Category Archives: Data Link Layer

Token Ring

Token Ring is a form of LAN (Local Area Network) protocol, which is embedded in a Data Link Layer or DLL. This is different from Ethernet since it utilizes a Ring topology wherein the data is transferred from one machine to another. This in turn will revert to the initial location, thus circling around the Ring.

Today, a Token passing control is also being implemented in order to limit access to a network to those that have control of the Token. This ensures that no collision will take place since only one machine is allowed to use the network within a given time.

The Basics

The process of Ring insertion goes through five phases until it is allowed into the Ring network. Therefore, when any one of the five phases fails, it refuses the insertion of the Token Ring station into the Ring. In fact, it may even register an error in the driver.

  1. Lobe Check – The station performs the standard lobe media check. A station sends test frames to its transmit pair, which will then follow a loop that will lead it back to its receive pair. This enables the station to validate whether it will be able to receive frames free of error.
  2. Physical Insertion – The station sends signal to the Multi Station Access Unit (MSAU) to initiate a relay.
  3. Address Verification – The station sends Media Access Control (MAC) frames using its MAC address to a specified destination address of a token ring frame. As soon as the frame returns and the address is copied, the station will then take part in a Ring poll process which will run every 7 seconds. This is also the part wherein the station identifies which ones are parts of the network within the MAC management functions.
  4. Participation in Ring Poll – Once the station has determined its NAUN address and has shared its address to the nearest downstream neighbor, a Ring map is then created. Then, the station will wait until the SMP or AMP frame is received. After the station has successfully participated in the Ring Poll process, it is now time to proceed to the request initialization, which is the last part of this 5-part process.
  5. Request Initialization – This is the phase wherein a station sends its parameter server a special request so that the configuration information can be obtained. Then the frame is forwarded to a special function address, usually a Token ring bridge. The said frame will consist of a Ring number information and timer that is sent to the new station.

EtherCAT

EtherCAT is an Ethernet-based family of computer network protocols that is essential for real-time control. Its goal is to use the Ethernet (Local Area Network connections) for automated applications that need cycle times, with low hardware costs, low fieldbus costs, and low communication jitters. It has a functional safety system and complies with international standards (ISO, SEMI and IEC).

An Overview

Most LAN automated networks work with short data per node (less than minimum payload of Ethernet frame). A frame for a node for one cycle means poor bandwidth utilization and, therefore, the network performs poorly. With EtherCAT, the frame for every node is not received, interpreted, and copied at every node. It simultaneously reads the data addressed to it, enters the device and inputs information. The frames work faster now because they are delayed by only less than a microsecond for each node. This means the entire network can use only one frame.

Where EtherCAT is Useful

EtherCAT is highly useful in technological processes that use heavy machinery. Quicker and better control of these machines is afforded through EtherCAT. Examples are metal forming, molding, printing machines, robotics, assembly systems, etc.

How the EtherCAT Works

The EtherCAT instruction/protocol is first transported within the standard IEEE 802.3 frame, using type 0x88a4. It may be composed of a couple of sub-datagrams, each of which serves a defined memory area of the logical process images (up to 4G). The sequence of data does not necessarily follow the physical arrangement of the network nodes.

A master device executes more complex operations like broadcast communication between slaves and multicast, and many more. In IP (Internet Protocol) routing, the EtherCAT protocol may be injected into the UDP (User Datagram Protocol)/ IP datagrams.

The EtherCAT Technology Group

The ETG, or EtherCAT Technology Group, is a worldwide organization where the OEM, technology service providers, and end users collaborate to encourage and support fast-moving technological developments of the EtherCAT. Founded in November 2003, it provides a forum for end users from different organizations. Today, it has bases in China, USA, Korea, and Japan.

Contrary to popular belief, you do not have to be a member of ETG to enjoy EtherCAT. Membership to the ETG is free because the EtherCAT is an open source technology.

