Frame relay

Defination

In the context of computer networking, frame relay consists of an efficient data transmission technique used to send digital information quickly and cheaply in a relay of frames to one or many destinations from one or many end-points. Network providers commonly implement frame relay for voice and data as an encapsulation technique, used between local area networks (LANs) over a wide area network (WAN). Each end-user gets a private line (or leased line) to a frame-relay node. The frame-relay network handles the transmission over a frequently-changing path transparent to all end-users.

As of 2006 native IP-based networks have gradually begun to displace frame relay. With the advent of MPLS, VPN and dedicated broadband services such as cable modem and DSL, the end may loom for the frame relay protocol and encapsulation. However many rural areas remain lacking DSL and cable modem services. In such cases the least expensive type of "always-on" connection remains a 128-kilobit frame-relay line. Thus a retail chain, for instance, may use frame relay for connecting rural stores into their corporate WAN.

Frame Relay description

The designers of frame relay aimed at a telecommunication service for cost-efficient data transmission for intermittent traffic between local area networks (LANs) and between end-points in a wide area network (WAN). Frame relay puts data in variable-size units called "frames" and leaves any necessary error-correction (such as re-transmission of data) up to the end-points. This speeds up overall data transmission. For most services, the network provides a permanent virtual circuit (PVC), which means that the customer sees a continuous, dedicated connection without having to pay for a full-time leased line, while the service-provider figures out the route each frame travels to its destination and can charge based on usage.

An enterprise can select a level of service quality - prioritizing some frames and making others less important. Frame relay can run on fractional T-1 or full T-carrier system carriers. Frame relay complements and provides a mid-range service between ISDN, which offers bandwidth at 128 kbit/s, and Asynchronous Transfer Mode (ATM), which operates in somewhat similar fashion to frame relay but at speeds from 155.520 Mbit/s to 622.080 Mbit/s.

Frame relay has its technical base in the older X.25 packet-switching technology, designed for transmitting analog data such as voice conversations. Unlike X.25, whose designers expected analog signals, frame relay offers a fast packet technology, which means that the protocol does not attempt to correct errors. When a frame relay network detects an error in a frame, it simply drops that frame. The end points have the responsibility for detecting and retransmitting dropped frames. (However, digital networks offer an incidence of error extraordinarily small relative to that of analog networks.)

Frame relay often serves to connect local area networks (LANs) with major backbones as well as on public wide-area networks (WANs) and also in private network environments with leased lines over T-1 lines. It requires a dedicated connection during the transmission period. Frame relay does not provide an ideal path for voice or video transmission, both of which require a steady flow of transmissions. However, under certain circumstances, voice and video transmission do use frame relay.

Frame relay relays packets at the data link layer (layer 2) of the Open Systems Interconnection (OSI) model rather than at the network layer (layer 3). A frame can incorporate packets from different protocols such as Ethernet and X.25. It varies in size up to a thousand bytes or more.

Frame Relay originated as an extension of Integrated Services Digital Network (ISDN). Its designers aimed to enable a packet-switched network to transport the circuit-switched technology. The technology has become a stand-alone and cost-effective means of creating a WAN.

Frame Relay switches create virtual circuits to connect remote LANs to a WAN. The Frame Relay network exists between a LAN border device, usually a router, and the carrier switch. The technology used by the carrier to transport the data between the switches is variable and changes between carrier (i.e. Frame Relay does not rely directly on the transportation mechanism to function.)

The sophistication of the technology requires a thorough understanding of the terms used to describe how Frame Relay works. Without a firm understanding of Frame Relay, it is difficult to troubleshoot its performance.

Frame Relay has become one of the most extensively-used WAN protocols. Its cheapness (compared to leased lines) provided one reason for its popularity. The extreme simplicity of configuring user equipment in a Frame Relay network offers another reason for Frame Relay's popularity.

Frame-relay frame structure essentially mirrors almost exactly that defined for LAP-D. Traffic analysis can distinguish frame relay format from LAP-D by its lack of a control field.

Each frame relay PDU consists of the following fields:

1. Flag Field. The flag is used to perform high level data link synchronization which indicates the beginning and end of the frame with the unique pattern 01111110. To ensure that the 01111110 pattern does not appear somewhere inside the frame, bit stuffing and destuffing procedures are used.
2. Address Field. Each address field may occupy either octet 2 to 3, octet 2 to 4, or octet 2 to 5, depending on the range of the address in use. A two-octet address field comprising the EA=ADDRESS FIELD EXTENSION BITS and the C/R=COMMAND/RESPONSE BIT.
3. DLCI-Data Link Connection Identifier Bits. The DLCI serves to identify the virtual connection so that the receiving end knows which information connection a frame belongs to. Note that this DLCI has only local significance. A single physical channel can multiplex several different virtual connections.
4. FECN, BECN, DE bits. These bits report congestion:
   • FECN=Forward Explicit Congestion Notification bit
   • BECN=Backward Explicit Congestion Notification bit
   • DE=Discard Eligibility bit
5. Information Field. A system parameter defines the maximum number of data bytes that a host can pack into a frame. Hosts may negotiate the actual maximum frame length at call set-up time. The standard specifies the maximum information field size (supportable by any network) as at least 262 octets. Since end-to-end protocols typically operate on the basis of larger information units, frame relay recommends that the network support the maximum value of at least 1600 octets in order to avoid the need for segmentation and reassembling by end-users.
6. Frame Check Sequence (FCS) Field. Since one cannot completely ignore the bit error-rate of the medium, each switching node needs to implement error detection to avoid wasting bandwidth due to the transmission of erred frames. The error detection mechanism used in frame relay uses the cyclic redundancy check (CRC) as its basis.

