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WHAT IS TDM, ETHERNET ATM?

OSI

The OSI (Open Systems Interconnection) model is a standard developed by ISO (International Organization for Standardization) to communicate different computers. The OSI model consists of 7 layers.

THESE LAYERS ARE IN ORDER;

  • Physical (Physical Layer)
  • Data Link (Data Link Layer)
  • Network (Network Layer)
  • Transport (Transport Layer)
  • Session (Session Layer)
  • Presentation (Presentation Layer)
  • Application (Application Layer).

Each layer performs different tasks within itself. Also, each protocol operates at a different layer. Each layer mentioned here provides a service to the layer above.

OSI LAYER 2 TECHNOLOGIES

The “Data Link” layer or “Data Link Layer” sets the rules for accessing and using the physical layer. It is the layer that ensures that the data to be sent is converted into electronic signals and transmitted to the physical layer and that the electronic signals from this layer are converted into data. This conversion process may vary according to the network technology used.

At this layer, a series of 0s and 1s from the physical layer are converted into frames and packets, and it is at this layer that the electrical signals are checked to see if they can travel smoothly over the cable. Physical addressing is also done at this layer.

THE DATA LINK LAYER IS DIVIDED INTO TWO SUB-SECTIONS

  • Media Access Control (MAC)
  • Logical Link Control (LLC)

The MAC sublayer packets the data with the error checking code (CRC-Cyclic Redundancy Check) and the MAC addresses of the sender and receiver and transfers it to the physical layer. At the receiving end, the MAC sub-layer does the same in reverse and transfers the data to the LLC, the second sub-layer in the data link.

The LLC sublayer acts as a transition to the next layer, the network layer (Layer 3). It creates protocol-specific logical ports (Service Access Points, SAPs). This allows the same protocols to communicate on the source machine and the destination machine (e.g. TCP/IP<->TCP/IP). The LLC is also responsible for re-sending data packets that are corrupted (or received for the other side). Flow Control, i.e. preventing the receiver from being overwhelmed by sending more data packets than it can process, is also the task of the LLC.

ATM, Ethernet, Wi-fi, TDM are technologies at this layer.

ATM

Asynchronous Transfer Mode (ATM) is a technology that dates back to the development of broadband ISDN in the 70s and 80s. It is a network technique that transmits data in cells of a fixed size of 53 bytes. It is a connection-based technology and uses the cell relay technique for data transmission, which can be considered a form of packet switching. This technique also takes advantage of circuit switching by creating virtual circuits. As with packet switching (e.g., X. 25, Frame Relay, TCP/IP), ATM integrates multiplexing and switching functions, is suitable for bursty traffic (unlike circuit switching), and allows devices operating at different speeds to communicate. Unlike packet switching alone, ATM is designed for high-performance multimedia networks. Contrary to expectations, its use in local area networks has been limited, and today it is mostly used to create fast backbone structures between communication and computer networks.

ATM-2

Since ATM networks are connection-based, a party first sends a connection setup packet to initiate data communication. The setup packet registers information about the existence of the connection and the resources it requires with the ATM switches it passes through. This connection is called a virtual circuit and the path information is called a virtual path. If the need for the connection is not temporary, the information is stored permanently in the switching tables. This type of permanent connection is called a permanent virtual circuit. Each connection has only its own credentials.

Once the connection is established, either party can start sending data. The data is converted into 53 byte cells, 5 bytes of header and 48 bytes of information. Since the header contains the connection ID, ATM switches know which side to forward incoming cells to. All cells therefore follow the same path. Although the cells follow a certain sequence, it is usually not checked whether the cells arrive at the destination.

ADVANTAGES OF ATM OVER OTHER TECHNOLOGIES:

  • Dynamic bandwidth for bursty traffic
  • Class of service support for multimedia
  • Scalability in speed and network size
  • Common LAN/WAN architecture
  • International standard compliance
  • Switched Ethernet and Token Ring working group hubs
  • High performance in combination with hardware switching
  • Possibilities for simplification through VC architecture

ETHERNET

Ethernet was developed at Xerox’s Palo Alto Research Center (PARC) in 1976. Ethernet provided the technological basis for the Institute of Electrical and Electronics Engineers (IEEE) 802.3 standard, first published in 1980. Shortly thereafter, Digital Equipment Corporation, Intel Corporation and Xerox Corporation jointly developed and announced an Ethernet (version 2.0) standard compatible with IEEE 802.3. Together, Ethernet and IEEE 802.3 currently have the largest market share in the local area network protocols market. Today, the term Ethernet is generally used for all Carrier Signal Sensing Multiple Access Collision Detection (CSMA/CD-CarrierSense Multiple Access/Collision Detection) networks that conform to Ethernet standards, including IEEE 802.3.

When it was first developed, Ethernet was designed to bridge the gap between long-distance low-speed networks and private, high-speed data-carrying but distance-constrained computer room networks. Ethernet is suitable for applications where the local communications environment must carry a high volume of heavy traffic, sometimes dispersed.

