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Katherine Johnson’s mathematical calculations helped guide some of America’s most important early space missions while she confronted the racial and gender barriers faced by Black women in twentieth-century America.
MORE →Reflects the personal views, recollections, and perspective of the author, Mike Davis.
This is a personal recollection on the Move fire on May 13, 1985
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A collision domain is a network segment (connected by a shared medium or through repeaters) where simultaneous data transmissions collide with one another as a result of more than one device attempting to send a packet on the network segment at the same time. The collision domain applies particularly in wireless networks, but also affected early versions of Ethernet. Members of a collision domain may be involved in collisions with one another. Devices outside the collision domain do not have collisions with those inside.
A channel access method dictates that only one device in the collision domain may transmit at any one time, and the other devices in the domain listen to the network and refrain from transmitting while others are already transmitting in order to avoid collisions. Because only one device may be transmitting at any one time, total network bandwidth is shared among all devices on the collision domain. Collisions also decrease network efficiency in a collision domain as collisions require devices to abort transmission and retransmit at a later time.
Since data bits are propagated at a finite speed, simultaneously is to be defined in terms of the size of the collision domain and the minimum packet size allowed. A smaller packet size or a larger dimension would make it possible for a sender to finish sending the packet without the first bits of the message being able to reach the most remote node. So, that node could start sending as well, without a clue to the transmission already taking place and destroying the first packet. Unless the size of the collision domain allows the initial sender to receive the second transmission attempt – the collision – within the time it takes to send the packet, they would neither be able to detect the collision nor to repeat the transmission – this is called a late collision.
On Ethernet using shared media, collisions are resolved using carrier-sense multiple access with collision detection (CSMA/CD) in which the competing packets are discarded and re-sent one at a time. This becomes a source of inefficiency in the network.[1]
Early Ethernet variants (10BASE5, 10BASE2) were based on a shared wire and inherently half-duplex, representing a single, potentially large collision domain. Collision domains are also found in an Ethernet hub or repeater environment where each host segment connects to a hub, and all segments represent only one collision domain within one broadcast domain. Collision domains are also found in other shared medium networks, e. g. wireless networks such as Wi-Fi.
Modern wired networks use a network switch to reduce or eliminate collisions. By connecting each device directly to a port on the switch, either each port on a switch becomes its own collision domain (in the case of half-duplex links), or the possibility of collisions is eliminated in the case of full-duplex links. For Gigabit Ethernet and faster, no hubs or repeaters exist and all devices require full-duplex links.
Most wireless LAN networks use the carrier-sense multiple access with collision avoidance (CSMA/CA) method. In addition to the requirements of a shared wire medium, wireless networks add the hidden node problem where two senders can't hear each other's transmissions, but they cause a collision at the receiver between them. Multiple Access with Collision Avoidance for Wireless is one such approach used, specifically in 802.11 RTS/CTS. Central coordination is another means of solving this problem for a collision domain. This technique is employed by Wireless Multimedia Extensions. Point coordination function and distributed coordination function are specific implementations.
Source: Wikipedia. Article content is retrieved live through the MediaWiki API.
A collision domain is a network segment (connected by a shared medium or through repeaters) where simultaneous data transmissions collide with one another as a result of more than one device attempting to send a packet on the network segment at the same time. The collision domain applies particularly in wireless networks, but also affected early versions of Ethernet. Members of a collision domain may be involved in collisions with one another. Devices outside the collision domain do not have collisions with those inside. A channel access method dictates that only one device in the collision domain may transmit at any one time, and the other devices in the domain listen to the network and refrain from transmitting while others are already transmitting in order to avoid collisions. Because only one device may be transmitting at any one time, total network bandwidth is shared among all devices on the collision domain. Collisions also decrease network efficiency in a collision domain as collisions require devices to abort transmission and retransmit at a later time. Since data bits are propagated at a finite speed, simultaneously is to be defined in terms of the size of the collision domain and the minimum packet size allowed. A smaller packet size or a larger dimension would make it possible for a sender to finish sending the packet without the first bits of the message being able to reach the most remote node. So, that node could start sending as well, without a clue to the transmission already taking place and destroying the first packet. Unless the size of the collision domain allows the initial sender to receive the second transmission attempt – the collision – within the time it takes to send the packet, they would neither be able to detect the collision nor to repeat the transmission – this is called a late collision.
A broadcast domain is a logical division of a computer network, in which all nodes can reach each other by broadcast at the data link layer. A broadcast domain can be within the same LAN segment or it can be bridged to other LAN segments. In terms of current popular technologies, any computer connected to the same Ethernet repeater or switch is a member of the same broadcast domain. Further, any computer connected to the same set of interconnected switches or repeaters is a member of the same broadcast domain. Routers and other network-layer devices form boundaries between broadcast domains. The notion of a broadcast domain can be compared with a collision domain, which would be all nodes on the same set of inter-connected repeaters and divided by switches and network bridges. Collision domains are generally smaller than and contained within broadcast domains. While some data-link-layer devices are able to divide the collision domains, broadcast domains are only divided by network-layer devices such as routers or layer-3 switches. Separating VLANs divides broadcast domains as well.
A collision is the situation that occurs when two or more demands are made simultaneously on equipment that can handle only one at any given instant. It may refer to: Collision domain, a physical network segment where data packets can "collide" Carrier-sense multiple access with collision avoidance, (CSMA/CA) used for example with wireless LANs Carrier-sense multiple access with collision detection, (CSMA/CD) used with Ethernet Late collision, a specific type of collision that should not occur on properly operating networks Local collision is a collision that occurs in the network interface rather than on the network itself
10BASE5 (also known as thick Ethernet or thicknet) is a version of Ethernet providing 10 Mbit/s bandwidth over a thick coaxial cable having a maximum length of 500 m (1,600 ft). It was the first commercially available version of Ethernet. The technology was standardized first by Ecma International in early 1982 based on an IEEE draft standard, then by IEEE in 1983 as IEEE 802.3. Up to 100 stations can be connected to the cable in a single collision domain. The standard specifies CSMA/CD to resolve collisions. 10BASE5 was superseded by much cheaper and more convenient alternatives: first by 10BASE2 based on a thinner coaxial cable (1985), and then, once Ethernet over twisted pair was developed, by 10BASE-T (1990) and its successors 100BASE-TX and 1000BASE-T. In 2003, the IEEE 802.3 working group deprecated 10BASE5 for new installations.
Before the 1921 destruction of Tulsa’s Greenwood District, Black residents had created a remarkable center of business and community life. The district included stores, professional offices, entertainment venues and homes owned by Black citizens. Understanding Greenwood means learning what was built—not only what was burned.
MORE →Shirley Chisholm, elected in 1968.