CN - Access Methods

Access Methods

In computing, an access method is a program or a hardware mechanism that moves data between the computer and an outlying device such as a hard disk (or other form of storage) or a display terminal.

It is sometimes used to refer to the mechanics of placing or locating specific data at a particular place on a storage medium and then writing the data or reading it. Access Method is also used to describe the way that data is located within a larger unit of data such as a data set or file.

Types of Access Methods

1. Ethernet
• CSMA/CD
• Switched Ethernet
• Ethernet Frame
• PoE (Power over Ethernet)
2. Token Ring
3. FDDI
4. ATM

Ethernet - CSMA/CD

In CSMA/CD (Carrier Sense Multiple Access/Collision Detection) Access Method, every host has equal access to the wire and can place data on the wire when the wire is free from traffic. When a host want to place data on the wire, it will “sense” the wire to find whether there is a signal already on the wire. If there is traffic already in the medium, the host will wait and if there is no traffic, it will place the data in the medium. But, if two systems place data on the medium at the same instance, they will collide with each other, destroying the data. If the data is destroyed during transmission, the data will need to be re-transmitted. After collision, each host will wait for a small interval of time and again the data will be re-transmitted, to avoid collision again.

CSMA/CD Events

Switched Ethernet

An Ethernet LAN that uses switches to connect individual hosts or segments. In the case of individual hosts, the switch replaces the repeater and effectively gives the device full 10 Mbps bandwidth (or 100 Mbps for Fast Ethernet) to the rest of the network. This type of network is sometimes called a desktop switched Ethernet. In the case of segments, the hub is replaced with a switching hub.

Ethernet Frame

 • A Ethernet Frame is a physical layer communications transmission, comprised of 6 fields which are assembled to transmit any higher layer protocol over an Ethernet fabric. 

An Ethernet frame is preceded by a preamble and start frame delimiter (SFD), which are both part of the Ethernet packet at the physical layer. Each Ethernet frame starts with an Ethernet header, which contains destination and source MAC addresses as its first two fields. The middle section of the frame is payload data including any headers for other protocols (for example, Internet Protocol) carried in the frame. The frame ends with a frame check sequence (FCS), which is a 32-bit cyclic redundancy check used to detect any in-transit corruption of data.

• A IP Packet is a formatted unit of data which can be transmitted across numerous physical topologies including Ethernet, Serial, SONET and ATM. 

• The important thing to understand is that a frame always refers to the physical medium.

PoE (Power over Ethernet) 

What is Power over Ethernet?

Power over Ethernet (POE) is a technology that lets network cables carry electrical power.

For example, a digital security camera normally requires two connections to be made when it is installed:

A network connection, in order to be able to communicate with video recording and display equipment

A power connection, to deliver the electrical power the camera needs to operate

However, if the camera is POE-enabled, only the network connection needs to be made, as it will receive its electrical power from this cable as well.

Why use POE?

Specifying Power over Ethernet brings many advantages to an installation:

Time and cost savings - by reducing the time and expense of having electrical power cabling installed.  Network cables do not require a qualified electrician to fit them, and can be located anywhere.

Flexibility - without being tethered to an electrical outlet, devices such as IP cameras and wireless access points can be located wherever they are needed most, and repositioned easily if required.

Safety - POE delivery is intelligent, and designed to protect network equipment from overload, underpowering, or incorrect installation.

Reliability - POE power comes from a central and universally compatible source, rather than a collection of distributed wall adapters.  It can be backed-up by an uninterruptible power supply, or controlled to easily disable or reset devices.

Scalability - having power available on the network means that installation and distribution of network connections is simple and effective.

Token Ring

Token ring or IEEE 802.5 is a network where all computers are connected in a circular fashion. The term token is used to describe a segment of information that is sent through that circle; when a computer on the network can decode that token, it receives data. The token ring is used by ARCNET, token bus and FDDI. Today, 802.5 and Token ring are considered inactive.

The token ring LAN process is delineated by the following sequence of events:

• A token continually circulates inside the toke ring LAN
• To transmit a message, a node inserts a message and destination address inside an empty token.
• The token is examined by each successive node. 
• The destination node copies the message data and returns the token to the source with the source address and a data receipt message.
• The source receives the returned token, verifies copied and received data and empties the token.
• The empty token now changes to circulation mode, and the process continues.

FDDI

FDDI (Fiber Distributed Data Interface) is a set of ANSI and ISO standards for data transmission on fiber optic lines in a local area network (LAN) that can extend in range up to 200 km (124 miles). The FDDI protocol is based on the token ring protocol.

Advantages

• provides high-speed network backbones that can be used to connect and extend LANs.
• has error-detection and correction capabilities.
• Fiber-optic cable such as the cable used with Fiber Distributed Data Interface (FDDI) can support very large volumes of data over large distances.

Disadvantage:

• expensive technology to set up because the network devices require a special network card and also fiber-optic cabling is required

ATM

ATM is a connection-oriented, unreliable (does not acknowledge the receipt of cells sent), virtual circuit packet switching technology. Unlike most connectionless networking protocols, ATM is a deterministic networking system — it provides predictable, guaranteed quality of service. From end to end, every component in an ATM network provides a high level of control. ATM technology includes:

• Scalable performance. ATM can send data across a network quickly and accurately, regardless of the size of the network. ATM works well on both very low and very high-speed media.
  
• Flexible, guaranteed Quality of Service (QoS). ATM allows the accuracy and speed of data transfer to be specified by the client. This feature distinguishes ATM from other high-speed LAN technologies such as gigabit Ethernet. The QoS feature of ATM also supports time dependent (or isochronous) traffic. Traffic management at the hardware level ensures that quality service exists end-to-end. Each virtual circuit in an ATM network is unaffected by traffic on other virtual circuits. Small packet size and a simple header structure ensure that switching is done quickly and that delays due to high traffic are minimized.
  
• Unobstructed speed. ATM imposes no architectural speed limitations. Its pre-negotiated virtual circuits, fixed-length cells, message segmentation and re-assembly in hardware, and hardware-level switching all help support extremely fast forwarding of data.
  
• Integration of different traffic types. ATM supports integration of voice, video, and data services on a single network. ATM over Asymmetric Digital Subscriber Line (ADSL) enables residential access to these services.

Connection Types: LAN vs. ATM

Traditional LANs, such as Ethernet and Token Ring, use a connectionless, unreliable approach, that cannot guarantee successful transmission when sending information across the network. Likewise, TCP/IP data transfers between networks are connectionless and unreliable. ATM, which is a connection-oriented, circuit-based technology, differs from the traditional approaches to networking.

Further research here

Referrence:

• http://ecomputernotes.com/fundamental/input-output-and-memory/what-is-access-method-explain-different-type-of-access-methods
• http://www.linktionary.com/a/access_method.html
• http://www.omnisecu.com/basic-networking/network-access-methods.php
• http://computer.howstuffworks.com/ethernet14.htm
• http://www.slideshare.net/KathiravanB1/ethernet-frames-25196930
• http://searchnetworking.techtarget.com/definition/ATM
• https://technet.microsoft.com/en-us/library/cc783455(v=ws.10).aspx