ADSL Router

In order to use the ADSL (Assymetric Digital Subscriber Line) service, you will need the ADSL Router (also DSL modem) for connecting the DSL phone line to the computer. In other countries, the router is termed as NTBBA, which stands for Network Termination BroadBand Access.

Meanwhile, there is another type of ADSL routers that is capable of sharing an Internet connection to various computers within a network. This system of sharing connection over a network is called residential gateway.

About ADSL Router

The ADSL Terminal Unit-Remote is essential as a functional block for every ADSL Router. Mainly, this helps in modulation, demodulation, and framing. Meanwhile, you can also get other blocks that will perform bridging and IP routing.

Your ADSL router interface could be either USB or Ethernet.

Placement of Router

Most routers are not placed within the computer; instead they function either as a USB or Ethernet and are connected via the computer’s port. This is unlike other modems, such as voiceband, which are placed inside. There are various operating systems like the Windows operating systems that do not acknowledge ADSL routers into their system.

You cannot manage these internally since both the transceiver and the computer are from separate nodes in the LAN. Hence, the ADSL router is not controlled by the computer the way it does other components like the keyboard or mouse.

Configuration of ADSL Router

An ADSL router may be configured by opening a web page browser. Others, though, do not require configuration since they are already incorporated into the network’s physical layer.

As for the frequency of the router range, it can vary between 25 kHz to 1 MHz. Therefore, the router does not interfere with the voice service. Basically, it is possible for you to talk to someone over the phone while you have the router connected and you are using the Internet at the same time. This is not possible with Voiceband modems though, since its frequency is the same with that of the telephone.

Speed of ADSL Router

Your ADSL Router’s speed depends on your purchased plan. However, it can range from hundreds of kilobits to megabits per second. While routers are configured on particular protocols, you might not have the same level of efficiency with one line in the same company or house.

Components of ADSL

As for the hardware components of the ADSL Router, it comprises the following:

  • transformer
  • data connection (whether USB or Ethernet)
  • line driver
  • digital data pump
  • micro controller

802.11a

802.11a is a standard used under WLAN (Wireless Local Area Network) and Ethernet. It is a wireless network protocol standard defined by the IEEE (Institute of Electrical and Electronics Engineers). 802.11a is part of a series developed to offer wireless connectivity for home, office, and commercial computing.

IEEE 802.11a wireless networks, theoretically, can sustain a maximum bandwidth of up to 54 Mbps. On the other hand, 802.11a performs better than its successor, 802.11b in terms of efficiency and capacity. Consequently, the APs (Access Points) and adapters accompanying 802.11a cost considerably more than the 802.11b complementary devices.

Radio transmission signals of 802.11a are in the frequency range of 5 GHz and higher. This span is ‘designated,’ which means that 802.11a machinery uses frequencies not used by other products like wireless home phones. In comparison, the 802.11b standard uses frequencies in the more-populated 2.4 GHz range thus, meeting more interference from other devices.

Due to the less availability of its more costly 5 GHz components, 802.11a devices reached the markets much slower than its 802.11b equivalents. Performance of the first generation 802.11a merchandise was poor and burdened with bugs. When the second wave of 802.11a products started to sell, the consumer market did not embrace the technology because the cheaper 802.11b was already in widespread use. 802.11a, however, penetrated enterprise network systems in spite of larger initial costs. This standard was more compatible with some networks, allowing greater capacity and increased reliability than that of 802.11b-driven networks.

Note that an 802.11a device’s signal is limited because it uses the high 5 GHz frequency, although network performance significantly improves and interference lessens. An 802.11a AP transmitter device may encompass less than a fourth of the area of a similar 802.11b device. Physical obstructions, such as brick walls, affect 802.11a networks at a higher degree than similar 802.11b networks.

Proxy ARP

Proxy ARP refers to a technique wherein one host, usually a router, answers an ARP (Address Resolution Protocol) request originally intended for another work station.

The host performing the Proxy ARP procedure does the said task by “faking” its identity. It then accepts the responsibility of routing packets to the machine for which the ARP request is intended for.