The frame relay network uses a simplified protocol at each switching node. It achieves simplicity by omitting link-by-link flow-control. As a result, the offered load has largely determined the performance of frame relay networks. In the case of high offered load is high, due to the bursts in some services, temporary overload at some frame relay nodes causes a collapse in network throughput. Therefore, frame-relay networks require some effective mechanisms to control the congestion.

Congestion control in frame-relay networks includes the following elements:

1. Admission Control. This provides the principle mechanism used in frame relay to ensure the guarantee of resource requirement once accepted. It also serves generally to achieve high network performance. The network decides whether to accept a new connection-request, based on the relation of the requested traffic-descriptor and the network's residual capacity. The traffic descriptor consists of a set of parameters communicated to the switching nodes at call set-up time or at service-subscription time, and which characterizes the connection's statistical properties. The traffic descriptor consists of three elements:
2. Committed Information Rate (CIR). The average rate (in bit/s) at which the network guarantees to transfer information units over a measurement interval T. This T interval is defined as: T = Bc/CIR .
3. Committed Burst Size (BC). The maximum number of information units transmittable during the interval T.
4. Excess Burst Size (BE). The maximum number of uncommitted information units (in bits) that the network will attempt to carry during the interval

Once the network has established a connection, the edge node of the frame relay network must monitor the connection's traffic flow to ensure that the actual usage of network resources does not exceed this specification. Frame relay defines some restrictions on the user's information rate. It allows the network to enforce the end user's information rate and discard information when the subscribed access rate is exceeded.

Explicit congestion notification is proposed as the congestion avoidance policy. It tries to keep the network operating at its desired equilibrium point so that a certain QOS for the network can be met. To do so, special congestion control bits have been incorporated into the address field of the frame relay: FECN and BECN. The basic idea is to avoid data accumulation inside the network. FECN means Forward Explicit Congestion Notification. FECN bit can be set to 1 to indicate that congestion was experienced in the direction of the frame transmission, so it informs the destination that congestion has occurred. BECN means Backwards Explicit Congestion Notification. BECN bit can be set to 1 to indicate that congestion was experienced in the network in the direction opposite of the frame transmission, so it informs the sender that congestion has occurred.

Speeds

Frame Relay is available in the following speeds (type and speed of frame relay may vary by ILEC): 56 kbit/s, 64 kbit/s, 128 kbit/s, 256 kbit/s, 512 kbit/s, 1.5 Mbit/s, and 2 Mbit/s.

Frame Relay versus X.25

The design of X.25 aimed to provide error-free delivery over links with high error-rates. Frame relay takes advantage of the new links with lower error-rates, enabling it to eliminate many of the services provided by X.25. The elimination of functions and fields, combined with digital links, enables frame relay to operate at speeds 20 times greater than X.25.

X.25 specifies processing at layers 1, 2 and 3 of the OSI model, while frame relay operates at layers 1 and 2 only. This means that frame relay has significantly less processing to do at each node, which improves throughput by an order of magnitude.

X.25 prepares and sends packets, while frame relay prepares and sends frames. X.25 packets contain several fields used for error and flow control, none of which frame relay needs. The frames in frame relay contain an expanded address field that enables frame relay nodes to direct frames to their destinations with minimal processing .

X.25 has a fixed bandwidth available. It uses or wastes portions of its bandwidth as the load dictates. Frame relay can dynamically allocate bandwidth during call setup negotiation at both the physical and logical channel level.

Virtual circuits

As a WAN protocol, frame relay is most commonly implemented at Layer 2 (data link layer) of the Open Systems Interconnection (OSI) seven layer model. Two types of circuits exist: permanent virtual circuits (PVCs) which are used to form logical end-to-end links mapped over a physical network, and switched virtual circuits (SVCs). The latter analogous to the circuit-switching concepts of the public-switched telephone network (or PSTN), the global phone network we are most familiar with today. While SVCs exist and are part of the frame relay specification, they are rarely applied to real-world scenarios. SVCs are most often considered harder to configure and maintain and are generally avoided without appropriate justification.

X.25 origins

Frame relay began as a stripped-down version of the X.25 protocol, releasing itself from the error-correcting burden most commonly associated with X.25. When frame relay detects an error, it simply drops the offending packet. Frame relay uses the concept of shared-access and relies on a technique referred to as "best-effort", whereby error-correction practically does not exist and practically no guarantee of reliable data delivery occurs. Frame relay provides an industry-standard encapsulation utilizing the strengths of high-speed, packet-switched technology able to service multiple virtual circuits and protocols between connected devices, such as two routers.

