The Atm And Telephone Networks

Published Date: 02 Nov 2017

INTRODUCTION:

ATM services was designed with the aim to support various services with a reasonable cost.

But cost and complexity grew higher than the expectation which resulted in IP based technology.

Driven by the integration of data and telephony networks, it is called broad band integrated service vision.

-The information is divided into small packets called cells.

- These cells are transmitted asynronously.

-The ATM network is connection oriented.

-The use of fixed sized cells made it efficient.

-In a ATM, first message is sent to setup the connection.

- After the connection is made or path is setup all the cells follow the same path.

-ATM handles both constant rate traffic and variable rate traffic. It means ATM carries all kinds of traffic with end to end quality of service.

-ATM is independent of transmission medium.

-The information may be sent on a wire or fiber themselves or they may also be packaged inside the payload of other carrier systems.

-The guarantee of packet delivery is not reliable but if the packet reaches the destination the order is guaranteed.

FEATURES OF ATM:

-It uses virtual circuits.

-It uses fixed sized packets called cells which results in fast hardware switching.

-It uses statistical multiplexing.

-Different qualities of services can co-exist at a same time.

-It has good management and traffic engineering features.

-There is scalability in speed and network size.

ATM CELL

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Fig-1: ATM basic cell

Cell is the basic unit of information used by the ATM. Its size is 53 bytes. The 5 bytes contains header information and other 48 bytes contains user data or payload.

ATM and TELEPHONE NETWORKS

In telephone networks all the data rates are multiples of 64kbps whereas in ATM the data rate can be varied as per the needs of the clients. In telephone networks all the high speed circuits are manually setup but ATM allows dialing at any speed and rapid provisioning since this is done through software and signaling. ATM networks have greater advantage to telephone networks.

ATM and DATA NETWORKS

ATM is based on virtual circuit. Once the path is setup between the sender and receiver the resources are reserved for the connection before transmission whereas internet protocol is connectionless and resource reservation is not possible. ATM has fixed size cells whereas packets of internet protocol networks are of variable size.

ATM devices and interfaces:

Different types of interfaces are designated in this standard. One is computer connecting to a private switch. For this there is a private user network interface. In the same say, if a private switch is connected to the public switch , there is a public user network interface. When the public switch is connected to the other public switch then it is called network-network interface.

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Fig-2: Components of an ATM network

WORKING OF ATM NETWORK:

ATM is connection oriented so the connection between end to end link is necessary before data transmission.

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Fig-3: Establishing a connection in a ATM network

ATM uses virtual circuits. Each of the cell contains a local label so that path is setup. Setting up of a path in ATM means each of the intermediate switch would know that a flow of cells is going to go through them from one source to some destination. Once the virtual circuit is setup in the starting phase, each of the cells need not contain specific identifier or destination address. It contains the virtual identifier which is used to find out the path used in the virtual circuit. This virtual identifier is divided into two parts VCI and VPI. In ATM cell, the data is preceded by the VCI. There are two types of virtual circuits permanent virtual circuits and switched virtual circuits.

-In the fig-3, Router A sends a signaling request packet switch to ATM switch 1.

-ATM switch 1 reassembles the signaling packet from Router A and then examines it.

-Every switch along the path to Router B reassembles and examines the signaling packet and then forwards it to the next switch if the QoS parameters can be supported.

-When the signaling packet arrives at Router B, Router B reassembles its and evaluates the packet. Then, an accept message is sent back to Router A, the switches set up a virtual circuit.

FUNCTIONAL LAYERS OF ATM:

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Fig-4: Layers of ATM

Physical layer: It transports ATM cells in a communication channel and defines mechanical specifications. It is further divided into two sub layers.

-Physical medium dependent layer: This sub-layer does medium dependent functions like bit transfer, bit alignment, optically electrical optical functions etc.

It synchronizes transmission and reception by sending and receiving a continuous flow of bits associated with timing information and specifies the format used by the physical medium.

-Transmission Convergence layer: This sub-layer maps cells into physical layer format on transmit and delineates ATM cells in the received bit stream. It generates HEC on transmit and also generates idle cells for cell rate decoupling or speed matching

ATM LAYER:

This layer matches with the network layer of OSI model though it contains the features of data link layer also. It uses globally unique addresses using network service access points format of ISO. This layer is responsible for setting up connections. Once the connection is established , path and circuit identifiers are used.

-This layer establishes connection and passes the cells through the ATM network.

-It multiplexes and de-multiplexes cells of different connections.

-It translates VPI/VCI values at the switches and at cross connections.

-It extracts and inserts the header before or after the cell is delivered to the ATM adaptation layer.

