Network interface device
In telecommunications, a network interface device is a device that serves as the demarcation point between the carrier's local loop and the customer's premises wiring. Outdoor telephone NIDs provide the subscriber with access to the station wiring and serve as a convenient test point for verification of loop integrity and of the subscriber’s inside wiring. Generically, an NID may be called a network interface unit, telephone network interface, system network interface, or telephone network box. Australia's National Broadband Network uses the term network termination device or NTD. A smartjack is a type of NID with capabilities beyond simple electrical connection, such as diagnostics. An optical network terminal is a type of NID used with fiber-to-the-premises applications; the simplest NIDs are just a specialized set of wiring terminals. These will take the form of a small, weather-proof box, mounted on the outside of the building; the telephone line from the telephone company will be connected to one side.
The customer connects their wiring to the other side. A single NID enclosure may contain termination for multiple lines. In its role as the demarcation point, the NID separates the telephone company's equipment from the customer's wiring and equipment; the telephone company owns the NID itself, all wiring up to it. Anything past the NID is the customer's responsibility. To facilitate this, there is a test jack inside the NID. Accessing the test jack disconnects the customer premises wiring from the public switched telephone network and allows the customer to plug a "known good" telephone into the jack to isolate trouble. If the telephone works at the test jack, the problem is the customer's wiring, the customer is responsible for repair. If the telephone does not work, the line is faulty and the telephone company is responsible for repair. Most NIDs include "circuit protectors", which are surge protectors for a telephone line, they protect customer wiring and personnel from any transient energy on the line, such as from a lightning strike to a telephone pole.
Simple NIDs contain no digital logic. They have no capabilities beyond wiring termination, circuit protection, providing a place to connect test equipment. Several types of NIDs provide more than just a terminal for the connection of wiring; such NIDs are colloquially called smartjacks or Intelligent Network Interface Devices as an indication of their built-in "intelligence", as opposed to a simple NID, just a wiring device. Smartjacks are used for more complicated types of telecommunications service, such as T1 lines. Plain old telephone service lines cannot be equipped with smartjacks. Despite the name, most smartjacks are much more than a simple telephone jack. One common form for a smartjack is a printed circuit board with a face plate on one edge, mounted in an enclosure. A smartjack may provide signal conversion, converting codes and protocols to the type needed by the customer equipment, it may buffer and/or regenerate the signal, to compensate for signal degradation from line transmission, similar to what a repeater does.
Smartjacks typically provide diagnostic capabilities. A common capability provided by a smartjack is loopback, such that the signal from the telephone company is transmitted back to the telephone company; this allows the company to test the line from the central telephone exchange, without the need to have test equipment at the customer site. The telephone company has the ability to remotely activate loopback, without needing personnel at the customer site; when looped back, the customer equipment is disconnected from the line. Additional smartjack diagnostic capabilities include alarm indication signal, which reports trouble at one end of the line to the far end; this helps the telephone company know if trouble is present in the line, the smartjack, or customer equipment. Indicator lights to show configuration and alarms are common. Smartjacks derive their operating power from the telephone line, rather than relying on premises electrical power, although this is not a universal rule. In fiber-to-the-premises systems, the signal is transmitted to the customer premises using fiber optic technologies.
Unlike many conventional telephone technologies, this does not provide power for premises equipment, nor is it suitable for direct connection to customer equipment. An optical network terminal is used to terminate the fiber optic line, demultiplex the signal into its component parts, provide power to customer telephones; as the ONT must derive its power from the customer premises electrical supply, many ONTs have the option for a battery backup, to maintain service in the event of a power outage. According to Telcordia GR-49, requirements for telecommunications NIDs vary based on three categories of environmental conditions: Normal conditions: This refers to a normal environment, expected in most areas of any service provider. Temperatures are expected to be in the range of −20–32 °C, humidity is expected to be less than 90% RH. No unusual contamination is expected. Severe climatic conditions: These cover environments more severe than those of a normal environment. Temperatures are expected to be in the range of −40–43 °C, humidity may exceed 90% RH.