Proxy ARP is helpful in enabling machines in a subnet to connect with other remote subnets. In addition, through proxy ARP, the said machines do not need to configure routing or identify a default gateway.

To better understand how proxy ARP works, below is an example of an interaction between two hosts: Host A in Subnet A and Host C in Subnet B.

  • Host A needs to send packets to Host C. Host A believes that it is directly connected to Host C’s subnet so Host A sends an ARP request to Host C.
  • In order for Host A to connect to Host C, Host A should first determine Host C’s MAC address. To do this, Host A broadcasts a new ARP request on Subnet A.
  • The ARP request is included in an Ethernet frame with Host A’s MAC address. The ARP request reaches all nodes in Subnet A, including the interface of the router. The request, however, does not actually reach Host C.
  • The router then replies to Host A with the router’s own MAC address. This is called the proxy ARP reply given by the router to Host A.
  • Host A then updates its ARP table. It sends the packets to the router, then the router forwards the packets to Host C.

MTTR

Mean Time to Repair or Mean Time to Recovery (MTTR) is the average time needed to return a faulty component or system to its proper operation. In making this prediction, the system’s maintainability, or the length of time for repair and maintenance in the event of system failure, is analyzed.

MTTR is also a factor in other reliability and maintainability forecasts and analyses. It can help calculate a product or system’s availability, the probability of an item being operable at any given time. This is based on a formula involving the Mean Time between Failures (MTBF) and the MTTR.

MTTR can range from a few milliseconds (in the event of a glitch in an Uninterrupted Power Supply or UPS) to hours or days (in application software or various complex machineries).

The duration of the component or system/s return to normal operation includes the diagnosis period and resolution of the problem. MTTR may decrease significantly if the failure rate is documented and predictable. Conversely, if the circumstances surrounding the system failure are unforeseen, the diagnosis will require a longer period of time. Inaccurate analyses may result to further disrepair, and thus extend the period before recovery. Factors such as these add to an increase in system MTTR.

If a maintenance contract includes MTTR, a shorter MTTR logically entails a larger cost because the provider ensures system restoration within a shorter span of time. The buyer or user pays a larger amount for quicker turnover.

MTTR has an immense bearing on the overall stability and the efficient operation of systems and businesses. System maintainability is an integral part to the operation of various industries, such as aeronautics or software production. Also, system reliability concerns the users as well as the manufacturers of a system. These are some of the reasons major businesses and international corporations rely heavily on MTTR.

ISDN PRI

ISDN PRI refers to a telecommunication standard used for carrying multiple voice and data transmissions from one physical location to another. This standard is used in ISDN (Integrated Services Digital Network), a digital phone connection system developed for simultaneous transfers of voice, data, and video. PRI stands for Primary Rate Interface.

ISDN PRI is designed for industrial and large-scale users. It is suitable for corporations with several branches and manufacturing firms with a nationwide network. This is in contrast to ISDN BRI (Basic Rate Interface), a standard used in homes and smaller businesses.

What are the components that make up ISDN PRI? This standard consists of two channels: the B channel and the D channel. The B channel manages data transmission including voice. The D channel manages control and signaling of data transfer. In a T-1 configuration, a PRI is made of 23 B channels and a single 64 Kbps D channel. On the other hand, a PRI using an E1 line consists of 30 B Channels and one D channel.

A flexible bulk transfer is the main advantage of using ISDN PRI. This is because the 23 or 30 B channels can be used in several combinations to suit a specific data transfer need.

ISDN PRI is applied in numerous business processes. The most common of these is the telephone exchange between a phone company’s customer and the phone company’s main office. Communication is established between the two said entities and the data is sent and received by the customer’s phone through the telephone company. Voice and video can then be transmitted by the customer’s phone to the desired number.

ISDN BRI

ISDN BRI is a standard service provided by the Integrated Services Digital Network (ISDN). BRI stands for Basic Rate Interface. ISDN BRI was developed for Internet connections used in small businesses and homes. This service is popular in most European countries, and is relatively accepted in certain parts of America.

