Asynchronous Transfer Mode And Fiber Channel

Published Date: 02 Nov 2017

ABSTRACT

In this paper I explained about ATM and fiber channel technologies. Firstly I explained about the ATM with the following concepts like the origin of ATM, protocol architecture ( the layered architecture was explained),logical connection (VCC and VPN connections) in detail ,call establishment was illustrated with a logical diagram and was also explained in detail. And also I mentioned about the concepts of ATM cell format, services that are being provided and also the applications of ATM in concise.

Secondly I exploited fiber channel by explaining about the origin of fiber channel and also illustrated combined advantages of network and channel which are present in fiber channel. These topics are followed by the topologies and architecture of fiber channel. And I have given a brief description about the services that are being provided by the fiber channel.

These concepts which are mentioned above about ATM and fiber channel are explained in this paper.

ATM

Why ATM came into existence?

Before discussing about ATM we need to discuss few points relating to frame relay. Frame relay, a packet switching technology in which data transfer is done using frames. Depending on the type of data, frame size varies with fixed header size of 48 bytes which reduces overhead within the packet and results in efficient data transmission. For voice and video traffic, fragmentation of data takes place into smaller components which results in overhead and also the effective transmission rate reduces significantly. So for handling voice, video and data traffic simultaneously a new technology is designed known as ATM.

What is ATM?

ATM is an acronym for Asynchronous Transfer Mode, which is a virtual packet switching technology. It is connection oriented and uses fixed size packets called cells for the transmission of data which is of 53bytes of which 5 bytes is allocated to header and remaining 48 bytes for information.

Overview of ATM Technology:

It is a cell switching technology which combines the benefits of circuit switching and packet switching technology such as flexibility, efficiency and constant transmission delay. It is an interface technology between host and the network equipment in both public and private networks. As it is asynchronous, clocking of signals is not necessary. There are no particular time slots allotted for transmission of data but it can be made available on demand with the help of labels identifying the source which are contained in the cells. It also provides both real time and non-real time services such as compressed voice and video using real time variable bit rate, traffic with specific quality of service requirements using non-real time variable bit rate.

Protocol Architecture of ATM:

The ATM architecture consists of the following:

Physical layer: It supports the flow of packets in transmission medium and signal encoding scheme. It also supports the data range from 25.6 Mbps to 622.08 Mbps. It is further sub divided into two sub layers as physical medium sub layer and transmission convergence sub layer. The physical medium provides dependent functions such as bit alignment and transmission. The medium can be optical fibre, twisted pair and coaxial cables.

On the other hand, the transmission convergence sub layer provides five different functions such as frame generation and recovery, cell delineation, adaptation, descrambling and scrambling, decoupling of cell rate.

ATM layer: It defines the transmission of data in fixed-size cells and also the use of logical connections. It supports information transfer protocols which are not based on ATM, so it creates the need for adaptation layer. It also provides 4 different functions such as generation or extraction of cell header, multiplexing and demultiplexing of cell, VPI and VCI translation and generic flow control.

AAL (ATM Adaptation Layer): Mapping of higher layer information to ATM cells that are to be transported over ATM network and then collects information from cells for deliver to higher layers is done by this layer. It is further sub divided into two layers as

Segmentation and reassembly sub layer: Segmentation of higher layer information into payload suitable size of ATM cell and then reassembles the content of cells of VC into data bits to delivery to higher layers.

Convergence sub layer: performed by this layer are message identification and time recovery. It is divided into two sub layers as common convergence sub layer and service specific convergence sub layer in order to support functions related to data over network.

Figure (a) Protocol Architecture of ATM

It also involves three separate planes as follows:

User Plane: user information transfer with associated controls such as flow control and error control are provided by this plane.

Control Plane: Call control and connection control functions are provided by this plane.

Management Plane: This plane is further divide into two as plane management and layer management. The plane management performs management functions related to system and provides coordination between all the planes. On the other hand, layer management performs functions related to resources and parameters present in its entities.

Logical Connections of ATM:

The logical connections in ATM are called as Virtual Channel Connections (VCC) which are analogous to virtual circuits in X.25 is a basic unit of switching in ATM network. The ATM adaptation layer deals with the concept of virtual path.

