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In telecommunications and computer networks, a virtual circuit (VC), synonymous to virtual connection and virtual channel, is a connection oriented communication service that is delivered by means of packet mode communication. After a connection or virtual circuit is established between two nodes or application processes, a bit stream or byte stream may be delivered between the nodes. A virtual circuit protocol hides the division into segments, packets or frames from higher level protocols.

Virtual circuit communication resembles circuit switching, since both are connection oriented, meaning that in both cases data is delivered in correct order, and signalling overhead is required during a connection establishment phase. However, circuit switching provides constant bit rate and latency, while these may vary in a virtual circuit service due to reasons such as:
- varying packet queue lengths in the network nodes,
- varying bit rate generated by the application,
- varying load from other users sharing the same network resources by means of statistical multiplexing, etc.

Many virtual circuit protocols, but not all, provide reliable communication service, by means of data retransmissions due to error detection and automatic repeat request (ARQ).

Layer 4 virtual circuits
Connection oriented transport layer datalink protocols such as TCP may rely on a connectionless packet switching network layer protocol such as IP, where different packets may be routed over different paths, and thus be delivered out of order. However, a virtual circuit is possible since TCP includes segment numbering and reordering on the receiver side to prevent out-of-order delivery.

Layer 2/3 virtual circuits
Network layer and datalink layer virtual circuit protocols are based on connection oriented packet switching, meaning that data is always delivered along the same network path, i.e. through the same nodes. Advantages with this over connectionless packet switching are:
- Bandwidth reservation during the connection establishment phase is supported, making guaranteed Quality of Service (QoS) possible. For example, a constant bit rate QoS class may be provided, resulting in emulation of circuit switching.
- Less overhead is required, since the packets are not routed individually and complete addressing information is not provided in the header of each data packet. Only a small virtual channel identifier (VCI) is required in each packet. Routing information is only transferred to the network nodes during the connection establishment phase.
- The network nodes are faster and have higher capacity in theory, since they are switches that only perform routing during the connection establishment phase, while connectionless network nodes are routers that perform routing for each packet individually. Switching only involves looking up the virtual channel identifier in a table rather than analyzing a complete address. Switches can easily be implemented in ASIC hardware, while routing is more complex and requires software implementation. However, due to the large market of IP routers, and because advanced IP routers support layer 3 switching, modern IP routers may today be faster than switches for connection oriented protocols.

Reference:
http://en.wikipedia.org/wiki/Virtual_circuit

In computer networking, Fast Ethernet is a collective term for a number of Ethernet standards that carry traffic at the nominal rate of 100 Mbit/s, against the original Ethernet speed of 10 Mbit/s. Of the 100 megabit Ethernet standards 100baseTX is by far the most common and is supported by the vast majority of Ethernet hardware currently produced. Full duplex fast Ethernet is sometimes referred to as "200 Mbit/s" though this is somewhat misleading as that level of improvement will only be achieved if traffic patterns are symmetrical. Fast Ethernet was introduced in 1995 and remained the fastest version of Ethernet for three years before being superseded by gigabit Ethernet.

A fast Ethernet adapter can be logically divided into a media access controller (MAC) which deals with the higher level issues of medium availability and a physical layer interface (PHY). The MAC may be linked to the PHY by a 4 bit 25 MHz synchronous parallel interface known as MII. Repeaters (hubs) are also allowed and connect to multiple PHYs for their different interfaces.

The MII interface may (rarely) be an external connection but is usually a connection between ICs in a network adapter or even within a single IC. The specs are written based on the assumption that the interface between MAC and PHY will be MII but they do not require it.

The MII interface fixes the theoretical maximum data bit rate for all versions of fast Ethernet to 100 Mbit/s. The data signaling rate actually observed on real networks is less than the theoretical maximum, due to the necessary header and trailer (addressing and error-detection bits) on every frame, the occasional "lost frame" due to noise, and time waiting after each sent frame for other devices on the network to finish transmitting.

Copper
100BASE-T is any of several Fast Ethernet standards for twisted pair cables, including: 100BASE-TX (100 Mbit/s over two-pair Cat5 or better cable), 100BASE-T4 (100 Mbit/s over four-pair Cat3 or better cable, defunct), 100BASE-T2 (100 Mbit/s over two-pair Cat3 or better cable, also defunct). The segment length for a 100BASE-T cable is limited to 100 metres (328 ft) (as with 10BASE-T and gigabit Ethernet). All are or were standards under IEEE 802.3 (approved 1995).