Eric Scace, an engineer at Sprint International, invented Frame Relay. He based the design on earlier work of his in the development of AX.25, a frame relay-like packet switching protocol created by amateur radio operators. Sprint International (as of 2005 a part of Sprint Nextel) subsequently contracted with StrataCom for the first implementations, and deployed StrataCom hardware in its public data network to offer the first frame relay public service.

Local Management Interface (LMI)

Initial proposals for Frame Relay were presented to the Consultative Committee on International Telephone and Telegraph (CCITT) in 1984. Lack of interoperability and standardisation, prevented any significant Frame Relay deployment until 1990 when Cisco, Digital Equipment Corporation (DEC), Northern Telecom, and StrataCom formed a consortium to focus on its development. They produced a protocol that provided additional capabilities for complex inter-networking environments. These Frame Relay extensions are referred to as the Local Management Interface (LMI).

Datalink Connection Identifiers (or DLCIs) are numbers that refer to paths through the frame relay network. They are only locally significant, which means that when device-A sends data to device-B it will most-likely use a different DLCI than device-B would use to reply. Multiple virtual circuits can be active on the same physical end-points (performed by using subinterfaces).

Committed Information Rate (CIR)

Frame relay connections are often given a Committed Information Rate (CIR) and an allowance of burstable bandwidth known as the Extended Information Rate (EIR). The provider guarantees that the connection will always support the CIR rate, and sometimes the EIR rate should there be adequate bandwidth. Frames that are sent in excess of the CIR are marked as "discard eligible" (DE) which means they can be dropped should congestion occur within the frame relay network. Frames sent in excess of the EIR are dropped immediately.

Market reputation

Frame relay aimed to make more efficient use of existing physical resources, which allow for the underprovisioning of data services by telecommunications companies (telcos) to their customers, as clients were unlikely to be utilizing a data service 100 percent of the time. In more recent years, frame relay has acquired a bad reputation in some markets because of excessive bandwidth overbooking by these telcos.

Telcos often sell frame relay to businesses looking for a cheaper alternative to dedicated lines; its use in different geographic areas depended greatly on governmental and telecommunication companies' policies. Some of the early companies to make frame relay products included StrataCom (later acquired by Cisco Systems) and Cascade Communications (later acquired by Ascend Communications and then by Lucent Technologies).

(From wikipedia)

Asymmetric Digital Subscriber Line (ADSL)

Defination

Asymmetric Digital Subscriber Line (ADSL) is a form of DSL, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. It does this by utilizing frequencies that are not used by a voice telephone call. By using a splitter or micro filters this allows a single telephone connection to be used for both ADSL service and voice calls at the same time. As phone lines are so varied in quality and weren't initially provisioned with ADSL in mind it can generally only be used over short distances, typically less than 5 km.

At the telephone exchange the line generally terminates at a DSLAM where another frequency splitter separates the voice band signal for the conventional phone network. The ATM stream carried by the ADSL physical layer is typically routed over the telephone company's data network to service center where the encapsulated IP packets are eventually routed onto a conventional internet network.

Explanation

The distinguishing characteristic of ADSL over other forms of DSL is that the volume of data flow is greater in one direction than the other, i.e. it is asymmetric. Providers usually market ADSL as a service for consumers to connect to the Internet in a relatively passive mode: able to use the higher speed direction for the "download" from the Internet but not needing to run servers that would require high speed in the other direction.

There are both technical and marketing reasons why ADSL is in many places the most common type offered to home users. On the technical side, there is likely to be more crosstalk from other circuits at the DSLAM end (where the wires from many local loops are close together) than at the customer premises. Thus the upload signal is weakest at the noisiest part of the local loop, while the download signal is strongest at the noisiest part of the local loop. It therefore makes technical sense to have the DSLAM transmit at a higher bit rate than does the modem on the customer end. Since the typical home user in fact does prefer a higher download speed, the telephone companies chose to make a virtue out of necessity, hence ADSL. On the marketing side, limiting upload speeds limits the attractiveness of this service to business customers, often causing them to purchase higher cost Digital Signal 1 services instead. In this fashion, it segments the digital communications market between business and home users.

On the wire

ADSL uses two separate frequency bands, referred to as the upstream and downstream bands. The upstream band is used for communication from the end user to the telephone central office. The downstream band is used for communicating from the central office to the end user. With standard ADSL (annex A), the band from 25,875 kHz to 138 kHz is used for upstream communication, while 138 kHz – 1104 kHz is used for downstream communication.

Frequency plan for ADSL. The red area is the frequency range used by normal voice telephony, the green and blue areas are used for ADSL.

