Asynchronous transfer mode (ATM)

ATM is a member of the fast packet−switching family called cell relay. As part of its heritage, it is an evolution from many other sets of protocols. In fact, ATM is a statistical time−division multiplexed (TDMed) form of traffic that is designed to carry any form of traffic and enables the traffic to be delivered asynchronously to the network.


How an ATM sends data?

Speed comparison between ATM over other technology and protocols.

                With this information we can derive is that the ATM technique is a combination of TDM, with cells using pre-assigned slots, and Statistical TDM, with cells using whatever slots are available or needed to handle a particular traffic flow. It is also a connection−oriented protocol much the same as dialup voice communications services, but it uses virtual circuits, such as permanent virtual circuits (PVCs) and switched virtual circuits (SVC), to handle the connection.

Advantage of ATM over other technology and protocols

                ATM was designed from the ground up to work across the various places where we communicate: the Local Area Network (LAN), the Campus Area Network (CAN), and the Metropolitan

Area Network (MAN), also the Wide Area Network (WAN).

Speed comparison between ATM over other technology and protocols.

ATM features and functions

ATM Protocols

It takes many protocols to support an ATM network, which is one of the issues that continually Comes up as a negative from the supporters of the gigabit Ethernet crowd. The actual protocols needed depend on where the traffic originates, what transport mechanisms must be traversed, and where the traffic will terminate.

To understand more about this protocol here is an image that will show you the graphic representation of the ATM protocol interfaces and a table that shows where protocols are used for ATM

The problem with ATM is that in order to support the older legacy systems, many protocol points and interfaces are necessary. To get around the problem of "forklift" changes, the necessary protocols have been developed.

Mapping Circuits through an ATM Network

ATM uses one of two connection types, namely PVC and SVC. There is actually no permanency to the circuits. They are logically mapped through the network and are used when Needed for PVC or dial−connected when using the SVC, the carriers promise only to make a best attempt to serve the needs of the end user when the time is appropriate.

The concept is that the network provider will provide a committed bandwidth available to the user on demand whenever the user wants to use it. This forms the basis of what ATM networks are all about: on−demand, high−speed communications networks.

The connection is built into a routing table in each of the switches involved with the connection from end to end. As such, the switches only need to look up a table for the incoming port and channel and then determine the mapping (in the same table) for the output port and channel. Using virtual path identifiers (VPI) and virtual channel identifiers (VCI), the carrier maps the table.

This figure shows a full virtual connection is mapped through the various switches across the network. Here the end−user device is connected across an ATM access link through a switch. The switches provide the cross connect ion and link to the next downstream node. Note that the connection from the end user to the network may be on a T1, T3, or OC−n. From the first switch out, the network will use Synchronous Optical Network (SONET) or synchronous Digital Hierarchy (SDH) capabilities possibly mapped onto a Dense Wave Division Multiplexer (DWDM). The network carrier will use whatever services and bandwidth is available at the connection points.

The network switches handle the mapping on the basis of VPI switching.  VPI switching means that the switches use the virtual path for mapping through the network and will remap from one virtual path to another, while the virtual channel number is held consistent through the entire network.

A second alternative is to use VPI/VCI Switching, In this case, the ATM switches along the route will switch and remap both on a virtual path and a virtual channel, the virtual path and virtual channel switches process and remap both elements.

The OSI and ATM Layered Architecture

This is the upper-layer services of ATM.

The types of ALL and services offered.

ATM Traffic Management

                Some requirements are needed for the ATM to manage the traffic.

• ATM must be flexible. It must meet the constantly changing demands of the user population. These goals mean that the demands for traffic will rise or fall as necessary, and therefore managing this traffic is of paramount importance.
• ATM must meet the diverse needs of the end−user population. Many users will have varying demands for both high− and low−speed traffic across the network. Using a QoS capability throughout the ATM network, a user can determine the performance and the capabilities of how the ATM network will meet their demands.
• Cost efficiency is a must, If ATM is truly to succeed, and traffic management must also include the effective usage of all of the circuitry available. ATM is designed to reduce the inefficient circuit usage by efficiently mapping cells into dead spaces, particularly when data is involved.
• Robustness in the event of failure or in the event of excess demand is a requirement of the traffic management goals. s. If the network is to be readily available for all users to be able to transmit information on demand, then the network must be very robust to accommodate failures,   link downtime, and so on.

Traffic in the form of asynchronous bursts of information (cells) enters the network at random times. This randomness is what causes the confusion and the unpredictability of the data. To manage the traffic flow, buffers are used to enable the flow and ebb of traffic volumes. Because data tends to be very bursty, it is extremely difficult to predict the demands of the network and the capacity needed at a given time.