Jacks installed in NIDs in such environments are known to become contaminated and develop low insulation resistances and low dielectric brea
General Services Administration
The General Services Administration, an independent agency of the United States government, was established in 1949 to help manage and support the basic functioning of federal agencies. GSA supplies products and communications for U. S. government offices, provides transportation and office space to federal employees, develops government-wide cost-minimizing policies and other management tasks. GSA employs about 12,000 federal workers and has an annual operating budget of $20.9 billion. GSA oversees $66 billion of procurement annually, it contributes to the management of about $500 billion in U. S. federal property, divided chiefly among 8,700 owned and leased buildings and a 215,000 vehicle motor pool. Among the real estate assets managed by GSA are the Ronald Reagan Building and International Trade Center in Washington, D. C. – the largest U. S. federal building after the Pentagon – and the Hart-Dole-Inouye Federal Center. GSA's business lines include the Federal Acquisition Service and the Public Buildings Service, as well as several Staff Offices including the Office of Government-wide Policy, the Office of Small Business Utilization, the Office of Mission Assurance.
As part of FAS, GSA's Technology Transformation Services helps federal agencies improve delivery of information and services to the public. Key initiatives include FedRAMP, Cloud.gov, the USAGov platform, Data.gov, Performance.gov, Challenge.gov. GSA is a member of the Procurement G6, an informal group leading the use of framework agreements and e-procurement instruments in public procurement. In 1947 President Harry Truman asked former President Herbert Hoover to lead what became known as the Hoover Commission to make recommendations to reorganize the operations of the federal government. One of the recommendations of the commission was the establishment of an "Office of the General Services." This proposed office would combine the responsibilities of the following organizations: U. S. Treasury Department's Bureau of Federal Supply U. S. Treasury Department's Office of Contract Settlement National Archives Establishment All functions of the Federal Works Agency, including the Public Buildings Administration and the Public Roads Administration War Assets AdministrationGSA became an independent agency on July 1, 1949, after the passage of the Federal Property and Administrative Services Act.
General Jess Larson, Administrator of the War Assets Administration, was named GSA's first Administrator. The first job awaiting Administrator Larson and the newly formed GSA was a complete renovation of the White House; the structure had fallen into such a state of disrepair by 1949 that one inspector of the time said the historic structure was standing "purely from habit." Larson explained the nature of the total renovation in depth by saying, "In order to make the White House structurally sound, it was necessary to dismantle, I mean dismantle, everything from the White House except the four walls, which were constructed of stone. Everything, except the four walls without a roof, was stripped down, that's where the work started." GSA worked with President Truman and First Lady Bess Truman to ensure that the new agency's first major project would be a success. GSA completed the renovation in 1952. In 1986 GSA headquarters, U. S. General Services Administration Building, located at Eighteenth and F Streets, NW, was listed on the National Register of Historic Places, at the time serving as Interior Department offices.
In 1960 GSA created the Federal Telecommunications System, a government-wide intercity telephone system. In 1962 the Ad Hoc Committee on Federal Office Space created a new building program to address obsolete office buildings in Washington, D. C. resulting in the construction of many of the offices that now line Independence Avenue. In 1970 the Nixon administration created the Consumer Product Information Coordinating Center, now part of USAGov. In 1974 the Federal Buildings Fund was initiated, allowing GSA to issue rent bills to federal agencies. In 1972 GSA established the Automated Data and Telecommunications Service, which became the Office of Information Resources Management. In 1973 GSA created the Office of Federal Management Policy. GSA's Office of Acquisition Policy centralized procurement policy in 1978. GSA was responsible for emergency preparedness and stockpiling strategic materials to be used in wartime until these functions were transferred to the newly-created Federal Emergency Management Agency in 1979.
In 1984 GSA introduced the federal government to the use of charge cards, known as the GMA SmartPay system. The National Archives and Records Administration was spun off into an independent agency in 1985; the same year, GSA began to provide governmentwide policy oversight and guidance for federal real property management as a result of an Executive Order signed by President Ronald Reagan. In 2003 the Federal Protective Service was moved to the Department of Homeland Security. In 2005 GSA reorganized to merge the Federal Supply Service and Federal Technology Service business lines into the Federal Acquisition Service. On April 3, 2009, President Barack Obama nominated Martha N. Johnson to serve as GSA Administrator. After a nine-month delay, the United States Senate confirmed her nomination on February 4, 2010. On April 2, 2012, Johnson resigned in the wake of a management-deficiency report that detailed improper payments for a 2010 "Western Regions" training conference put on by the Public Buildings Service in Las Vegas.