-It maintains flow control using the GFC bits of the header.

ATM adaptation layer:

-This layer communicates with the higher layer protocols. The AAL is divided into two sublayers:

=>Convergence sublayer(CS): It offers different kinds of services to the different applications. The types of services are constant bit rate-bandwidth guarantee and is suitable for real time traffic, available bit rate(ABR) which is suitable for bursty traffic and unspecified bit rate(UBR). UBR is cheaper and is suitable for bursty traffic. It also provides special AAL services at an AAL network service access point.

=>Segmentation and Reassembly sublayer: This layer segments higher layer user data into 48 byte cells at the sender's node and reassembles them in the receiver's node. This sub layer is usually implemented with ATM switch. It tears down the message passed from the upper layer and converts them into fixed size cells. To make the cells multiples of 48, padding is used.

References:

Fig-1:http://www.tamos.net/~rhay/wp/overhead/overhead_files/image003.gif

Fig-2:http://docwiki.cisco.com/w/images/c/c7/Nd200803.jpg

Fig-3: http://docwiki.cisco.com/wiki/File:Nd200804.jpg

Fig4:http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a0080093c9a.shtml

FIBRE CHANNEL

INTRODUCTION

Fiber channel is a technology that is reliable, economical and has very high speed. Fiber channel is good option for high speed communications and is the foundation of most of the storage area network installations. The features of fiber channel are:

i) They are economical for storage and network installations.

ii) They provide versatile connectivity with scalable performance.

iii)They are more trustworthy form of communications and they ensure information delivery to a greater extent.

iv) Fiber channel delivers data with a great speed as long as the receiver's buffer is able to receive it with less bit error rate.

v)The efficiency of fiber channel is very high.

FIBER CHANNEL PROTOCOL

The fiber channel is a transport layer protocol that consists of five layers.

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Fig-1: fiber channel protocol layers

The FC protocol layers are split into three groups:

i)FC-0 and FC-1 are physical layers.

ii)FC-2 is a network layer that defines main protocols.

iii)FC-3 and FC-4 are service layers.

FIBER CHANNEL FABRICS:

An fiber channel fabric is a switched network topology. It connects fiber channel devices with a fiber channel switch to get a storage area network. The fiber channel switch is a network layer switch and it is compatible with fiber channel protocol. Usually servers and storage devices are fiber channel devices. FCoE or fiber cahnnel forwarders are network layer switches and they are responsible for forwarding FC traffic but they do not provide network services. After configuring FCoE-FC gateway, the QFX series becomes a proxy for the functionality of fiber channel switch. The gateway gives Ethernet network access to the fiber channel network. It does not need FC switches to support Ethernet interfaces in the storage area network. The gateway is not responsible for providing FC services. Usually, fiber channel network uses two fabrics for redundancy. If at all one fabric stops working or goes down, the other fabric comes into play to provide connectivity service.

IMPLEMENTATION OF FCOE:

These days, Cisco Nexus 5000 series enables unified fabric solutions at the server access layer. Future availability of distribution and core switches with FCoE line cards and storage devices with FCoE interfaces will allow implementation of FCoE fabrics above the access layer and across the network. The benefits of implementing FCoE are:

- The number of adapter and network infrastructure devices are reduced greatly.

- The quantity of cables are reduced.

-Allows inexpensive copper and longer link optical technologies.

-Cost of cables installation is reduced.

-It can increase the server density if server deployment is impeded by cable bulk or airflow concerns.

FRAME:

The basic unit of information in FC are frames. The frame contains the information to be transmitted or payload, source and destination port address and link control information. The frames are divided as data frames and link control frames. Each frame begins and ends with a Frame delimiter. The frame header immediately follows the SOF delimiter. The frame header is used to control link applications, control device protocol transfers and detect missing or damaged frames. An optional header may contain further link control information. A maximum 2112 byte long field(payload) contains the information to be transferred from a source N_Port to a destination N_Port. The four bytes of cyclic redundancy check(CRC) precedes the EOF delimiter. The CRC is used for detecting transmission errors.

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Fig-2: frame structure

Sequence

A Sequence is formed by a set of one or more related frames transmitted unidirectionally from one N_Port to another. Each frame within a sequence is uniquely numbered with a Sequence Count. Error recovery, controlled by an upper protocol layer is usually performed at Sequence boundaries.

Exchange

An Exchange is composed of one or more non-concurrent sequences for a single operation. The Exchanges may be unidirectional or bidirectional between two N_Ports. Within a single Exchange, only one sequence may be active at any one time, but Sequences of different Exchanges may be concurrently active.