Each of these is further divided into smaller frequency channels of 4.3125 kHz. During initial training, the ADSL modem tests which of the available channels have an acceptable signal-to-noise ratio. The distance from the telephone exchange, noise on the copper wire, or interference from AM radio stations may introduce errors on some frequencies. By keeping the channels small, a high error rate on one frequency thus need not render the line unusable: the channel will not be used, merely resulting in reduced throughput on an otherwise functional ADSL connections.

Vendors may support usage of higher frequencies as a proprietary extension to the standard. However, this requires matching vendor-supplied equipment on both ends of the line, and will likely result in crosstalk issues that affect other lines in the same bundle.

There is a direct relationship between the number of channels available and the throughput capacity of the ADSL connection. The exact data capacity per channel depends on the modulation method used.

A common error is to attribute the A in ADSL to the word asynchronous. ADSL technologies use a synchronous framed protocol for data transmission on the wire.

Modulation

ADSL initially existed in two flavors (similar to VDSL), namely CAP and DMT. CAP was the de facto standard for ADSL deployments up until 1996, deployed in 90 percent of ADSL installs at the time. However, DMT was chosen for the first ITU-T ADSL standards, G.992.1 and G.992.2 (also called G.dmt and G.lite respectively). Therefore all modern installations of ADSL are based on the DMT modulation scheme.

ADSL standards

Standard name	Common name	Downstream rate	Upstream rate
ANSI T1.413-1998 Issue 2	ADSL	8 Mbit/s	1.0 Mbit/s
ITU G.992.1	ADSL (G.DMT)	8 Mbit/s	1.0 Mbit/s
ITU G.992.1 Annex A	ADSL over POTS	8 Mbit/s	1.0 MBit/s
ITU G.992.1 Annex B	ADSL over ISDN	8 Mbit/s	1.0 MBit/s
ITU G.992.2	ADSL Lite (G.Lite)	1.5 Mbit/s	0.5 Mbit/s
ITU G.992.3/4	ADSL2	12 Mbit/s	1.0 Mbit/s
ITU G.992.3/4 Annex J	ADSL2	12 Mbit/s	3.5 Mbit/s
ITU G.992.3/4 Annex L[1]	RE-ADSL2	5 Mbit/s	0.8 Mbit/s
ITU G.992.5	ADSL2+	24 Mbit/s	1.0 Mbit/s
ITU G.992.5 Annex L[1]	RE-ADSL2+	24 Mbit/s	1.0 Mbit/s
ITU G.992.5 Annex M	ADSL2+M	24 Mbit/s	3.5 Mbit/s

Annexes J and M shift the upstream/downstream frequency split up to 276 kHz (from 138 kHz used in the commonly deployed annex A) in order to boost upstream rates. Additionally, the "all-digital-loop" variants of ADSL2 and ADSL2+ (annexes I and J) support an extra 256 kbit/s of upstream if the bandwidth normally used for POTS voice calls is allocated for ADSL usage.

While the ADSL access utilizes the 1.1 MHz band, ADSL2+ utilizes the 2.2 MHz band.

The downstream and upstream rates displayed are theoretical maximums. Note also that because Digital subscriber line access multiplexers and ADSL modems may have been implemented based on differing or incomplete standards some manufacturers may advertise different speeds. For example, Ericsson has several devices that support non-standard upstream speeds of up to 2 Mbit/s in ADSL2 and ADSL2+.

Installation issues

Due to the way it uses the frequency spectrum, ADSL deployment presents some issues. It is necessary to install appropriate frequency filters at the customer's premises, to avoid interferences with the voice service, while at the same time taking care to keep a clean signal level for the ADSL connection.

ADSL Router by UTStarcom

In the early days of DSL, installation required a technician to visit the premises. A splitter was installed near the demarcation point, from which a dedicated data line was installed. This way, the DSL signal is separated earlier and is not attenuated inside the customer premises. However, this procedure is costly, and also caused problems with customers complaining about having to wait for the technician to perform the installation. As a result, many DSL vendors started offering a self-install option, in which they ship equipment and instructions to the customer. Instead of separating the DSL signal at the demarcation point, the opposite is done: the DSL signal is "filtered off" at each phone outlet by use of a low pass filter, also known as microfilter. This method does not require any rewiring inside the customer premises.

A side effect of the move to the self-install model is that the DSL signal can be degraded, especially if more than 5 voiceband devices are connected to the line. The DSL signal is now present on all telephone wiring in the building, causing attenuation and echo. A way to circumvent this is to go back to the original model, and install one filter upstream from all telephone jacks in the building, except for the jack to which the DSL modem will be connected. Since this requires wiring changes by the customer and may not work on some (poorly designed) household telephone wiring, it is rarely done. It is usually much easier to install filters at each telephone jack that is in use.

(From wikipedia)

Integrated Services Digital Network ( ISDN)

Defination

Integrated Services Digital Network (ISDN) is a circuit-switched telephone networkdigital transmission of voice and data over ordinary telephone copper wires, resulting in better quality and higher speeds than that is available with the PSTN system. More broadly, ISDN is a set of protocols for establishing and breaking circuit switched connections, and for advanced call features for the user. system, designed to allow

In a videoconference, ISDN provides simultaneous voice, video, and text transmission between individual desktop videoconferencing systems and group (room) videoconferencing systems.

