COS. 1. Class of service. An indication of how an upper-layer protocol requires that a lower-layer protocol treat its messages. In SNA subarea routing, COS definitions are used by subarea nodes to determine the optimal route to establish a given session. A COS definition comprises a virtual route number and a transmission priority field. Also called TOS (type of service). 2. Corporation for Open Systems. An organization that promulgates the use of OSI protocols through conformance testing, certification, and related activities.
COSINE. Cooperation for Open Systems Interconnection Networking in Europe. A project financed by the European Community (EC) to build a communication network between scientific and industrial entities in Europe. The project ended in 1994.
cost. An arbitrary value, typically based on hop count, media bandwidth, or other measures, that is assigned by a network administrator and used to compare various paths through an internetwork environment. Cost values are used by routing protocols to determine the most favorable path to a particular destination: The lower the cost, the better the path. Sometimes called path cost. See also routing metric.
count to infinity. A problem that can occur in routing algorithms that are slow to converge, in which routers continuously increment the hop count to particular networks. Typically, some arbitrary hop-count limit is imposed to prevent this problem.
CP. control point. In SNA networks, an element that identifies the APPN networking components of a PU 2.1 node, manages device resources, and can provide services to other devices. In APPN, CPs are able to communicate with logically adjacent CPs by way of CP-to-CP sessions. See also EN and NN.
CPCS. common part convergence sublayer. One of the two sublayers of any AAL. The CPCS is service independent and is further divided into the CS and the SAR sublayers. The CPCS is responsible for preparing data for transport across the ATM network, including the creation of the 48-byte payload cells that are passed to the ATM layer. See also AAL, ATM layer, CS, SAR, and SSCS.
CPE. customer premises equipment. Terminating equipment, such as terminals, telephones, and modems, supplied by the telephone company, installed at customer sites, and connected to the telephone company network.
CPI-C. Common Programming Interface for Communications. A platform-independent API developed by IBM and used to provide portability in APPC applications. See also APPC.
cps. cells per second.
CRC. cyclic redundancy check. An error-checking technique in which the frame recipient calculates a remainder by dividing frame contents by a prime binary divisor and compares the calculated remainder to a value stored in the frame by the sending node.
CREN. Corporation for Research and Educational Networking. The result of a merger of BITNET and CSNET. CREN is devoted to providing Internet connectivity to its members, which include the alumni, students, faculty, and other affiliates of participating educational and research institutions, via BITNET III. See also BITNET, BITNET III, and CSNET.
cross talk. Interfering energy transferred from one circuit to another.
CS. convergence sublayer. One of the two sublayers of the AAL CPCS, responsible for padding and error checking. PDUs passed from the SSCS are appended with an 8-byte trailer (for error checking and other control information) and padded, if necessary, so that the length of the resulting PDU is divisible by 48. These PDUs are then passed to the SAR sublayer of the CPCS for further processing. See also AAL, CPCS, SAR, and SSCS.
CSA. Canadian Standards Association. An agency in Canada that certifies products that conform to Canadian national safety standards.
CSLIP. Compressed Serial Link Internet Protocol. An extension of SLIP that, when appropriate, allows just header information to be sent across a SLIP connection, reducing overhead and increasing packet throughput on SLIP lines. See also SLIP.
CSMA/CD. carrier sense multiple access collision detect. Media-access mechanism wherein devices ready to transmit data first check the channel for a carrier. If no carrier is sensed for a specific period of time, a device can transmit. If two devices transmit at once, a collision occurs and is detected by all colliding devices. This collision subsequently delays retransmissions from those devices for some random length of time. CSMA/CD access is used by Ethernet and IEEE 802.3.
CSNET. Computer Science Network. A large internetwork consisting primarily of universities, research institutions, and commercial concerns. CSNET merged with BITNET to form CREN. See also BITNET and CREN.
CSNP. complete sequence number PDU. A PDU sent by the designated router in an OSPF network to maintain database synchronization.
CSU. channel service unit. A digital interface device that connects end-user equipment to the local digital telephone loop. Often referred to, together with DSU, as CSU/DSU. See also DSU.
CTS. 1. Clear To Send. A circuit in the EIA/TIA-232 specification that is activated when DCE is ready to accept data from DTE. 2. common transport semantic. A cornerstone of the IBM strategy to reduce the number of protocols on networks. CTS provides a single API for developers of network software and enables applications to run over APPN, OSI, or TCP/IP.
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