Multiplex communications – Communication techniques for information carried in plural... – Assembly or disassembly of messages having address headers
Reexamination Certificate
1999-02-10
2001-04-24
Nguyen, Chau (Department: 2733)
Multiplex communications
Communication techniques for information carried in plural...
Assembly or disassembly of messages having address headers
C370S395430, C370S469000
Reexamination Certificate
active
06222858
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to data transmission systems for transmitting various types of data, such as voice, video, and computer data. More particularly, the invention relates to inverse multiplexing for Asynchronous Transfer Mode (“ATM”) over communication links with different transmission rates and/or delays.
2. Description of the Related Art
Asynchronous Transfer Mode (ATM) is a high-speed connection oriented switching and multiplexing communication scheme that allows for high speed telecommunications. ATM is essentially a packet switched communication scheme which utilizes fixed length packets or cells. ATM utilizes fixed size cells that are 53 octets long, with 5 bytes being header information and 48 bytes being payload information. The 48 byte cell payload may contain up to four bytes of information for the ATM adaptation layer, leaving at least 44 bytes for user data.
The term “asynchronous transfer mode” (ATM) was coined to contrast with “synchronous transfer mode” (STM). ATM is based on a time slotted transmission scheme in which data from different applications are multiplexed in accordance with their particular bandwidth, delay, and loss requirements. In ATM each time slot carries exactly one ATM cell. STM is also time slotted, however, in contrast to ATM, time is divided into a fixed number of slots which are grouped together to form a frame which repeats in time. All the time slots that are located at the same relative position in each frame can be grouped to form a circuit consisting of a fixed number of time slots and a fixed bandwidth. STM is inefficient in that the bandwidth associated with each circuit is dedicated full-time to each particular user, regardless of whether the user has data which needs to be transmitted.
ATM addresses many of the deficiencies found in STM communication. ATM networks enable a wide variety of communication devices to share common carrier communication links on a demand driven, as needed basis. The carriers used in ATM typically include relatively slow speed metallic wire links, such as the T1 carrier in North America (1.544 Mbps) or the E1 carrier in Europe (2.048 Mbps). The carriers used for ATM may also include higher speed optical links, such as SDH/SONET OC-3 (155.52 Mbps) and OC-12 (622.08 Mbps). ATM networks utilize statistical multiplexing to provide bandwidth on an as needed basis to individual users. This obviates the need for each user to have a dedicated, wideband communication channel for occasional communication. Instead, wideband communication is a shared resource which may be allocated on demand. Of course, if a number of users require wideband communication all at the same time, the capacity of the network may be momentarily exceeded, resulting in lower performance. To protect a link from overload, the ATM network and the user agree on a description of the user's traffic (“traffic contract”) which the network uses to manage and allocate network resources (i.e., link bandwidth and buffer occupancy), as well as to monitor the user's traffic for compliance with the agreement.
The ATM header information identifies the Virtual Path (Virtual Path Identifier or VPI), Virtual Channel (Virtual Channel Identifier or VCI), payload type, and cell loss priority. The VPI and VCI together from a Virtual Circuit. The ATM header also provides flow control and header error control. All the cells of a Virtual Circuit (VPI/VCI) follow the same path through the network, which is determined during call set-up procedures or by assignment. The different users of the ATM network provide their cells to the ATM network interface where they are queued for cell assignment and transmission. Cell transmission in an ATM network is causal, i.e., the cells in a connection (cells with the same VPI/VCI) arrive in order at the destination or far end. This is because the cells travel over the same Virtual Circuit.
An ATM network can support different types of services, such as loss sensitive/delay sensitive, loss insensitive/delay sensitive, loss sensitive/delay insensitive and loss insensitive/delay insensitive. The required QoS (Quality of Service) is determined during call set-up.
T1 carriers are typically a cost effective way of user access to an ATM network, as well as connection between ATM network switches. However, with the proliferation of increased data transfer and transmission requirements, the need for transmission bandwidth greater than that of a T1 carrier is needed in many situations. T3 carriers may be used in such situations; however, the use of T3 carriers is disadvantageous in that their cost is still somewhat prohibitive. Also, the use of T3 carriers as dedicated communication links is inefficient in that oftentimes they are under-utilized in relation to their data transmission capabilities.
ATM inverse multiplexers (IMA) have been proposed which combine several communication lines, e.g., T1 carriers, into a higher bandwidth aggregate communication path. See, for example, U.S. Pat. Nos. 5,608,733 and 5,617,417 and the ATM Forum, “Inverse Multiplexing for ATM (IMA) Specification”, Version 1.0, July, 1997, the contents of which are incorporated herein by reference. ATM inverse multiplexing provides a modular bandwidth for user access to ATM networks and for connection between ATM network elements at rates between the traditional communication rates, for example, between the T1/E1 and T3/E3 rates. T3/E3 links may not be generally available throughout a network, and thus, ATM inverse multiplexing provides an effective method combining several T1/E1 links to collectively provide higher intermediate rates.
The general concept of ATM inverse multiplexing is shown in FIG.
1
. In the transmit direction, an ATM cell stream
100
is received from the ATM layer and distributed on a cell by cell basis by the ATM inverse multiplexer
102
to a number of physical links
103
,
104
, and
105
which collectively make up Virtual Link
106
. At the far end, a receiving ATM inverse multiplexer recombines the cells from each link, on a cell by cell basis, recreating the original ATM cell stream
110
which is then passed onto the ATM layer.
The transmit ATM inverse multiplexer periodically transmits special cells that contain information which permits reconstruction of the ATM cell stream at the receiving end. These cells, referred to as IMA Control Protocol (ICP) cells, also define an IMA frame. The transmitter aligns the transmission of IMA frames on all links to allow the receiver to adjust for differential link delays among the individual physical links. In this manner, the receiver can detect and adjust for differential delays by measuring the arrival times of the IMA frames on each link.
At the transmitting end, cells are transmitted continuously. If there are no ATM layer cells to be transmitted within a given IMA frame, then the ATM inverse multiplexer transmits Filler Cells to maintain a continuous stream of cells at the physical layer. The Filler Cells are discarded at the receiving end. A new physical layer OAM (operation administration and maintenance) cell is defined for use in ATM inverse multiplexing and includes codes which indicate whether a cell is an ICP or Filler Cell. The individual cell sequence for IMA framing for the case of three physical links is shown in FIG.
2
.
As shown in
FIG. 2
, the transmitter creates an IMA frame
158
on physical links
103
,
104
, and
105
by periodically transmitting an ICP cell
150
on each link. Although the ICP cell defines an IMA frame, the ICP cell
150
may be located anywhere within the IMA frame
158
. The position of the ICP cell
150
relative to the beginning of the frame (frame boundary) is referred to as the ICP cell offset. The use of ICP cell offsets is used to reduce cell delay variation (CDV) caused by the insertion of the frame marker, i.e., the ICP cell
150
itself. The ICP cell offset is conveyed as protocol information in each link's ICP cell. For the IMA framing shown in
FIG. 2
, link
103
has
Nguyen Chau
Suchyta Leonard Charles
Trinh D.
Verizon Laboratories Inc.
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