Asynchronous Transfer Mode (ATM)

Asynchronous Transfer Mode is a high-speed network technology that supports the transportation of voice, data, and video signals over a single stream. ATM combines both circuit and packet switching methods into one flexible technology that makes for simple network processing functions. That digital data is encoded in the form of small fixed size cells instead of the variable sized packets used by Internet Protocol or Ethernet. This ensures that the packets can be sent quickly and easily.

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 form of traffic that is designed to carry any form of traffic and enables the traffic to be delivered asynchronously to the network. When traffic in the form of cells arrives, these cells are mapped onto the network and are transported to their next destination. When traffic is not available, the network will carry empty (idle) cells because the network is synchronous.

ATM is connection oriented, which means that data sent through the ATM network will always follow the same pre-defined path with the data arriving in the order it was sent.

ATM Cells

An ATM cell is 53 bytes long with a 5-byte header possessing information for control and signaling, and 48 bytes of data payload. Having fixed-size cells may reduce queuing delays for high priority cells. Because one knows the size of a cell beforehand, it becomes easier to implement the switching mechanism in hardware for efficient switching. The header information is generated in the ATM Layer, while the ATM Adaptation Layer (AAL) breaks the entire message into 48-byte data chunks. The cell header contains fields to help deal with congestion, maintenance, and error control problems. It is broken up into the following fields:

• Generic Flow Control (GFC), a mechanism used to alleviate shortterm overload conditions in the network. It is intended to provide efficient and equal utilization of the link between all the users.
• Virtual Path Identifier (VPI), which allows for more virtual paths to be supported within the network.
• Virtual Channel Identifier (VCI), which functions as a service access point as it is used for routing to and from the end user.
• Payload Type (PT), which is used to distinguish between user information and connection-associated layer management information.
• Cell Loss Priority (CLP), which is used to provide guidance to the network to discard the cell in case of congestion.
• Header Error Control (HEC), which contains the information that can be used by the physical layer for error detection or correction. It is calculated from the first 32 bits of the header.

VCI/VPI Connections

The entire ATM network is based on virtual connections set up by the switches upon initialization of a call. Virtual Channel Identifiers (VCI) and Virtual Path Identifiers (VPI) are used to identify these virtual connections. They are used to route information from one switch to another. VCI and VPI are not addresses; they are explicitly assigned to each segment within a network.

A Virtual Channel Connection (VCC) is set up between two end users through the network and used for full-duplex flow of cells. They are also used for user-network exchange (control signaling) and network-network exchange (network management and routing). The VCI label identifies a VCC between two ATM switches and may change at intermediate nodes within a route.

Virtual channels having the same endpoints are often grouped together to form a Virtual Path Connection (VPC). This grouping of channels makes the task of network management easier without losing flexibility.

Layers and Their Functions

ATM is a layered architecture allowing multiple services—voice, data, and video—to be carried over the network. It consists of three layers: the physical layer, the ATM layer, and the ATM adaptation layer.

The physical layer of ATM is similar to layer 1 of the Open Systems Interconnections (OSI) model and performs bit level functions. It defines electrical characteristics and network interfaces. It is further divided into two layers: Physical Medium (PM) and Transmission Convergence (TC) sub-layer.

The PM sublayer contains physical medium dependent functions and provides bit transmission capability including bit alignment.

The TC sublayer performs five primary functions. The lowest function is the generation and recovery of the transmission frame. Transmission frame adaptation adapts the cell flow according to the used payload structure of the transmission system in the sending direction, and extracts the cell flow from the transmission frame in the receiving direction.

ATM Layer

The ATM layer is next above the physical layer. The ATM layer takes the data to be sent and adds the 5-byte header information. It performs the following four actions:

• Cell header generation/extraction, which adds the appropriate ATM cell header to the received cell information field from the upper layer in the transmit direction. It does the opposite in the receive direction.
• Cell multiplex and demultiplex function, which multiplexes cells from individual virtual channels and virtual paths into one resulting cell stream in the transmit direction. It divides the arriving cell stream into individual cell flows to VCs or VPs in the receive direction.
• VPI and VCI translation, which is performed at the ATM switching and/or cross-connect nodes.
• Generic Flow Control (GFC), which supports control of the ATM traffic flow in a customer network.

ATM Adaptation Layer

The AAL performs the adaptation of OSI higher layer protocols, as most applications cannot deal directly with cells. The Adaptation Layer assures the appropriate service characteristics, and divides all types of data into the 48-byte payload that will make up the ATM cell. AAL is further divided into two sublayers: Segmentation and Reassembly (SAR) and Convergence Sublayer (CS).

