Multiplex communications – Channel assignment techniques – Carrier sense multiple access
Reexamination Certificate
1998-04-13
2002-11-26
Ngo, Ricky (Department: 2664)
Multiplex communications
Channel assignment techniques
Carrier sense multiple access
C370S217000, C370S257000
Reexamination Certificate
active
06487214
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to methods and apparatuses for implementing an Ethernet physical media using a digital bus that includes three wires.
2. Description of the Related Art
The growth of local-area networks (LANs) has been driven by the introduction of Ethernet Technology as well as the availability of powerful, affordable personal computers and workstations.
Ethernet
Ethernet is a Media Access Control (MAC) layer network communications protocol specified by the Institute of Electrical and Electronics Engineers (IEEE) in IEEE specification 802.3 (the “802.3 specification”). When the Ethernet protocol is used, network devices each listen on a network bus and transmit only when no other network device is transmitting. Occasionally, two nodes do transmit nearly simultaneously and a collision occurs. A network that that includes detection of such collisions is referred to as a Carrier Sense Multiple Access Collision Detection (CSMA/CD) network.
Early Ethernet applications used a coaxial cable that carried an analog signal for the bus. Collisions were detected by analyzing the analog signal on the line to determine when two network devices were transmitting. In one implementation, collisions were detected by measuring the energy on the bus. Later, Ethernet was implemented on a set of digital communication lines that included separate transmit and receive lines. In such systems, collisions are detected by each network device by noting when data appears on the receive line while the network device is transmitting. The separate transmit and receive digital lines for each device make it necessary to have a hub to connect each of the transmit lines of the network devices to all of the receive lines of other network devices. Ethernet hubs are also referred to as repeaters. An Ethernet repeater serves as a central station for connecting network devices included in an Ethernet network, hence the term “hub.” An Ethernet repeater receives messages from the transmission lines of network devices that are plugged into it and broadcasts (or “repeats”) the message to all of the devices on the network on their receive lines.
As the Ethernet standard has evolved, the basic CSMA/CD scheme has remained for the most part the same. In July 1993, a group of networking companies joined to form the Fast Ethernet Alliance. The charter of the group was to draft the 802.3u 100BaseT specification (“802.3u specification”) of the Institute of Electrical and Electronics Engineers (IEEE) and to accelerate market acceptance of Fast Ethernet technology. The final IEEE 802.3 specification was approved in June 1995. Among the other goals of the Fast Ethernet Alliance are: to maintain the Ethernet transmission protocol (CSMA/CD); to support popular cabling schemes; and to ensure that Fast Ethernet technology will not require changes to the upper-layer protocols and software that run on LAN workstations. For example, no changes are necessary to Simple Network Management Protocol (SNMP) management software or Management Information Bases (MIBs) in order to implement Fast Ethernet.
CSMA/CD
Carrier sense-collision detection is widely used in LANs. Many vendors use this technique with Ethernet and the IEEE 802.3 specification. A carrier sense LAN considers all stations as peers; the stations contend for the use of the channel on an equal basis. Before transmitting, the stations monitor the channel to determine if the channel is active (that is, if another station is sending data on the channel). If the channel is idle, any station with data to transmit can send its traffic onto the channel. If the channel is occupied, the stations must defer to the station using the channel.
FIG. 1A
depicts a carrier sense-collision detection LAN. Network devices
102
,
104
,
106
, and
108
are attached to a network bus
110
. Only one network device at a time is allowed to broadcast over the bus. If more than one device were to broadcast at the same time, the combination of signals on the bus would likely not be intelligible. For example, assume network devices
102
and
104
want to transmit traffic. Network device
108
, however, is currently using the channel, so network devices
102
and
104
must “listen” and defer to the signal from network device
108
, which is occupying the bus. When the bus goes idle, network devices
102
and
104
can then attempt to acquire the bus to broadcast their messages.
Because network device
108
's transmission requires time to propagate to other network devices, the other network devices might be unaware that network device
102
's signal is on the channel. In this situation, network device
102
or
104
could transmit its traffic even if network device
108
had already seized the channel after detecting that the channel was idle.
Each network device is capable of transmitting and listening to the channel simultaneously. When an analog communication line is used and two network device signals collide, they create voltage irregularities on the channel, which are sensed by the colliding network devices. The network devices then turn off their transmission and, through an individually randomized wait period, attempt to seize the channel again. Randomized waiting decreases the chances of another collision because it is unlikely that the competing network devices generate the same wait time. When digital lines are used, the network devices detect collisions by detecting data on the receive line at the same time as they are transmitting.
FIG. 1B
is a block diagram illustrating a topology used in a conventional analog Ethernet network. An analog bus line
100
connects network devices
112
,
114
, and
116
. Additional network devices may be added to the network by simply tapping into analog bus line
100
. Since each network device transmits and receives on the same line, there is no need to include a device to connect the transmit lines of one network device to the receive lines of another network device. Each network device listens on analog bus line
100
before transmitting to make sure that no other network device is already using the bus. When two network devices simultaneously or nearly simultaneously begin transmitting, a collision occurs. The collision is sensed by the network devices by analyzing the analog signal on line
100
.
FIG. 1C
is a block diagram illustrating a digital implementation of a digital Ethernet network that includes network devices
122
,
124
, and
126
and
128
. The digital Ethernet network does not include a common bus line as shown in
FIG. 1B
for the analog Ethernet network. That is because each network device includes both a transmit data line and a receive data line. Each network device therefore, must be connected to a Repeater
120
. Repeater
120
receives the data transmitted on each of the data transmit lines belonging to the different network devices and repeats the transmitted data onto a each of the data receive lines of the network devices. Repeater
120
is also referred to as a hub. The interface between each network device and the repeater includes
7
wires that carry a set of signals according to the Ethernet network standard.
It would be useful if a digital Ethernet network could be implemented without requiring the use of a hub. Furthermore, it would be useful if the number of wires required to provide connections to each of the network devices in such a network could be minimized.
SUMMARY OF THE INVENTION
Accordingly, a digital Ethernet network is disclosed that allows transmission of data by multiple stations on a shared bus and does not require a hub or repeater. In one embodiment, the network uses only two wires to transmit and receive data, and a third wire for a common clock. The two data wires are logical complements of each other while data is being transmitted. Collisions are detected based on the signals transmitted on the two wires before corrupted data is transferred.
It should be appreciated that the present invention can be implemented in nume
Cisco Technology Inc.
Ngo Ricky
Tran Phuc
Van Pelt & Yi LLP
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