In July 1991 GSA contractors began the excavation of what is now the Ted Weiss Federal Building in New York City. The planning for that buildin
Digital subscriber line
Digital subscriber line is a family of technologies that are used to transmit digital data over telephone lines. In telecommunications marketing, the term DSL is understood to mean asymmetric digital subscriber line, the most installed DSL technology, for Internet access. DSL service can be delivered with wired telephone service on the same telephone line since DSL uses higher frequency bands for data. On the customer premises, a DSL filter on each non-DSL outlet blocks any high-frequency interference to enable simultaneous use of the voice and DSL services; the bit rate of consumer DSL services ranges from 256 kbit/s to over 100 Mbit/s in the direction to the customer, depending on DSL technology, line conditions, service-level implementation. Bit rates of 1 Gbit/s have been reached. In ADSL, the data throughput in the upstream direction is lower, hence the designation of asymmetric service. In symmetric digital subscriber line services, the downstream and upstream data rates are equal. Researchers at Bell Labs have reached speeds over 1 Gbit/s for symmetrical broadband access services using traditional copper telephone lines, though such speeds have not yet been deployed elsewhere.
It was thought that it was not possible to operate a conventional phone line beyond low-speed limits. In the 1950s, ordinary twisted-pair telephone cable carried four megahertz television signals between studios, suggesting that such lines would allow transmitting many megabits per second. One such circuit in the United Kingdom ran some 10 miles between the BBC studios in Newcastle-upon-Tyne and the Pontop Pike transmitting station, it was able to give the studios a low quality cue feed but not one suitable for transmission. However, these cables had other impairments besides Gaussian noise, preventing such rates from becoming practical in the field; the 1980s saw the development of techniques for broadband communications that allowed the limit to be extended. A patent was filed in 1979 for the use of existing telephone wires for both telephones and data terminals that were connected to a remote computer via a digital data carrier system; the motivation for digital subscriber line technology was the Integrated Services Digital Network specification proposed in 1984 by the CCITT as part of Recommendation I.120 reused as ISDN digital subscriber line.
Employees at Bellcore developed asymmetric digital subscriber line by placing wide-band digital signals at frequencies above the existing baseband analog voice signal carried on conventional twisted pair cabling between telephone exchanges and customers. A patent was filed in 1988. Joseph W. Lechleider's contribution to DSL was his insight that an asymmetric arrangement offered more than double the bandwidth capacity of symmetric DSL; this allowed Internet service providers to offer efficient service to consumers, who benefited from the ability to download large amounts of data but needed to upload comparable amounts. ADSL supports two modes of transport -- interleaved channel. Fast channel is preferred for streaming multimedia, where an occasional dropped bit is acceptable, but lags are less so. Interleaved channel works better for file transfers, where the delivered data must be error-free but latency incurred by the retransmission of error-containing packets is acceptable. Consumer-oriented ADSL was designed to operate on existing lines conditioned for Basic Rate Interface ISDN services, which itself is a digital circuit switching service, though most incumbent local exchange carriers provision rate-adaptive digital subscriber line to work on any available copper pair facility, whether conditioned for BRI or not.
Engineers developed high speed DSL facilities such as high bit rate digital subscriber line and symmetric digital subscriber line to provision traditional Digital Signal 1 services over standard copper pair facilities. Older ADSL standards delivered 8 Mbit/s to the customer over about 2 km of unshielded twisted-pair copper wire. Newer variants improved these rates. Distances greater than 2 km reduce the bandwidth usable on the wires, thus reducing the data rate, but ADSL loop extenders increase these distances by repeating the signal, allowing the LEC to deliver DSL speeds to any distance. Until the late 1990s, the cost of digital signal processors for DSL was prohibitive. All types of DSL employ complex digital signal processing algorithms to overcome the inherent limitations of the existing twisted pair wires. Due to the advancements of very-large-scale integration technology, the cost of the equipment associated with a DSL deployment lowered significantly; the two main pieces of equipment are a digital subscriber line access multiplexer at one end and a DSL modem at the other end.