The SAR sublayer performs segmentation of the higher layer information into a size suitable for the payload of the ATM cells of a virtual connection and, at the receiving side, it reassembles the contents of the cells of a virtual connection into data units to be delivered to the higher layers. The CS sublayer performs functions like message identification and time/clock recovery.

Mapping Circuits Through an ATM Network

ATM uses one of two connection types. The protocol is connection−oriented, so the two choices are a PVC or a 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 concept is that the network provider will provide a committed bandwidth available to the user on demand whenever the user wants to use it. The connection is built into a routing table in each of the switches involved with the connection from end to end.

ATM Traffic Management

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. These demands must be met in terms of the

delay or the actual delivery of the cells across the network.

Cost efficiency is a must. If ATM is truly to succeed, 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. 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. Through this process, the managing of traffic must

accommodate such diverse needs on a WAN.

Key Benefits of ATM

ATM offers significant benefits to users and those who design and maintain communications networks. Because network transport functions can be separated into those related to an individual logical connection and those related to a group of logical connections, ATM simplifies network management. ATM also allows for the integration of networks, improving efficiency and manageability and providing a single network for carrying voice, data, and video.

ATM increases network performance and reliability because the network is required to deal with fewer aggregated entities. There is also less processing needed and it takes less time to add new virtual channels because capacity is reserved beforehand on a virtual path connection. Finally, ATM offers a high degree of infrastructure compatibility. Because ATM is not based on a specific type of physical transport, it can be transported over twisted pair, coaxial, and fiber optic cables.

Two additional features of ATM that warrant discussion are its asynchronous operation and its connection-oriented operation. ATM cells are intermixed via multiplexing, and cells from individual connections are forwarded from switch to switch via a single-cell flow. However, the multiplexing of ATM cells occurs via asynchronous transfer, in which cells are transmitted only when data is present to send. In comparison, in conventional time division multiplexing, keep-alive or synchronization bytes are transmitted when there is no data to be sent. Concerning the connection-oriented technology used by ATM, this means that a connection between the ATM stations must be established before data transfer occurs. The connection process results in the specification of a transmission path between ATM switches and end stations, enabling the header in ATM cells to be used to route the cells on the required path through an ATM network.

Sources:

http://www.telecomdictionary.com/telecom_dictionary_ATM_definition.html

http://www.javvin.com/protocolATMLayer.html

Cisco - Creating ATM VLANs and Configuring LANE Services

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/atm.htm

Frame Relay, A Simple Discussion

Frame Relay is a simplified form of connection-based, packet-switching service in which synchronous frames of data are routed to destinations indicated on the header information. Frame Relay assumes an error-free physical link and therefore does not guarantee data integrity. Error detection and correction responsibility is left with the end devices. It uses the synchronous High-level Data Link Control (HDLC) frame format up to 4096 octets in length. Each frame contains a start flag, two octets that contain the information required for multiplexing across the link, the data information (payload), two octets generated by a cyclic redundancy check (CRC) of the rest of the octets between the flags, and the end flag.

Frame Relay can be used for various types of connections. It should be seen as a flexible protocol that lies on the Data Level of the Connections between routers. It can currently be used effectively for carrying all sorts of data, up to speeds of about 4 megabits per second.

Frame Relay Networks

In the diagram above, a mainframe communicates with each of the workstations (devices) on the LAN over a separate virtual circuit. The Frame Relay protocol identifies a virtual circuit by a 10-bit address called a Data Link Connection Identifier (DLCI). Each DLCI is unique on its local Frame Relay link. However, DLCIs are NOT unique throughout the network.

Since the DLCI is a 10-bit number, the Frame Relay protocol defines 1024 possible DLCIs. Of these, 2 (0 and 1023) have been reserved for signalling and 30 (1 to 15 and 1008 to 1022) have been reserved for future use.

For instance, networks that have implemented the optional multicasting feature reserve DLCIs 1019 to 1022 for that purpose. The remaining 992 DLCIs, (16 to 1007), are available to subscribers.

A Frame Relay network relies on the higher-layer protocols in its attached devices to recover from errors or congestion. In practice, this means that the higher layers must recognize that the network has discarded one or more frames of data.