Bundle of VCC which are having same end points are named as virtual path connections (VPC). The concept of virtual path is developed for high speed networking and due to its feasibility it has become more popular these days. The following are the advantages of virtual path connections:

Compact network architecture: The transport functions can be sub-divided according to their nature of behaviour and can be sent through individual logical connections, same type data can be sent through same individual path.

Network performance and reliability is increased.

Figure (b) ATM Connections

Reduced connection set up and processing time: The time is consumed while establishing a call and there is no processing time at transit nodes, which reduced the time required for the transmission of data in a network.

Services provided by network: The end users can define the closed user networks for groups since virtual path is visible to the end users.

Figure (c) Establishment of call using virtual path

In order to provide a virtual channel connection between the end users for a call establishment, the host request a connection with the ATM network. The ATM network receives a request and checks if a virtual path exists or not. If at all a VPC exists then it checks the call quality service else it establishes a new virtual path. If call quality service is satisfied then a connection is provided else it blocks the VCC and request for higher capacity. If request is granted then a connection is made else it rejects the VCC request.

Services provided by virtual channel connections:

Quality of service: This service is provided by VCC with parameters cell loss ratio and delay variation in a cell.

Semi permanent and switched VCC: For setting up a connection and terminating a call control signalling is required, which is provided by switched VCC. The semi permanent connection is set up by network management action. The duration of semi permanent connection is long where as for switched it is on demand.

The cell sequence integrity is preserved in a virtual channel connection.

Negotiation of traffic parameter and monitoring usage: Between the user and the network, the traffic parameters can be negotiated with in each VCC. The monitoring of cells to VCC is done by the network to make sure the negotiated parameters are not violated.

Cell Format of ATM:

An ATM cell header format can be of two types User Network Interface and Network Network Interface. The UNI header is used for communication between endpoints and switches in private networks where as NNI header is also used for communication between switches. The below diagram depicts the basic ATM cell format, the ATM UNI cell-header format, and the ATM NNI cell-header format.

Generic Flow Control (GFC) - Provides local functions as sharing of single ATM interface by identifying multiple stations. This field is typically not used and is set to its default value.

Virtual Path Identifier (VPI) - In conjunction with the VCI, the next destination of a cell is identified as it passes through a series of ATM switches on the way to its destination.

Virtual Channel Identifier (VCI) – it is used for routing from and to the user end.

Payload Type (PT) - The first bit of data indicates the contents of cell is either user data or control data. If the cell contains user data, the second bit indicates congestion, and the third bit indicates whether the cell is the last in a series of cells that represent a single AAL5 frame.

Congestion Loss Priority (CLP) - Indicates whether the cell should be discarded if it encounters extreme congestion as it moves through the network. If the CLP bit equals 1, the

cell should be discarded in preference to cells with the CLP bit equal to zero.

Header Error Control (HEC) - Calculates checksum only on the header itself.

Figure (d) Cell format of ATM

Performance Issues:

There are five parameters that characterize the performance of ATM switching systems. They are:

1. Throughput

2. Connection Blocking Probability

3. Cell Loss Probability

4. Switching Delay

5. Jitter on the Delay

Applications:

There are several practical applications using ATM Technology. ATM forms a backbone network for many broadband applications including the 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 Wireless Networks

Fiber Channel

Why fiber channel?

The requirement for higher speeds in delivering data across the networks is growing rapidly, as memory and speed of servers, personal computers, workstations etc have increasing in day to day life which effects two types of communications with processor are I/O and network communications. Combining the features of these two technologies a new technology is designed which is named as "Fiber Channel". ATM and Fiber Channel both technologies are designed to solve the problems associated with network bandwidth. For the transmission of voice, video and data simultaneously then ATM would be a better choice. On the other hand, for transmitting voice and video with high throughput then fiber channel is a best choice.

What is fiber channel?

Since fiber channel supports high level protocols such as SCSI, IPI, FDDI etc; it is said to be a high performance serial link which can transfer huge amounts of information with high speeds. It also provides a port which can support both network and channel interfaces which reduces the burden on systems with number of ports attached to it. Error checking and flow control services are also provided by the fiber channel. As it is hardware intensive, at high transmission rates it takes some time for making routing decisions and therefore to avoid congestion some buffer space should be added.

