B-NT1 functions are similar to Layer1 of the OSI Reference model and some of the functions are 1)Line Transmission Termination 2)Transmission Interface handling 3)OAM functions.
B-NT2 functions are similar to layer1 and higher layers of the OSI model. Some functions of B-NT2 are 1) Adaptation functions for different interface media and topology 2) Multiplexing & demultiplexing and concentration of traffic 3) Buffering of ATM cells 4) Resource allocation & Usage parameter control 5) Signalling protocol handling 6) Interface handling 7)Switching of internal connections. SB and TB indicate reference points between the terminal and the B-NT2 and between B-NT2 and B-NT1 respectively.
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ATM Signalling:
The Signalling capability for ATM Networks has to satisfy the following functions.
1. set up, maintain and release ATM virtual channel connections for information transfer.
2. Negotiate the traffic characteristics of a connection(CAC algorithms are considered for these functions.)
Signalling functions may also support multi-connection calls and multi-party calls. Multi-connection call requires the establishment of several connections to set up a composite call comprising various types of traffic like voice,video, image and data. It will also have the capability of not only removing one or more connections from the call but also adding new connections to the existing ones. Thus the network has to correlate the connections of a call. A multi-party call contains several connections between more than two end-users like conferencing calls.
Signalling messages are conveyed out-of band in dedicated signalling virtual channels in broadband networks. There are different types of signalling virtual channels that can be defined at the B-ISDN user-to-network interface. They can be described as follows. 1. Meta-signalling virtual channel is used to establish, check and release point-to-point and selective broadcast signalling virtual channels. It is bidirectional and permanent. 2.Point-to-point signalling channel is allocated to a signalling endpoint only while it is active. These channels are also bidirectional and are used to establish, control and release VCCs to transport user information. In a point-to- multipoint signalling access configuration, meta-signalling is needed for managing the signalling virtual channels.
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ATM Networks
ATM for LANs: Traditional Local Area Networks(LANs) like Ethernet, Token Ring and Token Bus are limited in speed(10Mb) and thus are limited to particular type of (mainly data) application. For multimedia applications, the bandwidth requirement is high and the information is a combination of voice, video and data and it requires a transfer mode capable of transporting and switching these different types of information. ATM satisfies this requirement for LANs and ATM Forum was created to specify the interfaces for ATM LANs. ATM LANs may be used to interconnect multiple LANs and also to connect to powerful workstations and servers. ATM Forum is a non-profit organization founded in 1991 and is actively involved in the definition of ATM LAN and also in the eventual deployment of a universal BISDN network. A simple ATM LAN can be shown as follows.
ATM allows transmission capacities upto 622 Mbit/s which is enough for most LAN applications. ATM LANs are expected to be mainly star configured and they have simpler network management functions. Deployment of ATM in LANs will also stimulate faster deployment of the public BISDN.
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Traffic Control in ATM Networks
There are many functions involved in the traffic control of ATM networks which are given as follows.
1. Connection Admission Control: This can be defined as the set of actions taken by the network during the call set-up phase to establish whether a VC/VP connection can be made. A connection request for a given call can only be accepted if sufficient network resources are available to establish the end-to-end connection maintaining its required quality of service and not affecting the quality of service of existing connections in the network by this new connection.
There are two classes of parameters which are to be considered for the connection admission control. They can be described as follows.
A. Set of parameters that characterize the source traffic i.e. Peak cell rate, Average cell rate, burstiness and peak duration etc.
B. Another set of parameters to denote the required quality of service class expressed in terms of cell transfer delay, delay jitter, cell loss ratio and burst cell loss etc.
2. Usage Parameter Control(UPC) and Network Parameter Control(NPC): UPC and NPC perform similar functions at User-to-Network Interface and Network-to- Node Interface respectively. They indicate the set of actions performed by the network to monitor and control the traffic on an ATM connection in terms of cell traffic volume and cell routing validity. This function is also known as “Police Function”. The main purpose of this function is to protect the network resources from malicious connection and to enforce the compliance of every ATM connection to its negotiated traffic contract. An ideal UPC/NPC algorithm meets the following features. 1. Capability to identify any illegal traffic situation. 2. Quick response time to parameter violations. 3. Less complexity and much simplicity of implementation.
3. Priority Control: CLP(Cell Loss priority) bit in the header of an ATM cell allows users to generate different priority traffic flows and the low priority cells are discarded to protect the network performance for high priority cells. The two priority classes are treated separately by the network Connection Admission Control and UPC/NPC functions to provide two requested QOS classes.
4. Network Resource Management: Virtual Paths can be employed as an important tool of traffic control and Network resource management in ATM networks. They are used to simplify Connection Admission Control(CAC) and Usage/ Network parameter control(UPC/NPC) that can be applied to the aggregate traffic of an entire virtual path. Priority control can also be implemented by segregating traffic types requiring different quality of service(QOS) through virtual paths. VPs can also be used to distribute messages efficiently for the operation of particular traffic control schemes like congestion notification. Virtual paths are also used in statistical multiplexing to seperate traffic to prevent statistically multiplexed traffic from being interfered with othe types of traffic, for example guaranteed bit rate traffic.
5.Traffic Shaping: Traffic shaping changes the traffic characteristics of a stream of cells on a VPC or VCC by properly spacing the cells of individual ATM connections to decrease the peak cell rate and also to reduce the cell delay variation. Traffic shaping must preserve the cell sequence integrity of an ATM connection. Traffic shaping is an optional function for both network operators and end users. It helps the network operator in dimensioning the network more cost-effectively and it is used to ensure conformance to the negotiated traffic contract across the user-to network interface in the customer premises network.
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Congestion Control in ATM
Congestion control plays an important role in the effective traffic management of ATM networks. Congestion is a state of network elements in which the network can not assure the negotiated quality of service to already existing connections and to new connection requests. Congestion may happen because of unpredictable statistical fluctuations of traffic flows or a network failure.
Congestion control is a network means of reducing congestion effects and preventing congestion from spreading. It can assign CAC or UPC/NPC procedures to avoid overload situations. To mention an example, congestion control can minimize the peak bit rate available to a user and monitor this. Congestion control can also be done using explicit forward congestion notification(EFCN) as is done in Frame Relay protocol. A node in the network in a congested state may set an EFCN bit in the cell header. At the receiving end, the network element may use this indication bit to implement protocols which will lower the cell rate of an ATM connection during congestion.
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Congestion Control in ATM
Congestion control plays an important role in the effective traffic management of ATM networks. Congestion is a state of network elements in which the network can not assure the negotiated quality of service to already existing connections and to new connection requests. Congestion may happen because of unpredictable statistical fluctuations of traffic flows or a network failure.
Congestion control is a network means of reducing congestion effects and preventing congestion from spreading. It can assign CAC or UPC/NPC procedures to avoid overload situations. To mention an example, congestion control can minimize the peak bit rate available to a user and monitor this. Congestion control can also be done using explicit forward congestion notification(EFCN) as is done in Frame Relay protocol. A node in the network in a congested state may set an EFCN bit in the cell header. At the receiving end, the network element may use this indication bit to implement protocols which will lower the cell rate of an ATM connection during congestion.
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ATM Applications
There are several practical applications using ATM Technology. ATM is going to be the Backbone Network for many broadband applications including Information SuperHighway. Some of the key applications can be mentioned as follows.
Video Conferencing
Desktop Conferencing
Multimedia Communications
ATM Over Satellite Communications
Mobile Computing over ATM for Wire-less Networks
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