The use of "leaky buckets" in the buffering of the traffic helps to manage and control the flow of           traffic onto and through the network. The leaky bucket, as the name implies, is a buffer that is constantly flowing.

Traffic enters into the buffers and is tagged, based on the amount of cells enabled by the carrier. If the user exceeds the amount of cell flow per increment (per second, and so on), then the buffer is filled and begins to empty out the bottom side. If more cells enter the buffer than are allowed, the cells are flagged for discard.

Shaping the Traffic

The ITU−TSS defines four possible situations when a cell enters an ATM network:

• Successfully delivered the cell arrives at the destination with less than time T−cell delay.
• An error cell occurs a cell arrives with at least one detected bit error in the information field in the cell. Another possibility is the severely error cell with information bits errors equal to n or n>1
• Lost cells a cell either never arrives or arrives after the time T−cell delay, in which case it is discarded at the destination.
•       An inserted cell a cell contains an undetected error or is misdirected by an ATM node and therefore shows up at the wrong destination.

This figure shows that if the user exceeds the network rate, the cells will be discarded, and the user will, therefore, have to retransmit at a later time.

And if the user does not exceeds the network rate the network should deliver all available cells delivered to the network. As shown in the figure bellow.

Source

Broadband telecommunication handbook 2nd edition by Regis J. "Bud" Bates

Frame Relay

Frame Relay:

It is a standardized wide area network technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching methodology. Originally designed for transport across integrated services digital network (ISDN) infrastructure, it may be used today in the context of many other network interfaces.

Network providers commonly implement Frame Relay for voice (VoFR) and data as an encapsulation technique, used between local area networks (LANs) over a wide area networks (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.

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 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.

Frame:

When Frame Relay was developed, the important part of the data−carrying capacity was the use of the frame to carry the traffic and not have the same overhead as an older technology (such as X.25). The frame was filled with data as necessary, but it handled the speed and throughput via the high−speed communications and lower overhead.

Frame relay vs OSI:

Frame Relay works at the data link layer to reduce the overhead associated

with the movement of data across the wide area. Because we refer to Frame Relay as a WAN

technology, it is natural that the protocols will work with the improvements made in the network over

the past decades.

In the older days, data was shipped across the layer three protocols (such as X.25) to assure the

reliability and integrity of the data.The X.25 protocol worked at layer three.The overhead associated with the transmission and reception of the data on the X.25 networks was inordinate. To facilitate better data throughout and eliminate some of the overhead, Frame Relay was developed. 

• The networks were now improved through the mass deployment of fiber−based networking technologies and the use of SONET protocols.
• The networking st rategies of  many end users were based on  router   technologies and LAN−to−WAN communications instead of the older terminal−to−host inter communications.

These two changes actually revamped the way we communicate. No longer did we have to use a

timing   relationship,  as in the older data networks. Any form of data transmission could be

accommodated across the newer improved techniques and protocols

With this comparison in mind, one will note that the Layer 2 protocol (in this case, Frame Relay)

eliminates some of the overhead associated with the transmission of data. The need for network

addressing using Layer 3 is reduced because many of the link architectures are based on

point−to−point circuits or private networking techniques.

Frame Relay Speed:

Carriers' Implementation of IP−Enabled Frame Relay:

Carriers are now offering IP−enabled services, enabling a customer to use an existing Frame Relay

access link to tap into a connection less Multiprotocol Label Switching (MPLS)−based IP backbone

or a private IP network. The primary benefit is that achieving mesh connectivity within a customer's

VPN requires just a single "access" PVC from each remote site.

Frame Relay vs IP:

Source:


www.wikipedia.com
Broadband Telecommunications Handbook second edition by Regis J. "BUD" BATES

ISDN(Integrated Services Digital Network)

Integrated Services Digital Network (ISDN) is a set of communications standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switch telephone network

ISDN is a public switch telephone network system, which also provides access to packet switch network, designed to allow digital transmission of voice and data over ordinary telephone copper wire, resulting in potentially better voice quality than an analog phone can provide. It offers circuit-switched connections (for either voice or data), and packet-switched connections (for data), in increments of 64 kbps.

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 (apart from broadband internet access and entertainment television) at a much higher transmission rate, without forcing the purchase of multiple analog phone lines.

The world's telephone companies conceived ISDN in the early 1980s as the next generation network. The existing voice networks didn't deal well with data for the following reasons:

·         One had to use modems to transmit data.