In the early days of Fast Ethernet, much vendor advertising centered on claims by competing standards that "ours will work better with existing cables than theirs." In practice, it was quickly discovered that few existing networks actually met the assumed standards, because 10-megabit Ethernet was very tolerant of minor deviations from specified electrical characteristics and few installers ever bothered to make exact measurements of cable and connection quality; if Ethernet worked over a cable, it was deemed acceptable. Thus most networks had to be rewired for 100-megabit speed whether or not there had supposedly been CAT3 or CAT5 cable runs. The vast majority of common implementations or installations of 100BASE-T are done with 100BASE-TX.

100BASE-TX
100BASE-TX is the predominant form of Fast Ethernet, and runs over two pairs of category 5 or above cable (a typical category 5 cable contains 4 pairs and can therefore support two 100BASE-TX links). Like 10BASE-T, the proper pairs are the orange and green pairs (canonical second and third pairs) in TIA/EIA-568-B's termination standards, T568A or T568B. These pairs use pins 1, 2, 3 and 6.

In T568A and T568B, wires are in the order 1, 2, 3, 6, 4, 5, 7, 8 on the modular jack at each end. The color-order would be green/white, green, orange/white, blue, blue/white, orange, brown/white, brown for T568A, and orange/white, orange, green/white, blue, blue/white, green, brown/white, brown for T568B.

Each network segment can have a maximum distance of 100 metres (330 ft). In its typical configuration, 100BASE-TX uses one pair of twisted wires in each direction, providing 100 Mbit/s of throughput in each direction (full-duplex). See IEEE 802.3 for more details.

The configuration of 100BASE-TX networks is very similar to 10BASE-T. When used to build a local area network, the devices on the network (computers, printers etc.) are typically connected to a hub or switch, creating a star network. Alternatively it is possible to connect two devices directly using a crossover cable.

With 100BASE-TX hardware, the raw bits (4 bits wide clocked at 25 MHz at the MII) go through 4B5B binary encoding to generate a series of 0 and 1 symbols clocked at 125 MHz symbol rate. The 4B5B encoding provides DC equalization and spectrum shaping (see the standard for details). Just as in the 100BASE-FX case, the bits are then transferred to the physical medium attachment layer using NRZI encoding. However, 100BASE-TX introduces an additional, medium dependent sublayer, which employs MLT-3 as a final encoding of the data stream before transmission, resulting in a maximum "fundamental frequency" of 31.25 MHz. The procedure is borrowed from the ANSI X3.263 FDDI specifications, with minor discrepancies.

100BASE-T4
100BASE-T4 was an early implementation of Fast Ethernet. It requires four twisted copper pairs, but those pairs were only required to be category 3 rather than the category 5 required by TX. One pair is reserved for transmit, one for receive, and the remaining two will switch direction as negotiated. A very unusual 8B6T code is used to convert 8 data bits into 6 base-3 digits (the signal shaping is possible as there are three times as many 6-digit base-3 numbers as there are 8-digit base-2 numbers). The two resulting 3-digit base-3 symbols are sent in parallel over 3 pairs using 3-level pulse-amplitude modulation (PAM-3).

100BASE-T2
In 100BASE-T2, the data is transmitted over two copper pairs, 4 bits per symbol. First, a 4 bit symbol is expanded into two 3-bit symbols through a non-trivial scrambling procedure based on a linear feedback shift register; see the standard for details. This is needed to flatten the bandwidth and emission spectrum of the signal, as well as to match transmission line properties. The mapping of the original bits to the symbol codes is not constant in time and has a fairly large period (appearing as a pseudo-random sequence). The final mapping from symbols to PAM-5 line modulation levels obeys the table on the right.

Fiber
100BASE-FX
100BASE-FX is a version of Fast Ethernet over optical fibre. It uses a 1300 nm near-infrared (NIR) light wavelength transmitted via two strands of optical fibre, one for receive(RX) and the other for transmit(TX). Maximum length is 400 metres (1,310 ft) for half-duplex connections (to ensure collisions are detected) or 2 kilometres (6,600 ft) for full-duplex over multimode optical fiber. Longer distances are possible when using single-mode optical fiber. 100BASE-FX uses the same 4B5B encoding and NRZI line code that 100BASE-TX does. 100BASE-FX should use SC, ST, or MIC connectors with SC being the preferred option.

100BASE-FX is not compatible with 10BASE-FL, the 10 MBit/s version over optical fibre.

100BASE-SX
100BASE-SX is a version of Fast Ethernet over optical fibre. It uses two strands of multi-mode optical fibre for receive and transmit. It is a lower cost alternative to using 100BASE-FX, because it uses short wavelength optics which are significantly less expensive than the long wavelength optics used in 100BASE-FX. 100BASE-SX can operate at distances up to 300 metres (980 ft).

