Multiplex communications – Channel assignment techniques – Carrier sense multiple access
Utility Patent
1998-01-07
2001-01-02
Pham, Chi H. (Department: 2731)
Multiplex communications
Channel assignment techniques
Carrier sense multiple access
C370S229000, C370S462000, C709S235000
Utility Patent
active
06169744
ABSTRACT:
FIELD OF INVENTION
The present invention relates to communications in computer networks. More specifically, it relates to a method and protocol for a Medium Access Control layer for computer networks with multiple priority traffic classes.
BACKGROUND OF THE INVENTION
Local Area Networks (“LAN”) operate using a variety of data link layer protocols. As is known in the art, the Open Systems Interconnection (“OSI”) model is used to describe computer networks. The OSI model consists of seven layers including, from lowest-to-highest, a physical, data link, network, transport, session, application and presentation layer. The physical layer transmits bits over a communication link. The data link layer establishes and maintains point-to-point connections for the transmission of data frames.
A widely used scheme for managing a LAN is Carrier Sense Multiple Access with Collision Detection (“CSMA/CD”). CSMA/CD is organized around layered protocols that include a data link layer with a Logical Link Control (“LLC”) sub-layer and a Medium Access Control (“MAC”) sub-layer. The LLC sub-layer handles logical links between stations while the MAC controls access to a transmission medium via a physical layer. A CSMA/CD standard for Ethernet LANs is defined by Institute of Electrical and Electronic Engineers (“IEEE”) 802.3 standard.
The CSMA/CD structure is a peer-to-peer network structure and stations on the network are vying for use of a channel when they have data to transmit. The contention can result in collisions that distort the transmission signals. Distorted collision signals cannot be used by stations on the network. A “collision window” is typically used to handle CSMA/CD collisions. A collision window describes the length of time required for a signal to propagate through the channel and to be detected by each station in the network.
Token bus LANs use a bus topology and provide access to a channel as if it were a ring. The token bus protocol is a collision-free protocol that uses a special frame called a “token” to govern which network station is allowed to send data. Capacity can be allocated through the use of priority classes. IEEE 802.4 defines a token bus LAN standard.
Token ring LANs connect participating stations in a ring topology; each station is connected to medium via a ring interface unit. The ring interface unit is responsible for monitoring data passing through it as well as regenerating a signal and passing it to the next station. Token ring LANs use a token to determine which station may transmit, and also include a priority scheme. The object of the priority scheme is to give each station an opportunity to reserve the use of the ring for the next transmission around the ring. IEEE 802.5 defines a token ring LAN standard.
The CSMA/CD Ethernet LANs, token bus LANs and token ring LANs and other 802.x standards use LLC and MAC in the data link layer. However, there are several problems associated with using CSMA/CD Ethernet LANs, token bus LANs and token ring LANs that use LLC and MAC in their data link layer.
CSMA/CD protocols with LLC and MAC work reasonably well with large numbers of stations, but do not easily or efficiently support multiple priority classes or priority schemes. Token bus and token ring protocols with LLC and MAC are typically more complex than CSMA/CD protocols. Token bus and ring related protocols with LLC and MAC in their data link layer can support multiple priority classes or priority schemes but have poor latency characteristics as the number of network stations grow.
It is desirable, especially in the area of real-time services (e.g., teleconferencing), to have a MAC layer protocol which is simple, efficient, fair and distributed that can be used on a variety of computer network types (e.g., CSMA/CD, token bus and token ring). The MAC layer protocol should allow for a large number of network stations with differing priority class levels and delay requirements.
SUMMARY OF THE INVENTION
In accordance with an illustrative embodiment of the present invention, the problems associated with MAC layer protocols are overcome. A method for contention resolution and a new MAC layer protocol is provided. The new MAC layer protocol is used in networks with physical layers that employ frequency modulation schemes. Frequency domain information from the physical layer is used to quickly resolve contention during a very short interval that precedes all transmissions.
The new MAC layer protocol is based on three principals: 1) contention is definitively resolved before data transmission proceeds; 2) use of the frequency domain of a physical layer allows all contending stations to simultaneously broadcast their intent to transmit; and 3) partitioning of the utilized frequency bandwidth provides a simple process for introducing new priority levels into the contention resolution process.
The new MAC layer protocol includes: (1) a dead-time-silence-interval, which is used to begin an open-contention-interval; (2) an open-contention-interval, to allow all stations with data to transmit to contend for a access to the physical medium; (3) an intent-to-transmit-signal, which is a single signal frequency from a set of frequencies available on a selected physical layer; (4) combined-contention-signal, which is a simultaneous superposition of multiple intent-to-transmit-signal frequencies and is used to resolve contentions; (5) a restricted-contention-interval, which is used only for stations colliding in a previous contention-open-interval; and (6) a restricted-contention-delay-interval, which is used to begin a restricted-contention-interval and is shorter than a dead-time-silence-interval. The intent-to-transmit-signal can also be used with a set of partitions created from the set of frequencies available on the selected physical layer. The set of partitions allows use of multiple priority classes among contending stations. However, more or fewer protocol elements could also be used in the new MAC layer protocol.
The method includes broadcasting a first signal called an intent-to-transmit-signal from a first station to all stations active in the network during an open-contention-interval. The first signal is a frequency signal selected independently and at random from a predetermined set of frequencies used by a physical layer in the network. A second signal called a combined-contention-signal is received from active stations on the network. The first and second signals are temporally coincident (i.e., simultaneous). The second signal is decoded by each station and the randomly selected frequencies present in the combined signal are determined. A station determines that it has gained access to the transmission medium based on a comparsion of the frequencies present in the combined-contention-signal with its own selected frequency. A station that gains access is allowed to transmit data until the transmission is complete, without interference from other network stations. If two of more stations determine that they have gained access to the transmission medium, then a collision will occur. In this case, a restricted contention interval will be used to resolve contention only between the colliding stations. This process is identical to the open contention process except that only colliding stations may participate, and restricted contention is preceeded by a restricted-contention-only interval.
The method and new MAC layer protocol provides fast contention resolution for a large number of network stations with differing priority class levels and delay requirements. The method and new MAC layer protocol is simple, efficient and can be used on a variety of computer networks that use wideband modulation in the physical layer.
The foregoing and other features and advantages of an illustrative embodiment of the present invention will be more readily apparent from the following detailed description, which proceeds with references to the accompanying drawings.
REFERENCES:
patent: 4667323 (1987-05-01), Engdahl et al.
patent: 4943980 (1990-07-01), Dobson et al.
patent: 5128964 (1
Grabelsky David
Sidhu Ikhlaq
3Com Corporation
Lesavich Stephen
McDonnell & Boehnen Hulbert & Berghoff
Pham Chi H.
Tran Maikhanh
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