A DSL connection can be deployed over existing cable. Such deployment including equipment, is much cheaper than installing a new, high-bandwidth fiber-optic cable over the same route and distance; this is true both for SDSL variations. The commercial success of DSL and similar technologies reflects the advances made in electronics over the decades that have increased performance and reduced costs while digging trenches in the ground for new cables remains expensive. In the case of ADSL, competition in Internet access caused subscription fees to drop over the years, thus making ADSL more economical than dial up access. Telephone companies were pressured into moving to A
A telephone exchange is a telecommunications system used in the public switched telephone network or in large enterprises. An exchange consists of electronic components and in older systems human operators that interconnect telephone subscriber lines or virtual circuits of digital systems to establish telephone calls between subscribers. In historical perspective, telecommunication terms have been used with different semantics over time; the term telephone exchange is used synonymously with central office, a Bell System term. A central office is defined as a building used to house the inside plant equipment of several telephone exchanges, each serving a certain geographical area; such an area has been referred to as the exchange. Central office locations may be identified in North America as wire centers, designating a facility from which a telephone obtains dial tone. For business and billing purposes, telephony carriers define rate centers, which in larger cities may be clusters of central offices, to define specified geographical locations for determining distance measurements.
In the United States and Canada, the Bell System established in the 1940s a uniform system of identifying central offices with a three-digit central office code, used as a prefix to subscriber telephone numbers. All central offices within a larger region aggregated by state, were assigned a common numbering plan area code. With the development of international and transoceanic telephone trunks driven by direct customer dialing, similar efforts of systematic organization of the telephone networks occurred in many countries in the mid-20th century. For corporate or enterprise use, a private telephone exchange is referred to as a private branch exchange, when it has connections to the public switched telephone network. A PBX is installed in enterprise facilities collocated with large office spaces or within an organizational campus to serve the local private telephone system and any private leased line circuits. Smaller installations might deploy a PBX or key telephone system in the office of a receptionist.
In the era of the electrical telegraph, post offices, railway stations, the more important governmental centers, stock exchanges few nationally distributed newspapers, the largest internationally important corporations and wealthy individuals were the principal users of such telegraphs. Despite the fact that telephone devices existed before the invention of the telephone exchange, their success and economical operation would have been impossible on the same schema and structure of the contemporary telegraph, as prior to the invention of the telephone exchange switchboard, early telephones were hardwired to and communicated with only a single other telephone. A telephone exchange is a telephone system located at service centers responsible for a small geographic area that provided the switching or interconnection of two or more individual subscriber lines for calls made between them, rather than requiring direct lines between subscriber stations; this made it possible for subscribers to call each other at businesses, or public spaces.
These made telephony an available and comfortable communication tool for everyday use, it gave the impetus for the creation of a whole new industrial sector. As with the invention of the telephone itself, the honor of "first telephone exchange" has several claimants. One of the first to propose a telephone exchange was Hungarian Tivadar Puskás in 1877 while he was working for Thomas Edison; the first experimental telephone exchange was based on the ideas of Puskás, it was built by the Bell Telephone Company in Boston in 1877. The world's first state-administered telephone exchange opened on November 12, 1877 in Friedrichsberg close to Berlin under the direction of Heinrich von Stephan. George W. Coy designed and built the first commercial US telephone exchange which opened in New Haven, Connecticut in January, 1878; the switchboard was built from "carriage bolts, handles from teapot lids and bustle wire" and could handle two simultaneous conversations. Charles Glidden is credited with establishing an exchange in Lowell, MA. with 50 subscribers in 1878.
In Europe other early telephone exchanges were based in London and Manchester, both of which opened under Bell patents in 1879. Belgium had its first International Bell exchange a year later. In 1887 Puskás introduced the multiplex switchboard.. Exchanges consisted of one to several hundred plug boards staffed by switchboard operators; each operator sat in front of a vertical panel containing banks of ¼-inch tip-ring-sleeve jacks, each of, the local termination of a subscriber's telephone line. In front of the jack panel lay a horizontal panel containing two rows of patch cords, each pair connected to a cord circuit; when a calling party lifted the receiver, the local loop current lit a signal lamp near the jack. The operator responded by inserting the rear cord into the subscriber's jack and switched her headset into the circuit to ask, "Number, please?" For a local call, the operator inserted the front cord of the pair into the called party's local jack and started the ringing cycle. For a long distance call, she plugged into a trunk circuit to connect to another operator in another bank of boards or at a remote central office.