100BASE-SX uses the same wavelength as 10BASE-FL, the 10 MBit/s version over optical fibre. Unlike 100BASE-FX, this allows 100BASE-SX to be backwards-compatible with 10BASE-FL.

Because of the shorter wavelength used (850 nm) and the shorter distance it can support, 100BASE-SX uses less expensive optical components (LEDs instead of lasers) which makes it an attractive option for those upgrading from 10BASE-FL and those who do not require long distances.

100BASE-BX
100BASE-BX is a version of Fast Ethernet over a single strand of optical fibre (unlike 100BASE-FX, which uses a pair of fibres). Single-mode fibre is used, along with a special multiplexer which splits the signal into transmit and receive wavelengths.

Reference:
http://en.wikipedia.org/wiki/Fast_ethernet

There are several standards for Ethernet over twisted pair or copper-based computer networking physical connectivity methods. The currently most widely used of these are 10BASE-T, 100BASE-TX, and 1000BASE-T (Gigabit Ethernet), running at 10 Mbit/s, 100 Mbit/s, and 1000 Mbit/s (1 Gbit/s) respectively. These three standards all use the same connectors. Higher speed implementations nearly always support the lower speeds as well, so that in most cases different generations of equipment can be freely mixed. They use 8 position modular connectors, usually (but incorrectly) called RJ45 in the context of Ethernet over twisted pair. The cables usually used are four-pair Category 5 or above twisted pair cable. Each of the three standards support both full-duplex and half-duplex communication. According to the standards, they all operate over distances of 'up to 100 meters'.

The common names of the standards are derived from several aspects of the physical media. The number refers to the theoretical maximum transmission speed in megabits per second (Mbit/s). The BASE is short for baseband, meaning that there is no frequency-division multiplexing (FDM) or other frequency shifting modulation in use; each signal has full control of wire, on a single frequency. The T designates twisted pair cable, where the pairs of wires are twisted together for purposes of reducing crosstalk (FEXT and NEXT) when the pulsing direct current goes across the wires and creates electromagnetic induction effects. Where there are several standards for the same transmission speed, they are distinguished by a letter or digit following the T, such as TX. Some higher-speed standards use twin-axial cable, designated by CX.

Reference:
http://en.wikipedia.org/wiki/10BASE-T

Bit time is a concept in computer networking. It is defined as the time it takes for one bit to be ejected from a Network Interface Card (NIC) operating at some predefined standard speed, such as 10 Mbit/s. The time is measured between the time the logical link control layer 2 sublayer receives the instruction from the operating system until the bit actually leaves the NIC. It is important to note that the bit time has nothing to do with the time it takes for a bit to travel on the network medium, but has to do with the internals of the NIC.

To calculate the bit time at which a NIC ejects bits, use the following:

        bit time = 1 / NIC speed

To calculate the bit time for a 10 Mbit/s NIC, use the formula as follows:

        bit time = 1 / (10 * 10^6)
                 = 10^-7
                 = 100 * 10^-9
                 = 100 nanoseconds

The bit time for a 10 Mbit/s NIC is 100 nanoseconds. That is, a 10 Mbit/s NIC can eject 1 bit every 100 nanoseconds.

Bit time is distinctively different from slot time, which is the time taken for a pulse to travel through the longest permitted length of network medium.

Reference:
http://en.wikipedia.org/wiki/Bit_time

802.1D is the IEEE MAC Bridges standard which includes Bridging, Spanning Tree, interworking for 802.11 and others. It is standardized by the IEEE 802.1 working group.

VLANs (virtual LANs) are not part of 802.1D, but specified in 802.1Q.

Publishing history:
1990 - Original publication (802.1D-1990), based on the ISO/IEC 10038 standard
1998 - Revised version (802.1D-1998), incorporating the extensions 802.1p, P802.12e, 802.1j and 802.6k.
2004 - Revised version (802.1D-2004), incorporating the extensions 802.1t and 802.1w, which were separately published in 2001, and removing the original Spanning tree protocol, instead incorporating the Rapid Spanning Tree Protocol (RSTP) from 802.1w.

Reference:
http://en.wikipedia.org/wiki/IEEE_802.1D

The application layer is the seventh level of the seven-layer OSI model, and the fifth layer of the five-layer TCP/IP model. It interfaces directly to and performs common application services for the application processes; it also issues requests to the presentation layer.

The common application layer services provide semantic conversion between associated application processes. Note: Examples of common application services of general interest include the virtual file, virtual terminal, and job transfer and manipulation protocols.

The application layer of the four-layer and five-layer TCP/IP models corresponds to the application layer, the presentation layer and session layer in the seven layer OSI model.

Reference:
http://en.wikipedia.org/wiki/Application_layer

The presentation layer is the sixth level of the seven layer OSI model. It responds to service requests from the application layer and issues service requests to the session layer.