·         The data rates were around 9600 bps.

·         Connections (worldwide) were unreliable.

ISDN Interfaces

The customer interface I.45x specifies the Basic Rate Interface (BRI). It was intended to become the standard subscriber interface.

The BRI specifies two bearer channels and a data channel. The two bearer channels would bear the customer's information. The initial concept had this as being everything from analog telephone calls (digitized) to teleconferencing data and these would be switched channels. The only difference between a conventional telephone circuit and a bearer channel is that the bearer channel would be 64 Kbps all the way to the customer. (Note in the current network, your analog telephone circuit is digitized to 64 Kbps at the local Telco office before being switched across the network. It is then turned back into analog at the far end before being delivered to the called party.)

Now with BRI, we have not one, but two such telephone circuits.  Since it is digital, we have switched digital 64 Kbps to (theoretically) anywhere in the world.

The problem with the existing Telco network is that the signaling information shares the telephone channel with the user information. With plain voice circuits, a customer doesn't notice or care. With the advent of modems, this represents a loss of channel bandwidth, and with digital transmission, it means a loss of usable bits per second. The customer is therefore stuck with 56 Kbps, instead of the actual channel rate of 64 Kbps.

The BRI interface therefore specifies a multifunctional data channel at 16 Kbps that could handle signaling (its primary function) and network data (X.25) when not needed for signaling. BRI is therefore referred to as 2B + D, two bearer channels and a data channel.

ISDN Interface Components

NT1:

It creates the T interface for premise devices (from the U interface). In the original CCITT concept, the NT1 was provided by the Telco as part of the ISDN service. The U interface was therefore only the concern of the Telco with open networks (which concerned North America at the time). It is now important to understand on general principles.

NT2:

This device would do the switching, permitting more than the standard eight devices to share the T bus by creating perhaps multiple S buses. Therefore, ISDN terminal equipment (TE) device can't really tell if it is connected to an NT1 or NT2.

TE1:

The terminal equipment type 1 (TE1) is a standard (there is that word again) ISDN terminal that is capable of dealing with the B and D channels. In other words, it can interface with the S/T bus.

TE2:

The terminal equipment type 2 (TE2) is a standard device having an RS−232 or V.35 interface. (In ITU parlance, this is called a V−series interface.) It may be intelligent, but it doesn't have an ISDN interface capable of handling the D and B channels.

TA:

The terminal adapter (TA) is the semi−intelligent device that lets a TE2 connect to the S/T ISDN interface. The primary function of the TA is to run the ISDN interface for our TE2. The functionality varies widely due to the manufacturers. Some are simple and support only one

TE2:

Others support two TE2s and an analog telephone. The TA need not be a stand−alone box; it can come on a PC card and plug into the computer's internal bus.  With the proper software to run the card, this instantly creates a TE1 out of your computer.

Thus, the BRI then was designed for small offices or home offices (SOHOs).

Applications of the ISDN Interface

Digital fax:

Fax machines now have to be digital. Therefore, the Group IV fax standard specifies 64 Kbps fax operation.

Telephone:

The obvious starting point is the telephone, which is now a digital   telephone. Instead of the telephone conversation being analog from the handset to the central office where it becomes digitized, the conversation can be digitized directly at the source and passed digitally all the way through the network to the other end.

Computer/video conference:

Our computer or video conferencing equipment can use one of the 64 Kbps or bond both bearer channels together for a 128 Kbps digital channel across the network.

Primary-rate ISDN

Primary Rate Interface (PRI) provides 23B + D and, in this case, all are 64 Kbps channels.

PRI is frequently used for a PBX interface where the full signaling capability of the D channel is needed

Signaling in D channel

The signaling packets on the D channel are the same for BRI and PRI.

As we indicated, Signaling System 7 (SS7) is the mechanism for managing the network and it's logical to simply let the D channel use SS7 packets

Conclusion:

ISDN, therefore, was a great technology−driven service that didn't really solve a business (or home need. It is little wonder that ISDN is not widely implemented or used, but there are, as we have noted, some clear exceptions. The most notable is video conferencing. Internet access is also a possibility, but ISDN can't compete with DSL technology in performance for the cost. The ISDN primary rate is used extensively in call centers, utilizing computer telephony integration to maximize their efficiency. PRI is also used in PBX applications, where the digital PBX can make use of the network control and status information provided by the PRI.
Source:
www.wikipedia.com
Broadband Telecommunications Handbook second edition by Regis J. "BUD" BATES