In 1918, the average time to complete the connection for a long-distance call was 15 minutes. Early manual switchboards required the operator to operate listening keys and ringing keys, but by the late 1910s and 1920s, advances in switchboard technology led to features which allowed the call to be automatic
In electronics, a multiplexer is a device that selects between several analog or digital input signals and forwards it to a single output line. A multiplexer of 2 n inputs has n select lines, which are used to select which input line to send to the output. Multiplexers are used to increase the amount of data that can be sent over the network within a certain amount of time and bandwidth. A multiplexer is called a data selector. Multiplexers can be used to implement Boolean functions of multiple variables. An electronic multiplexer makes it possible for several signals to share one device or resource, for example, one A/D converter or one communication line, instead of having one device per input signal. Conversely, a demultiplexer is a device taking a single input signal and selecting one of many data-output-lines, connected to the single input. A multiplexer is used with a complementary demultiplexer on the receiving end. An electronic multiplexer can be considered as a multiple-input, single-output switch, a demultiplexer as a single-input, multiple-output switch.
The schematic symbol for a multiplexer is an isosceles trapezoid with the longer parallel side containing the input pins and the short parallel side containing the output pin. The schematic on the right shows a 2-to-1 multiplexer on the left and an equivalent switch on the right; the s e l wire connects the desired input to the output. One use for multiplexers is economizing connections over a single channel, by connecting the multiplexer's single output to the demultiplexer's single input; the image to the right demonstrates this benefit. In this case, the cost of implementing separate channels for each data source is higher than the cost and inconvenience of providing the multiplexing/demultiplexing functions. At the receiving end of the data link a complementary demultiplexer is required to break the single data stream back down into the original streams. In some cases, the far end system may have functionality greater than a simple demultiplexer; this would be typical when: a multiplexer serves a number of IP network users.
A multiplexer and demultiplexer are combined together into a single piece of equipment, conveniently referred to as a "multiplexer". Both circuit elements are needed at both ends of a transmission link because most communications systems transmit in both directions. In analog circuit design, a multiplexer is a special type of analog switch that connects one signal selected from several inputs to a single output. In digital circuit design, the selector wires are of digital value. In the case of a 2-to-1 multiplexer, a logic value of 0 would connect I 0 to the output while a logic value of 1 would connect I 1 to the output. In larger multiplexers, the number of selector pins is equal to ⌈ log 2 ⌉ where n is the number of inputs. For example, 9 to 16 inputs would require no fewer than 4 selector pins and 17 to 32 inputs would require no fewer than 5 selector pins; the binary value expressed on these selector pins determines the selected input pin. A 2-to-1 multiplexer has a boolean equation where A and B are the two inputs, S is the selector input, Z is the output: Z = + Which can be expressed as a truth table: Or, in simpler notation: These tables show that when S = 0 Z = A but when S = 1 Z = B.
A straightforward realization of this 2-to-1 multiplexer would need 2 AND gates, an OR gate, a NOT gate. While this is mathematically correct, a direct physical implementation would be prone to race conditions that require additional gates to suppress. Larger multiplexers are common and, as stated above, require ⌈ log 2 ⌉ selector pins for n inputs. Other common sizes are 4-to-1, 8-to-1, 16-to-1. Since digital logic uses binary values, powers of 2 are used to maximally control a number of inputs for the given number of selector inputs; the boolean equation for a 4-to-1 multiplexer is: Z = + ( B
A network bridge is a computer networking device that creates a single aggregate network from multiple communication networks or network segments. This function is called network bridging. Bridging is distinct from routing. Routing allows multiple networks to communicate independently and yet remain separate, whereas bridging connects two separate networks as if they were a single network. In the OSI model, bridging is performed in the data link layer. If one or more segments of the bridged network are wireless, the device is known as a wireless bridge. There are four main types of network bridging technologies: simple bridging, multiport bridging, learning or transparent bridging, source route bridging. Transparent bridging uses a table called the forwarding information base to control the forwarding of frames between network segments; the table starts empty and entries are added as the bridge receives frames. If a destination address entry is not found in the table, the frame is flooded to all other ports of the bridge, flooding the frame to all segments except the one from which it was received.
By means of these flooded frames, a host on the destination network will respond and a forwarding database entry will be created. Both source and destination addresses are used in this process: source addresses are recorded in entries in the table, while destination addresses are looked up in the table and matched to the proper segment to send the frame to. Digital Equipment Corporation developed the technology in the 1980s. In the context of a two-port bridge, one can think of the forwarding information base as a filtering database. A bridge decides to either forward or filter. If the bridge determines that the destination host is on another segment on the network, it forwards the frame to that segment. If the destination address belongs to the same segment as the source address, the bridge filters the frame, preventing it from reaching the other network where it is not needed. Transparent bridging can operate over devices with more than two ports; as an example, consider a bridge connected to three hosts, A, B, C.
