Multiplex communications – Channel assignment techniques – Carrier sense multiple access
Reexamination Certificate
2001-08-16
2003-07-01
Patel, Ajit (Department: 2664)
Multiplex communications
Channel assignment techniques
Carrier sense multiple access
C375S222000
Reexamination Certificate
active
06587473
ABSTRACT:
BACKGROUND OF THE INVENTION
Computers and related devices are increasingly being connected into networks for communications between the devices. Typically, the networks comprise LANs (local area networks) which provide communications among devices within a relatively small geographical area different LANs being interconnected via MANs (metropolitan area networks) and WANs (wide area networks). This has resulted in a global computer information network which is generally known as the Internet. The term “Network” is used herein to refer generically to this global computer information network and to any other network of computers and related devices.
Different technologies can be used to facilitate communications on any LAN and throughout the Network, the most common being Carrier Sense Multiple Access with Collision Detection (CSMA/CD) technology. This is documented in IEEE Standard 802.3 entitled “Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications” which has been adopted by the International Organization for Standardization (ISO). The 802.3 Standard is based on the 1985 Version 2 Standard for Ethernet and, although there are some differences including different use of a length/type field, the two Standards are largely interchangeable and can be considered equivalent as far as this invention is concerned. The term “CSMA/CD” is used herein to refer generically to this technology. Using CSMA/CD, packets of data are communicated in frames that are generally referred to as Ethernet frames. This term is also used herein, regardless of whether the frames comply with the 802.3 Standard or the Ethernet Standard (i.e. regardless of the value contained in the length/type field of the frame).
The OSI (Open Systems Interconnection) reference model established by the ISO defines packetized communications protocols in seven layers, of which Layer 1 is the physical layer which is concerned with the physical interfaces between devices and the communications medium, and Layer 2 is the data link layer which is concerned with sending and receiving blocks of data together with information for example for synchronization and error and flow control. For LANs, the data link layer is generally considered as comprising two sub-layers, referred to as the LLC (logical link control) layer and the MAC (medium access control) layer. The LLC layer (Layer 2) is addressed by IEEE Standard 802.2. The CSMA/CD Standards address communications at the MAC and physical layers (Layers 2 and 1).
A particularly convenient and popular physical medium for LAN communications is twisted pair wiring as is commonly used for telephone communications. Such wiring typically consists of 0.4 mm to 0.6 mm diameter (26 AWG to 22 AWG) unshielded wires twisted together in pairs in a multipair cable. For example, one of the options for the physical layer documented for CSMA/CD is referred to as 10BASE-T and provides baseband communications at a data rate of 10 Mb/s over twisted pair wiring. The performance specifications are generally met by up to 100 m (meters) of 0.5 mm telephone twisted pair wire without the use of a repeater. Longer wiring lengths are permitted as long as the performance specifications, in particular a maximum delay, are met.
Accordingly, devices that are located relatively close to one another, for example within a building, can be relatively easily connected in a LAN using twisted pair wiring. For CSMA/CD communications via the LAN and for access to the Network, each device is easily equipped with an Ethernet interface card, which is connected via a respective twisted pair of wires to a repeater or CSMA/CD hub, and with TCP/IP (Transmission Control Protocol/Internet Protocol) software that handles the packetized communications at Layers 3 and 4 of the OSI model (Network and Transport Layers, respectively).
Increasingly, access to the Network is required from devices that are relatively distant from existing Network facilities. For example, such devices may be located within residences and small businesses, and they may be isolated computers or they may be connected in a LAN that is not connected to the rest of the Network. Such devices may for example comprise general-purpose computers or specific-purpose devices such as a Network browser, game machine, and/or entertainment device, and may also comprise related and/or ancillary equipment such as workstations, printers, scanners, bridges, routers, etc. that it may be desired to connect to the Network. The generic term “terminal device” and its abbreviation “TD” is used below to embrace all such devices.
It is known to provide for access to the Network from a relatively distant terminal device, or TD, via a communications path between a router on the Network and the distant TD, the communications path typically being constituted by a telephone line.
A simple form of such a communications path is a serial link comprising modem communications via a conventional two-wire telephone line. At Layers 1 and 2 of the OSI model the CSMA/CD communication, which can not be used on the serial link because of its length and characteristics, is replaced for the communication with each distant TD by modem communications via the respective telephone line and a point-to-point protocol, such as PPP (Point to Point Protocol) or SLIP (Serial Link Internet Protocol). Currently, modem communications generally provide a maximum data rate of 28.8 kb/s, and may typically operate in practice at lower, fall-back, data rates such as 19.2 or 14.4 kb/s. Such data rates are increasingly insufficient to meet demands imposed on communications for Network access, in particular for rapid downloading of relatively large amounts of data, e.g. for graphics. In addition, use of such modem communications prevents simultaneous use of the telephone line for telephone communications. Furthermore, such a communications path is set up as a dialled connection via the public switched telephone network (PSTN), which involves the inconvenience to the distant TD user of having to establish the dialled connection and the disadvantage of long connection times via the PSTN.
An alternative form of telephone communications path comprises an ISDN (Integrated Services Digital Network) telephone line. This provides two 64 kb/s B-channels each of which can be used for carrying voice communications or data A TD can be connected to the ISDN line via a terminal adapter, which can thereby provide a total bit rate of 128 kb/s for data on both B-channels, or 64 kb/s for data on one B-channel simultaneously with digital telephone voice communications on the other B-channel. While this provides a significant increase in data rate compared with using a conventional two-wire telephone line, it requires an ISDN telephone line which, in the relatively limited areas in which it is available, involves additional cost, and the connection still has the disadvantage of being a dialled connection via the telephone network. In addition, a terminal adapter is generally more costly than a modem. Furthermore, even data rates of 64 kb/s or 128 kb/s are likely to be increasingly insufficient with evolution of the Network.
Higher speed telecommunications lines may be available for lease to provide high data rate communications, but these are not economical for TDs in residences and most small businesses. Cable modems have also been proposed for providing Network access via coaxial (coax) or hybrid fiber-coax (HFC) cable television distribution networks that provide bidirectional communications. While such proposals offer the possibility of high data rates, they are also limited to their own serving areas and are likely to involve relatively high costs for both the modem equipment and the ongoing use of the service.
Accordingly, there is an increasing need to facilitate access from terminal devices to the Network at relatively low cost both for equipment and ongoing service, that is not restricted to particular areas, that provides for high data rates, and that desirably does not preempt
Richards Roger St. Patrick
Terry John Brian
Elastic Networks, Inc.
Patel Ajit
Thomas Kayden Horstemeyer & Risley
LandOfFree
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