Most higher-layer protocols use rotating sequence numbers to recognize frames that have been discarded. When a device receives a sequence number out of order, it requests that its partner retransmit all frames in order since the last frame it received with a correct sequence number.

In a well-tuned network, this typically includes the missing frame and all frames that its originator had transmitted in the time the destination device took to recognize the discard and send a message across the network requesting retransmission. In most cases, the originating device retransmits more data than would have been necessary.

This is a very reliable way to recover data lost through occasional transmission errors. However, when data's been discarded because of traffic congestion, bulk retransmission can only make the problem worse.

Fortunately, most higher-layer protocols use some form of throttling or flow control mechanism to recognize and prevent congestion.

The Frame Relay protocol also provides a way for the network to alert its subscribers when it becomes congested. The header of each Frame Relay frame contains two Explicit Congestion Notification bits that the network can set if it transmits that frame over a congested path. Each of these bits signifies congestion in a specific direction on the virtual route.

   

The Frame Relay frame header is illustrated above. The first octet is a flag field that delimits the frame from another frame or from idle time on the circuit. The second octet contains the first 6 bits of the 10-bit DLCI followed by a Command/Response bit (C/R) and the frame's first Extended Address (EA) bit.

The next octet contains the remaining four bits of the DLCI followed by the FECN and BECN bits described above, a Discard Eligibility (DE) bit, and the frame's second EA bit.

The subscriber or the network may set the value of the DE bit to 1 to indicate that the network may discard this frame in preference to frames in which the value of the DE bit is 0. (This occurs only after it has discarded all frames transmitted in excess of their subscribers' CIR and Bc).

The subscriber's data follows the Frame Relay header in most Frame Relay frames, and the data is followed in turn by the 2- octet Frame Check Sequence (FCS) and a final flag octet. A frame must contain at least one octet of user data for a total of 5 octets between flags.

A frame may not exceed 8192 octets between flags, counting header and FCS. The latest Frame Relay standards recommend a maximum frame size of 1600 octets overall.

Pro's and Cons of Frame Relay.

PRO: In many scenario's involving long haul, high speed connections, it is cheaper than dedicated lines.

PRO: There is a cheap solution to incorporate redundancy in the network.

PRO: Mixed speeds can be converted, traffic bursts can be buffered.

PRO: Less hardware is needed to for the same amount of connections

CON: There may be jams; no guaranteed bandwidth

CON: In a point-to-point scenario it is not economically feasible.

CON: In short haul, it is not economically feasible.

Sources:

http://www.myhome.org/pg/frame.htm

http://www.etisalat.ae/index.jsp?lang=en&type=service&currentid=10d3d1f598540310VgnVCM1000000c24a8c0____&contentid=a5aaa34673d3b010VgnVCM1000000c24a8c0RCRD

http://www.techfest.com/networking/wan/frrel.htm

http://www.dcbnet.com/notes/framerly.html

Broadband Communications Handbook

Integrated Services Digital Network (ISDN)

Integrated Services Digital Network (ISDN) refers to a set of communication protocols proposed by telephone companies to permit telephone networks to carry data, voice, graphics, music, and video. ISDN was developed to permit faster access over existing telephone systems without the additional call setup time. It enables the simultaneous transmission of voice and data signals using end-to-end digital connectivity.

Because ISDN uses existing phone lines, it requires that the central office be within a certain distance, which limits service availability

With ISDN, bearer channels (B channels) carry voice and data signals. These channels occupy a bandwidth of 64 kilobits per second (kbps). Some switches limit B channels to a capacity of 56 kbps. A data channel (D channel) handles signaling at 16 kbps or 64 kbps, depending on the service type.

ISDN Components

ISDN components include:

• Terminal equipment (TE)
• Terminal adapters (TAs)
• Network-termination devices

ISDN terminals come in two types terminal equipment (TE) and terminal adapters (TAs). TE refers to end-user devices such as digital telephones or workstations:

• Native ISDN terminals are referred to as terminal equipment type 1 (TE1). TE1s connect to the ISDN network through a four-wire, twisted-pair digital link.
• Non-ISDN terminals such as DTE that predate the ISDN standards are referred to as terminal equipment type 2 (TE2). TE2s connect to the ISDN network through terminal adapters. The ISDN TA can be either a standalone device or a board inside the TE2.

If the TE2 is implemented as a standalone device, it connects to the TA through a standard physical layer interface. Examples include EIA/TIA-232-C, V.24, and V.35. The TA performs the necessary protocol conversion to allow non-ISDN (TE2) equipment to access the ISDN network.