ISDN elements

The English term is a backronym that was thought to be better for English-language advertisements than the original, "Integriertes Sprach- und Datennetz" (German for "Integrated Speech and Data Net").

• Integrated Services refers to ISDN's ability to deliver at minimum two simultaneous connections, in any combination of data, voice, video, and fax, over a single line. Multiple devices can be attached to the line, and used as needed. That means an ISDN line can take care of most people's complete communications needs at a much higher transmission rate, without forcing the purchase of multiple analog phone lines.

• Digital refers to its purely digital transmission, as opposed to the analog transmission of plain old telephone service (POTS). Use of an analog telephone modem for Internet access requires that the Internet service provider's (ISP) modem has converted the website's digital content to analog signals before sending it back and the modem then converts those signals back to digital when receiving. When connecting with ISDN there is no analog conversion. ISDN transmits data digitally, resulting in a very clear transmission quality. There is none of the static and noise of analog transmissions that can cause slow transmission speed.

• Network refers to the fact that ISDN is not simply a point-to-point solution like a leased line. ISDN networks extend from the local telephone exchange to the remote user and includes all of the telecommunications and switching equipment in between.

The purpose of the ISDN is to provide fully integrated digital services to the users. These services fall under three categories: bearer services, supplementary services and teleservices.

Consumer and industry perspectives

There are two points of view into the ISDN world. The most common viewpoint is that of the end user, who wants to get a digital connection into the telephone/data network from home, whose performance would be better than an ordinary analog modem connection. The typical end-user's connection to the Internet is related to this point of view, and talk about the merits of various ISDN modems, carriers' offerings and tarriffing (features, pricing) are from this perspective. Much of the following discussion is from this point of view, but it should be noted that as a data connection service, ISDN has been mostly superseded by DSL.

There is, however, a second viewpoint: that of the telephone industry, where ISDN is not a dead issue. A telephone network can be thought of as a collection of wires strung between switching systems. The common electrical specification for the signals on these wires is T1 or E1. On a normal T1, the signalling is done with A&B bits to indicate on-hook or off-hook conditions and MF and DTMF tones to encode the destination number. ISDN is much better because messages can be sent much more quickly than by trying to encode numbers as long (100 ms per digit) tone sequences. This translated to much faster call setup times, which is greatly desired by carriers who have to pay for line time and also by callers who become impatient while their call hops from switch to switch.

It is also used as a smart-network technology intended to add new services to the public switched telephone network (PSTN) by giving users direct access to end-to-end circuit-switched digital services.

ISDN BRI (Basic Rate Interface) has never gained popularity as a telephone access technology in North America and today remains a niche product. However, most modern non-VoIP PBXs use PRI (Primary Rate Interface) T1 lines to communicate with a Telco Class 5 central office switch, replacing older analog two-way and Direct Inward Dialing (DID) trunks. PRI is capable of delivering caller ID in both directions so that the telephone number of an extension, rather than a company's main number, can be sent. It is still commonly used in recording studios, when a voice-over actor is in one studio, but the director and producer are in a studio at another location. ISDN is used because of its "guaranteed" real-time, not-over-the-Internet service, and its superior audio fidelity as compared to POTS service. A few companies make video conference call equipment that combine three BRI lines and six 64K channels to create a good quality picture.

In Japan, it became popular to some extent from around 1999 to 2001, but now that ADSL has been introduced, the number of subscribers is in decline. NTT, a dominant Japanese telephone company, provides an ISDN service with the names INS64 and INS1500, which are much less recognized than ISDN.

In the UK, British Telecom (BT) provides ISDN2e (BRI) as well as ISDN30 (PRI). Until April 2006, they also offered Home Highway and Business Highway, which are BRI ISDN-based services that offer integrated analogue connectivity as well as ISDN. Later versions of the Highway products also included built-in USB sockets for direct computer access. Home Highway has been bought by many home users, usually for Internet connection, although not as fast as ADSL, because it was available before ADSL and in places where ADSL does not reach.

France Télécom offers ISDN services under their product name Numeris (2 B+D), of which a professional Duo and home Itoo version is available. ISDN is generally known as RNIS in France and has widespread availability. The introduction of ADSL is reducing ISDN use for data transfer and Internet access, although it is still common in more rural and outlying areas, and for applications such as business voice and point-of-sale terminals.

In Germany, ISDN is very popular with an installed base of 25 million channels (29% of all subscriber lines in Germany as of 2003 and 20% of all ISDN channels worldwide). Due to the success of ISDN, the number of installed analog lines is decreasing. Deutsche Telekom (DTAG) offers both BRI and PRI. Competing phone companies often offer ISDN only and no analog lines. Because of the widespread availability of ADSL services, ISDN is today primarily used for voice traffic, but is still very popular thanks to the pricing policy of German telcos. Today ISDN (BRI) and ADSL/VDSL are often bundled on the same line.

