path signal label) byte value inside a Path Over-Head (POH) of the TDM packets
12
a
-
12
n
, ATM packets
14
a
-
14
n
, the POS packets
16
a
-
16
n
, of PDH traffic. The nodes at different points in the SONET fiber line
18
have different types of data to send on the network.
Referring to
FIG. 2
, various data transmissions are shown in a SONET ring
30
. Assume, for example, that a node A has ATM cells
14
a
-
14
n
to transmit on the SONET ring
30
. The SONET ring
30
forms a SONET synchronous payload envelope (SPE), sets the PSL value for ATM cells (in the POH) and sends the SONET frame down a link
32
a
-
32
n
. Even if the SPE is only partially full, the entire SPE frame size is transmitted. If a node B has IP packets to send, the node cannot use the partially filled SONET frame received from the node A to add POS packets, because the PSL value identifies only one type of data (ATM cells, in this example). The node must now wait 125 &mgr;S for the partially filled packet to transfer down the link
32
b
-
32
n
. Furthermore, the SONET ring
30
cannot handle T1 or T3 channels entirely in a payload space. The only way it can simultaneously support T1/T3 channels is by using virtual tributaries and by using some of the tributaries for non-T1/T3traffic.
Statistically, a significant percentage of network traffic comprises IP packets that are quite small in size. Because higher-speed concatenated SONET frames are large in size, many times a SONET SPE may not be completely filled with packets or cell
Cypress Semiconductor Corp.
George Keith M.
Maiorana P.C. Christopher P.
Pham Chi
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