Beyond the TE1 and TE2 devices, the next connection point in the ISDN network is the network termination type 1 (NT1).

At the customer site, the ISDN local loop is terminated using a network termination type 1 (NT1). These are network-termination devices that connect the four-wire subscriber wiring to the conventional two-wire local loop. The NT1's responsibilities include line performance monitoring, timing, physical signaling protocol conversion, power transfer, and multiplexing of the B and D channels.

To delineate areas of responsibility within an ISDN network, ISDN defines a number of reference points within the network. Reference points define logical interfaces between functional groupings, such as TAs and NT1s. Reference points provide for a common term usage when troubleshooting a component of the local loop part of an ISDN network. Vendors and providers of ISDN equipment use the reference points R, S, T, and U. Table 12-1 illustrates the relationship between devices and ISDN reference points.

ISDN interfaces

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.

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, an ISDN (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. It may be intelligent but 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 manu-facturers. Some are simple and
support only one TE2; others support two TE2s and an analog telephone. 

The U Interface
The U interface is unique to North America and the “open” telephone network
interconnection. This U interface can be either a two-wire or a four-wire connection.


The Physical Interface
Another clever design feature of both the S/T and U interfaces is that they all use the
same RJ-45 type connector. 


The S/T and U interfaces carefully select the pin assignments so that accidentally
plugging an S/T connector into a U interface and vice versa doesn’t hurt anything.

Applications of the ISDN Interface

Multiple channels

The plan is to provide access to every possible home device. The original concept was for up to eight devices. 

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.

Digital Fax

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

KB/s fax operation.

Analog Fax

Analog fax machines use a modem, so it has to plug into the telephone that would take the analog modem tones and digitize them at 64 KB/s. 

Computer/Video Conferencing

Our computer or video conferencing equipment can use one of the 64 KB/s or bond both

Bearer channels together for a 128 KB/s digital channel across the network.

Signaling

The primary function of the data channel is to provide for signaling, that is, the setting up

and tearing down of the switched Bearer channels. At 16 KB/s, the data channel has more

bandwidth than is needed for signaling alone. Therefore, when it is not being used for its

primary and high priority signaling function, it could be used for other things.

Telemetry

This feature has never been well defined. The concept is that many household devices

can be connected to the data channel. This can include an energy management system

that would let the power company selectively turn off the refrigerator or air conditioner

for an hour or so at peak usage time. 

Packet Switching

The 16-KB/s data channel has bandwidth to spare. Therefore, the local carrier can

provide a data service on this excess bandwidth. X.25 is just maturing and is the logical

packet switching technology to offer. As it turns out, all the data on the data channel,

whether it be signaling data, telemetry data, or X.25 data, are always sent in packets

anyway. 

DDR Concepts

Dial-on-Demand Routing (DDR) addresses the need for intermittent network connections over circuit-switched WANs such as ISDN. With DDR, all traffic is classified as either interesting or uninteresting. If traffic is interesting, the packet is passed to the interface, and the router then connects by dialing to the remote router thus, the name dial-on-demand routing. DDR is used to cause the router to dial or receive a dialed connection.

Configuring  DDR

 It is important to remember that DDR cannot dial until some traffic is directed out the dialing interface. For this to occur, two steps are required:

1. Define what traffic will trigger the link to dial. This is known as specifying interesting traffic.
2. Specify how DDR traffic is routed out the dial interface. This involves configuring a static route so that the router will queue packets to be sent out the dialing interface.

Next, how will the dialing occur, and for how long? DDR must know what number to call to make the dialed connection and must know how long the call should remain up. These details are defined with the following two steps:

1. Define a dialer string on the dialing interface. (Dialer maps may be used when dialing to multiple destinations.)
2. Define a dialer idle-timeout.

The dialer string defines the number to call to get to the next-hop router. After a dialed connection is made, any type of packets can be routed across the link; however, only interesting traffic will keep the link up. The idle timer counts the time since the last interesting packet traversed the link. If the time specified as the idle-timeout expires, meaning that no interesting packets have traversed the link for the specified amount of time, the connection is brought down

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 xDSL technology in performance for the cost. 

Sources:

http://netcert.tripod.com/ccna/wan/isdn.html

http://www.inetdaemon.com/tutorials/telecom/isdn/index.shtml

http://www.birds-eye.net/definition/i/isdn-integrated%20_service%20_digital_network.shtml

Broadband Telecommunications Handbook