In India, ISDN was very popular until the introduction of ADSL. Bharat Sanchar Nigam Limited (A Govt. Of India Ent.), the largest communication service provider in India, is offering both ISDN BRI and PRI services across the country over its ISDN network. After the introduction of ADSL broadband technology with static IPs, the data transfer load is taken up by ADSL. But ISDN still plays a very big role as a backup network for point-to-point leased line customers and low cost reliable data network for organisations located all over India, such as Banks,E-seva centres, LIC, and so on.

Configurations

In ISDN, there are two types of channels, B (for "Bearer") and D (for "Delta"). B channels are used for data (which may include voice), and D channels are intended for signaling and control (but can also be used for data).

There are three ISDN implementations. Basic rate interface (BRI) — also Basic rate access (BRA) — consists of two B channels, each with bandwidth of 64 kbit/s, and one D channel with a bandwidth of 16 kbit/s. Together these three channels can be designated as 2B+D. Primary rate interface (PRI) — also Primary rate access (PRA) — contains a greater number of B channels and a D channel with a bandwidth of 64 kbit/s. The number of B channels for PRI varies according to the nation: in North America and Japan it is 23B+1D, with an aggregate bit rate of 1.544 Mbit/s (T1); in Europe, India and Australia it is 30B+1D, with an aggregate bit rate of 2.048 Mbit/s (E1). Broadband Integrated Services Digital NetworkBISDN) is another ISDN implementation and it is able to manage different types of services at the same time. It is primarily used within network backbones and employs ATM. (

Another alternative ISDN configuration can be used in which the B channels of an ISDN basic rate interface are bonded to provide a total duplex bandwidth of 128 kbit/s. This precludes use of the line for voice calls while the internet connection is in use.

Using bipolar with eight-zero substitution encoding technique, call data is transmitted over the data (B) channels, with the signalling (D) channels used for call setup and management. Once a call is set up, there is a simple 64 kbit/s synchronous bidirectional data channel between the end parties, lasting until the call is terminated. There can be as many calls as there are data channels, to the same or different end-points. Bearer channels may also be multiplexed into what may be considered single, higher-bandwidth channels via a process called B channel bonding.

The D channel can also be used for sending and receiving X.25 data packets, and connection to X.25 packet network, this is specified in X.31. In practice, X.31 was only commercially implemented in France and Japan.

Types of communications

Among the kinds of data that can be moved over the 64 kbit/s channels are pulse-code modulated voice calls, providing access to the traditional voice PSTN. This information can be passed between the network and the user end-point at call set-up time. In North America, ISDN is now used mostly as an alternative to analog connections, most commonly for Internet access. Some of the services envisioned as being delivered over ISDN are now delivered over the Internet instead. In Europe, and in Germany in particular, ISDN has been successfully marketed as a phone with features, as opposed to a POTS phone (Plain Old Telephone Service) with few or no features. Meanwhile, features that were first available with ISDN (such as Three-Way Call, Call Forwarding, Caller ID, etc.) are now commonly available for ordinary analog phones as well, eliminating this advantage of ISDN. Another advantage of ISDN was the possibility of multiple simultaneous calls (one call per B channel), e.g. for big families, but with the increased popularity and reduced prices of mobile telephony this has become less interesting as well, making ISDN unappealing to the private customer. However, ISDN is typically more reliable than POTS, and has a significantly faster call setup time compared with POTS, and IP connections over ISDN typically have some 30–35ms round trip time, as opposed to 120–180ms (both measured with otherwise unused lines) over 56k or V.34 modems, making ISDN more pleasant for telecommuters.

Where an analog connection requires a modem, an ISDN connection requires a terminal adapterADSL-routers. (TA). The function of an ISDN terminal adapter is often delivered in the form of a PC card with an S/T interface, and single-chip solutions seem to exist, considering the plethora of combined ISDN- and

ISDN is commonly used in radio broadcasting. Since ISDN provides a high quality connection this assists in delivering good quality audio for transmission in radio. Most radio studios are equipped with ISDN lines as their main form of communication with other studios or standard phone lines.

(From wikipedia)

OSI model

Defination

The Open Systems Interconnection Basic Reference Model (OSI Reference Model or OSI Model for short) is a layered, abstract description for communications and computer network protocol design, developed as part of Open Systems Interconnection (OSI) initiative. It is also called the OSI seven layer model. The layers, described below, are, from top to bottom, Application, Presentation, Session, Transport, Network, Data Link and Physical.

Even though newer IETF and IEEE protocols, and indeed OSI protocol work subsequent to the publication of the original architectural standards have largely superseded it, the OSI model is an excellent place to begin the study of network architecture. Not understanding that the pure seven-layer model is more historic than current, many beginners make the mistake of trying to fit every protocol they study into one of the seven basic layers. Those beginners need to accept that protocols not designed against the OSI model may not cleanly fit into it.

History

In 1977, the International Organization for Standardization (ISO), began to develop its OSI networking suite. OSI has two major components: an abstract model of networking (the Basic Reference Model, or seven-layer model), and a set of concrete protocols. The standard documents that describe OSI are for sale and not currently available online.