Network, Channel and Fiber Channel:

Channel: A channel can transfer large amounts of data at high speeds over smaller distances with less overhead.

Network: A network transfers small amounts of data over long distances with overhead.

A fiber channel is formed by combining both channel and network protocol. All communications over fiber channel takes place through copper coax, twisted pair or optical fiber. It provides three different topologies which provides both channel and network usages. They are:

Switched fabric

Arbitrated loop

Point-to-point

(i) Switched-fabric topology:

The general topology supported by fiber channel is switched fabric topology. It permits dynamic interconnections between nodes through ports are connected to fabric. It consists of several switches forming a switched network supporting end nodes.

Figure (g) Switched fabric network topology

It provides the structure which enables the interconnection of multiple nodes. With the use of fiber channel, the storage unit supports increased connectivity extending the distance to thousands of miles with the help of routers and other network components.

(ii) Arbitrated loop:

It is a low cost topology for connecting nodes with fiber channel and a simple topology network. It can support maximum of 127 ports as a private loop. When a host or device is added or removed from the loop it goes to the loop initialization process which disrupts any I/O operations that are currently in progress. This is the reason why we need to have only a single host and a storage unit in a loop. Its operation is similar to token ring topology.

Figure (h) fibre channel arbitrated loop topology

(iii) Point-to-point topology:

Figure (i) point to point topology

It is also known as directly connected topology since the ports can be directly interconnected. With this topology number of connections can be limited that are made across the wires. The following figure is an illustration of a point-to-point topology configuration that includes one host system and one storage unit.

Architecture of Fiber Channel:

It is a layered architecture with five layers as FC-0, FC-1, FC-2, FC-3, and FC-4. The figure below shows the relationship between Fiber Channel layers and OSI layers.

http://techpubs.sgi.com/library/dynaweb_docs/0650/SGI_EndUser/books/FibreCh_OG/sgi_html/figures/fc2-3.gif

Figure 5 Fibre channel layers and OSI layers

A brief description of each layer is discussed below.

FC-0 Physical Media

This layer defines the physical media, receivers, transmitters, connectors that are used with fiber channel. A wide range of performance and cost alternatives are covered in this layer. For long-distance applications it includes optical fiber, coaxial cable for high speeds over short distances, and shielded twisted pair for lower speeds over short distances. 

FC-1 Transmission Protocol

This layer defines the signal encoding schemes used to transmit/receive data. In order to transmit data on a network it uses 8b/10b scheme.  

FC-2 Framing Protocol

This layer deals with defining topologies, frame format, flow and error control and grouping of frames into logical entities called sequences and exchanges. 

FC-3 Common Services

This layer includes multicasting. 

FC-4 Mapping

Defines the mapping of various channels and network protocols to Fibre Channel, including IEEE 802, ATM, IP, and the Small Computer System Interface (SCSI)

Services provided by Fiber Channel:

It was designed to provide wide range of communication needs which offers three classes of services as follows:

Class 1 Services: it provides a circuit connection which is often referred as selfish mode because when a connection between two nodes is established then that path cannot be used by another.

Class 2 services: A connectionless service is provided by this layer which is referred as unselfish mode as this path can be used by others. Delivery of data is guaranteed with acknowledgement. If it cannot deliver the data a busy signal is returned and host retries.

Class 3: it is referred as hopeful mode since it provides connectionless, frame-linked with no acks. An optional mode known as Intermix was developed to provide full bandwidth of fiber channel offered by class1 but allows connectionless traffic with spare capacity.

Conclusion:

In conclusion with ATM technology and fiber channel technology, ATM offers support for various multimedia applications using cell transfer technology and ability to move voice, video and data over a variety of physical media with high speeds, low cost and low delays makes ATM a solution for both LAN and WAN. Whereas Fiber Channel supports high level protocols such as SCSI, IPI, FDDI etc; it is said to be a high performance serial link which can transfer huge amounts of information with high speeds. It also provides a port which can support both network and channel interfaces which reduces the burden on systems with number of ports attached to it. The main advantage with fiber channel is its absence of topology dependencies. It avoids problems with topology dependencies by working under a closed system where data exchange on attributes and characteristics which determine communication capabilities.