The presentation layer is responsible for the delivery and formatting of information to the application layer for further processing or display. It relieves the application layer of concern regarding syntactical differences in data representation within the end-user systems. Note: An example of a presentation service would be the conversion of an EBCDIC-coded text file to an ASCII-coded file.

The presentation layer is the first one where people start to care about what they are sending at a more advanced level than just a bunch of ones and zeros. This layer deals with issues like how strings are represented - whether they use the Pascal method (an integer length field followed by the specified amount of bytes) or the C/C++ method (null-terminated strings, i.e. "thisisastring\0"). The idea is that the application layer should be able to point at the data to be moved, and the Presentation layer will deal with the rest.

Encryption is typically done at this level too, though it can be done at the application, session, transport, or network layer; each having its own advantages and disadvantages. Another example is representing structure, which is normally standardised at this level, often by using XML. As well as simple pieces of data, like strings, more complicated things are standardised in this layer. Two common examples are 'objects' in object-oriented programming, and the exact way that streaming video is transmitted.

In many widely used applications and protocols, no distinction is made between the presentation and application layers. For example, HTTP, generally regarded as an application layer protocol, has presentation layer aspects such as the ability to identify character encodings for proper conversion, which is then done in the application layer.

List of Presentation layer services
Encryption
Compression

Sublayers
The presentation layer is composed of two sublayers:
- CASE (Common Application Service Element)
- SASE (Specific Application Service Element)

CASE
The CASE sublayer provides services for Application layer and request services from Presentation layer. It provides support for common application services, such as:
- ACSE (Association Control Service Element)
- ROSE (Remote Operation Service Element)
- CCR (Commitment Concurrency and Recovery)
- RTSE (Reliable Transfer Service Element)

SASE
The SASE sublayer provides application specific services (protocols), such as
- FTAM (File Transfer, Access and Manager)
- VT (Virtual Terminal)
- MOTIS (Message Oriented Text Interchange Standard)
- CMIP (Common Management Information Protocol)
- JTM (Job Transfer and Manipulation) a former OSI standard
- MMS (Manufacturing Messaging Service)
- RDA (Remote Database Access)
- DTP (Distributed Transaction Processing)

Reference:
http://en.wikipedia.org/wiki/Presentation_layer

The session layer is level five of the seven level OSI model. It responds to service requests from the presentation layer and issues service requests to the transport layer.

The session layer provides the mechanism for opening, closing and managing a session between end-user application processes, i.e. a semi-permanent dialogue. Communication sessions consist of requests and responses that occur between applications. Session layers are commonly used in application environments that make use of remote procedure calls (RPCs).

An example of a session layer protocol is the OSI protocol suite Session Layer Protocol, also known as X.225 or ISO 8327. In case of a connection loss this protocol may try to recover the connection. If a connection is not used for a long period, the session layer protocol may close it and re-open it. It provides for either full duplex or half-duplex operation and provides synchronization points in the stream of exchanged messages.

Other examples of session-layer implementations include Zone Information Protocol (ZIP) – the AppleTalk protocol that coordinates the name binding process; and Session Control Protocol (SCP) – the DECnet Phase IV session layer protocol.

In brief: the session layer establishes, manages and terminates connections (sessions) among cooperating applications. It also adds traffic flow information.

Session layer support in the Internet
The Session layer does not exist as a separate protocol layer or process in the four or five layer TCP/IP model, but its functionality is partly provided by the TCP/IP model transport layer, and partly by some TCP/IP model application layer protocols. However, a major part of its functionality is typically unused in Internet applications.

The OSI model made the session layer responsible for "graceful close" of sessions, which is a property of the TCP protocol, provided by the four-way SYN handshake process.

The OSI model also made the session layer responsible for session checkpointing and recovery, which is not usually used in the Internet protocols suite. However, there are a few applications layer protocols where the concept is useful. The idea is to allow information on different streams, perhaps originating from different sources, to be properly combined. In particular, it deals with synchronization issues, and ensuring nobody ever sees inconsistent versions of data, and similar things.

One application which is fairly intuitively clear is web conferencing. Here, we want to make sure that the streams of audio and video match up - or in other words, that we do not have lip synch problems. We may also want to do floor control - ensuring that the person displayed on screen and whose words are relayed is the one selected by the speaker, or by some other criteria.

Another big application is in live TV programs, where streams of audio and video need to be seamlessly merged from one to the other so that we do not have half a second of blank airtime, or half a second when we transmit two pictures simultaneously.

List of Session layer services
Authentication
Permissions
Session restoration (checkpointing and recovery)

Reference:
http://en.wikipedia.org/wiki/Session_layer