Ethernet encodes data with an electrical signal. In 10 Mbps (Megabit per second – One Million Bits Per Second) systems, the coding format is called Manchester coding. This system displays binary numbers as zero and one by varying the voltage. The increase or decrease in voltage over a period of time, called the bit period, indicates the binary number value of the bit.

Ethernet is further classified according to the cable used and communication speeds. Those that communicate at 10 Mbps are generally called Ethernet, those that communicate at 100 Mbps are called Fast Ethernet, and those that communicate at 1000 Mbps are called Gigabit Ethernet.

ETHERNET NETWORK ELEMENTS

Ethernet local area networks include network nodes and physical connection media. They are mainly divided into two groups: Data Terminal Equipment (DTE) and Data Communication Equipment (DCE). Devices can be the source or destination of data frames. DTE devices can be personal computers, workstations and file servers. DCE devices are devices that forward incoming data frames to the network. Network switches, repeaters and modems are DCE devices.

CARRIER SENSE MULTIPLE ACCESS COLLISION DETECTION (CSMA/CD -CARRIER SENSE MULTIPLE ACCESS / COLLISION DETECTION)

In Ethernet, it is the technique used to access the data transmission path (access to the physical medium). Ethernet is a broadcast network technology. This technique allows all nodes connected to the data transmission path to access the network environment. Only one node is eligible to use the network at a given time. In this concept, when a station wants to transmit a frame to the network, it must ensure that the network is not being used by another station. Since the data transmission path is open for all connected nodes to transmit data, any attempt to transmit data by different nodes at the same time will cause collision.

In case of collision, all data is corrupted and needs to be retransmitted. Therefore, a node that sends data needs to listen to the line after transmission and recognize possible collisions. The collision checker recognizes the similarity of both stations that want to transfer data and can simultaneously detect that the network is empty. When both stations send frames to the network, milliseconds later both frames collide.

Collisions are normal events in Ethernet. When such a collision occurs, both stations stop the transport and resend the frame after a random delay. It is important that the delay is random, otherwise the same collision will occur many times. When both computers detect the collision, they pause the data transfer for a while and repeat it using the binary exponential backoff algorithm.

CSMA/CD used in Ethernet receives and transports data frames, decodes data frames before passing them to the OSI upper layer, checks the validity of addresses and detects errors in the network or data frame.

TDM (TİME-DİVİSİON MULTİPLEXİNG)

Time-division multiplexing is one of the (less common) types of digital or analog multiplexing. In this type of multiplexing, multiple signals or bitstreams are sent over the same channel, but on the physical side this is done by allocating a timeslot to each channel. These channels send data sequentially over the transfer channel.

TDM is often used in SDH and Synchronous Optical Networking (SONET), leased circuits and network transmission standards.

TDM MULTIPLEXING CAN BE ACHIEVED IN 2 WAYS;

  • synchronous
  • asynchronous

SYNCHRONOUS TDM

Synchronous TDM’de multiplexer aynı zaman birimini, kanalı kullanan cihazlara ayırır, bu ayrım olayı cihazın kanalı kullanıp kullanmadıgına baglı degildir, örnegin eğer TimeSlot-1 X cihazı için ayrılmışsa ve X cihazının kanalı kullanma ihtiyacı olmasa bile herhangi bir cihaz bu kanalı kullanamaz.

ASYNCHRONOUS TDM

In Asynchronous TDM, TimeSlots are not separated by device. Each slot holds the details of the device to which the signal is to be sent (index) and the message itself. Thus, the number of slots in each frame does not have to be equal to the number of devices sending the message. Asynchronous TDM helps to maximize the utilization of the transfer channel and also helps to convert multiple slow input channels into a fast channel.

EXAMPLES OF TDM IN COMMUNICATION

PDH (plesiochronous digital hierarchy) systems, also referred to as PCM systems, are a method of digitally transporting many telephone calls over the same 4-wire copper cable (T-carrier or E-carrier) or fiber cable to a circuit switched digital telephone network.

SDH and SONET (synchronous optical networking) network transport standards use PDH.

GSM TELEPHONE SYSTEMS

TDM is mainly used in ISDN, Telephony service network and SS7 technologies.

ADVANTAGES

  • It has a long history.
  • It is reliable and secure.
  • Guaranteed constant bandwidth and suitable for constant speed data flow.
  • Provides advanced management and control.
  • TDM-based networks have been widely used for a long time, so they are secure. It is recommended to be used where there is a constant traffic flow. Because of its structure, it cannot respond to sudden changes in data flow.
  • In ATM and Frame Relay technologies, the ability to adjust the bandwidth according to the requirement provides a great advantage in terms of cost.
  • It is cheaper than Frame Relay in urban circuits.

DISADVANTAGES

  • No gain in bandwidth when there is no data flow.
  • Supported Applications
  • Constant speed, time-sensitive traffic
  • Primarily voice and data traffic
  • Possibility to access a network at low speed
  • Video conferencing
  • Distributed applications (Computer Aided Design / Computer Aided Manufacturing)