Parts of OSI have influenced Internet protocol development, but none more than the abstract model itself, documented in ISO 7498 and its various addenda. In this model, a networking system is divided into layers. Within each layer, one or more entities implement its functionality. Each entity interacts directly only with the layer immediately beneath it, and provides facilities for use by the layer above it.

In particular, Internet protocols are deliberately not as rigorously architected as the OSI model, but a common version of the TCP/IP model splits it into four layers. The Internet Application Layer includes the OSI Application Layer, Presentation Layer, and most of the Session Layer. Its End-to-End Layer includes the graceful close function of the OSI Session Layer as well as the Transport Layer. Its Internetwork Layer is equivalent to the OSI Network Layer, while its Interface layer includes the OSI Data Link and Physical Layers. These comparisons are based on the original seven-layer protocol model as defined in ISO 7498, rather than refinements in such things as the Internal Organization of the Network Layer document.

Protocols enable an entity in one host to interact with a corresponding entity at the same layer in a remote host. Service definitions abstractly describe the functionality provided to a (N)-layer by an (N-1) layer, where N is one of the seven layers inside the local host.

Description of OSI layers

Layer 7: Application layer

The application layer is the seventh level of the seven-layer OSI model. It interfaces directly to and performs common application services for the application processes; it also issues requests to the presentation layer. Note carefully that this layer provides services to user-defined application processes, and not to the end user. For example, it defines a file transfer protocol, but the end user must go through an application process to invoke file transfer. The OSI model does not include human interfaces.

The common application services sublayer provides functional elements including the Remote Operations Service Element (comparable to Internet Remote Procedure Call), Association Control, and Transaction Processing (according to the ACID requirements).

Above the common application service sublayer are functions meaningful to user application programs, such as messaging (X.400), directory (X.500), file transfer (FTAM), virtual terminal (VTAM), and batch job manipulation (JTAM).

Layer 6: Presentation layer

The Presentation layer transforms the data to provide a standard interface for the Application layer. MIME encoding, data encryption and similar manipulation of the presentation are done at this layer to present the data as a service or protocol developer sees fit. Examples of this layer are converting an EBCDIC-coded text file to an ASCII-coded file, or serializing objects and other data structures into and out of XML.

Layer 5: Session layer

The Session layer controls the dialogues/connections (sessions) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for either full-duplex or half-duplex operation, and establishes checkpointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for "graceful close" of sessions, which is a property of TCP, and also for session checkpointing and recovery, which is not usually used in the Internet protocols suite.

Layer 4: Transport layer

The Transport layer provides transparent transfer of data between end users, providing reliable data transfer while relieving the upper layers of it. The transport layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state and connection oriented. This means that the transport layer can keep track of the segments and retransmit those that fail. The best known example of a layer 4 protocol is the Transmission Control Protocol (TCP). The transport layer is the layer that converts messages into TCP segments or User Datagram Protocol (UDP), Stream Control Transmission Protocol (SCTP), etc. packets. Perhaps an easy way to visualize the Transport Layer is to compare it with a Post Office, which deals with the dispatch and classification of mail and parcels sent. Do remember, however, that a post office manages the outer envelope of mail. Higher layers may have the equivalent of double envelopes, such as cryptographic Presentation services that can be read by the addressee only. Roughly speaking, tunneling protocols operate at the transport layer, such as carrying non-IP protocols such as IBM's SNA or Novell's IPXIPsec. While Generic Routing EncapsulationL2TP carries PPP frames inside transport packets. over an IP network, or end-to-end encryption with (GRE) might seem to be a network layer protocol, if the encapsulation of the payload takes place only at endpoint, GRE becomes closer to a transport protocol that uses IP headers but contains complete frames or packets to deliver to an endpoint.

Layer 3: Network layer

The Network layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport layer. The Network layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors. Routers operate at this layer—sending data throughout the extended network and making the Internet possible. This is a logical addressing scheme – values are chosen by the network engineer. The addressing scheme is hierarchical. The best known example of a layer 3 protocol is the Internet Protocol (IP). Perhaps it's easier to visualize this layer as managing the sequence of human carriers taking a letter from the sender to the local post office, trucks that carry sacks of mail to other post offices or airports, airplanes that carry airmail between major cities, trucks that distribute mail sacks in a city, and carriers that take a letter to its destinations. Think of fragmentation as splitting a large document into smaller envelopes for shipping, or, in the case of the network layer, splitting an application or transport record into packets.

Layer 2: Data Link layer

The Data Link layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical layer. The best known example of this is Ethernet. This layer manages the interaction of devices with a shared medium. Other examples of data link protocols are HDLC and ADCCP for point-to-point or packet-switched networks and Aloha for local area networks. On IEEE 802FDDI, this layer may be split into a Media Access Control (MAC) layer and the IEEE 802.2 Logical Link Control (LLC) layer. It arranges bits from the physical layer into logical chunks of data, known as frames. local area networks, and some non-IEEE 802 networks such as

This is the layer at which the bridges and switches operate. Connectivity is provided only among locally attached network nodes forming layer 2 domains for unicast or broadcast forwarding. Other protocols may be imposed on the data frames to create tunnels and logically separated layer 2 forwarding domain.