The bridge has three ports. A is connected to bridge port 1, B is connected to bridge port 2, C is connected to bridge port 3. A sends a frame addressed to B to the bridge; the bridge examines the source address of the frame and creates an address and port number entry for A in its forwarding table. The bridge examines the destination address of the frame and does not find it in its forwarding table so it floods it to all other ports: 2 and 3; the frame is received by hosts B and C. Host C ignores the frame. Host B recognizes a destination address match and generates a response to A. On the return path, the bridge adds an port number entry for B to its forwarding table; the bridge has A's address in its forwarding table so it forwards the response only to port 1. Host C or any other hosts on port 3 are not burdened with the response. Two-way communication is now possible between B without any further flooding in network. A simple bridge connects two network segments by operating transparently and deciding on a frame-by-frame basis whether or not to forward from one network to the other.
A store and forward technique is used so, as part of forwarding, the frame integrity is verified on the source network and CSMA/CD delays are accommodated on the destination network. In contrast to repeaters which extend the maximum span of a segment, bridges only forward frames that are required to cross the bridge. Additionally, bridges reduce collisions by creating a separate collision domain on either side of the bridge. A multiport bridge connects multiple networks and operates transparently to decide on a frame-by-frame basis whether to forward traffic. Additionally a multiport bridge must decide. Like the simple bridge, a multiport bridge uses store and forward operation; the multiport bridge function serves as the basis for network switches. The forwarding information base stored in content-addressable memory is empty. For each received ethernet frame the switch learns from the frame's source MAC address and adds this together with an ingress interface identifier to the forwarding information base.
The switch forwards the frame to the interface found in the CAM based on the frame's destination MAC address. If the destination address is unknown the switch sends the frame out on all interfaces; this behaviour is called unicast flooding. Once a bridge learns the addresses of its connected nodes, it forwards data link layer frames using a layer 2 forwarding method. There are four forwarding methods a bridge can use, of which the second through fourth methods were performance-increasing methods when used on "switch" products with the same input and output port bandwidths: Store and forward: the switch buffers and verifies each frame before forwarding it. Cut through: the switch starts forwarding after the frame's destination address is received. There is no error checking with this method; when the outgoing port is busy at the time, the switch falls back to store-and-forward operation. When the egress port is running at a faster data rate than the ingress port, store-and-forward is used. Fragment free: a method that attempts to retain the benefits of both store and forward and cut through.
Fragment free checks the first 64 bytes of the frame. According to Ethernet specifications, collisions should be detected during the first 64 bytes of the frame, so frames that are in error because of a collision will not be forwarded; this way
Data terminal equipment
Data terminal equipment is an end instrument that converts user information into signals or reconverts received signals. These can be called tail circuits. A DTE device communicates with the data circuit-terminating equipment; the DTE/DCE classification was introduced by IBM. V.35 is a high-speed serial interface designed to support both higher data rates and connectivity between DTEs or DCEs over digital lines. Two different types of devices are assumed on each end of the interconnecting cable for a case of adding DTE to the topology, which brings a less trivial case of interconnection of devices of the same type: DTE-DTE or DCE-DCE; such cases need crossover cables, such as for the Ethernet or null modem for RS-232. A DTE is the functional unit of a data station that serves as a data source or a data sink and provides for the data communication control function to be performed in accordance with the link protocol; the data terminal equipment may be a single piece of equipment or an interconnected subsystem of multiple pieces of equipment that perform all the required functions necessary to permit users to communicate.
A user interacts with the DTE. The DTE device is the terminal, the DCE is a modem or another carrier-owned device. A general rule is that DCE devices provide the clock signal and the DTE device synchronizes on the provided clock. D-sub connectors follow another rule for pin assignment. 25 pin DTE devices transmit on pin 2 and receive on pin 3. 25 pin DCE devices transmit on pin 3 and receive on pin 2. 9 pin DTE devices transmit on pin 3 and receive on pin 2. 9 pin DCE devices transmit on pin 2 and receive on pin 3. This term is generally used in the Telco and Cisco equipment context to designate a network device, such as terminals, personal computers but routers and bridges, that's unable or configured not to generate clock signals. Hence a direct PC to PC Ethernet connection can be called a DTE to DTE communication; this communication is done via an Ethernet crossover cable as opposed to a PC to DCE communication, done via an Ethernet straight cable. Internetworking Technology Handbook, Frame Relay, Cisco Systems