The data link layer might implement a sliding window flow control and acknowledgment mechanism to provide reliable delivery of frames; that is the case for SDLC and HDLC, and derivatives of HDLC such as LAPB and LAPD. In modern practice, only error detection, not flow control using sliding window, is present in modern data link protocols such as Point-to-Point Protocol (PPP), and, on local area networks, the IEEE 802.2 LLC layer is not used for most protocols on Ethernet, and, on other local area networks, its flow control and acknowledgment mechanisms are rarely used. Sliding window flow control and acknowledgment is used at the transport layers by protocols such as TCP.

Layer 1: Physical layer

The Physical layer defines all the electrical and physical specifications for devices. In particular, it defines the relationship between a device and a physical medium. This includes the layout of pins, voltages, and cable specifications. Hubs, repeaters, network adapters and Host Bus Adapters (HBAs used in Storage Area Networks) are physical-layer devices. The major functions and services performed by the physical layer are:

• Establishment and termination of a connection to a communications medium.
• Participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control.
• Modulation, or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling (such as copper and fiber optic) or over a radio link.

Parallel SCSI buses operate in this layer. Various physical-layer Ethernet standards are also in this layer; Ethernet incorporates both this layer and the data-link layer. The same applies to other local-area networks, such as Token ring, FDDI, and IEEE 802.11, as well as personal area networks such as Bluetooth and IEEE 802.15.4.

OSI Model
	Data unit	Layer	Function
Host layers	Data	Application	Network process to application
		Presentation	Data representation and encryption
		Session	Interhost communication
	Segments	Transport	End-to-end connections and reliability (TCP)
Media layers	Packets	Network	Path determination and logical addressing (IP)
	Frames	Data link	Physical addressing (MAC & LLC)
	Bits	Physical	Media, signal and binary transmission

Interfaces

In addition to standards for individual protocols in transmission, there are also interface standards for different layers to talk to the ones above or below (usually operating-system–specific). For example, Microsoft Windows' Winsock, and Unix's Berkeley sockets and System V Transport Layer Interface, are interfaces between applications (layers 5 and above) and the transport (layer 4). NDIS and ODI are interfaces between the media (layer 2) and the network protocol (layer 3).

OSI Service Specifications are abstractions of functionality commonly present in programming interfaces.

Examples

Layer		Misc. examples	TCP/IP suite	SS7	AppleTalk suite	OSI suite	IPX suite	SNA	UMTS
#	Name
7	Application	NNTP, HL7, Modbus, SIP, SSI	DHCP, DNS, FTP, Gopher, HTTP, NFS, NTP, RTP, SMPP, SMTP, SNMP, Telnet	ISUP, INAP, MAP, TUP, TCAP	AFP	FTAM, X.400, X.500, DAP		APPC
6	Presentation	TDI, ASCII, EBCDIC, MIDI, MPEG	MIME, XDR, SSL, TLS (Not a separate layer)		AFP	ISO 8823, X.226
5	Session	Named Pipes, NetBIOS, SAP, SDP	Sockets. Session establishment in TCP. SIP. (Not a separate layer with standardized API.)		ASP, ADSP, ZIP, PAP	ISO 8327, X.225	NWLink	DLC?
4	Transport	NetBEUI, nanoTCP, nanoUDP	TCP, UDP, SCTP		ATP, NBP, AEP, RTMP	TP0, TP1, TP2, TP3, TP4	SPX
3	Network	NetBEUI, Q.931	IP, ICMP, IPsec, ARP, RIP, OSPF	MTP-3, SCCP	DDP	X.25 (PLP), CLNP	IPX		RRC (Radio Resource Control)
2	Data Link	802.3 (Ethernet), 802.11a/b/g/n MAC/LLC, 802.1Q (VLAN), ATM, CDP, FDDI, Fibre Channel, Frame Relay, HDLC, ISL, PPP, Q.921, Token Ring	PPP, SLIP, PPTP, L2TP	MTP-2	LocalTalk, TokenTalk, EtherTalk, AppleTalk Remote Access, PPP	X.25 (LAPB), Token Bus	IEEE 802.3Ethernet II framing framing,	SDLC	RLC (Radio Link Control), MACMedia Access Control), PDCPPacket Data Convergence Protocol) and Broadcast/Multicast Control (BMC). ( (
1	Physical	RS-232, V.35, V.34, I.430, I.431, T1, E1, 10BASE-T, 100BASE-TX, POTS, SONET, DSL, 802.11a/b/g/n PHY		MTP-1	RS-232, RS-422, STP, PhoneNet	X.25 (X.21bis, EIA/TIA-232, EIA/TIA-449, EIA-530, G.703)		Twinax	UMTS L1 (UMTS Physical Layer)

(from Wikipedia)