ISDN BRI has unique and convenient features. This digital service can enable the simultaneous sending and receiving of any digital signal, may it be video, voice, or data. The 64 Kbps digital channels of ISDN BRI enable it to perform the said function. Through the ISDN Terminal Adapters, individuals can also create direct connections to and from telephones and data terminals. Another feature of ISDN BRI is its ability to connect to non-ISDN devices and achieve translation for the said equipment. This is done through its terminal adapters as well.

To carry out its functions, ISDN BRI makes use of certain channels. The following are the Bearer Channel or the B Channel to transmit data such as voice output, and the D Channel for control and signaling. D Channels can also transmit data.

ISDN BRI is also known as 2B+D. This is because it consists of 2 B Channels with a speed of 64 Kbps, and 1 D Channel with a speed of 16 Kbps. Since ISDN BRI has 2 64 Kbps B Channels, it has a 128 Kbps maximum data rate.

CSMA/CD

Carrier Sense Multiple Access with Collision Detection (CSMA/CD)is a protocol used for network control. This ensures that only a single network node transmits on the Ethernet wire at any given time in the Ethernet network.

Carrier Sense is the responsiveness of each Ethernet device to the activity that goes on in Ethernet wires before transmission attempts. The device will delay transmission if it observes transmission activity from another device.

Multiple Access is two or more Ethernet devices sensing (observation and waiting before transmitting) at any given time.

Collision Detection is the ability of different Ethernet devices to perceive the error when transmitting simultaneously.

Collision Occurrence in CSMA/CD

Assume that an Ethernet network is comprised of only two nodes. Each independent node opts to send the other an Ethernet frame. Both nodes observe the Ethernet wire and perceive that there is no carrier present. Both nodes then transmit at the same time, triggering a collision. Both nodes sense the collision and each delays the transmission for a random period of time.

Ethernet network collisions are usual, and a small number of collisions are typical to the protocol design.

Collision detection methods depend on the media. A comparison of transmitted information with the received information detects collisions for an electrical bus like the Ethernet. If the data differs and another transmitter overlays the other’s signal, the transmission is terminated immediately. This causes the sending of a jam signal, which sets transmitters back by random durations, consequently decreasing the probability of collision.

Logically, if numerous nodes transmit in an Ethernet network, the collision level can escalate to an insupportable extent. This means loss of accessible bandwidth in a network because retransmission wastes much bandwidth.

To optimize bandwidth and further reduce the marginal concerns with the CSMA/CD protocol, networks also utilize Ethernet switches.

MTU

MTU refers to Maximum Transmission Unit. This pertains to the maximum datagram or packet size that a network allows to be sent from one station to another. MTU is a parameter used by computer networks to control the flow of data and prevent excessive data transfer.

Normally, network technologies appropriate a certain MTU size. This parameter, however, can be changed by the network administrator. Stated below are a few examples of network technologies and their corresponding default MTU sizes. Note that the MTU size of each network technology is measured in bytes.

  • PPP or Point-to-Point Protocol has a default MTU of 296.
  • Ethernet has a default MTU of 1500.
  • 4Mb Token Ring has a default MTU of 4464.
  • Hyperchannel has a default MTU of 65,535.

In the event of a specific datagram being larger than the MTU of the network, the datagram will be fragmented into several smaller ones. Fragmentation is a process wherein data packets are divided in order for them to be permitted to transfer to another network location. There are cases where the MTU is smaller than most or all of the datagrams that need to be sent over the network. In this situation, all the datagrams have to be fragmented.

MTU can also increase bandwidth efficiency. This is done by increasing the MTU size in a network. However, increasing the size of the MTU may result to multiple sending of data due to errors that may take place.

Clear Arp Cache

A user needs to perform occasional maintenance procedures in his computer. One of these procedures involves the PC’s ARP cache.

The ARP cache is a virtual table in the computer’s system. The network layer addresses and data link layer addresses are being stored in this table. The ARP cache in the system can be displayed by the command arp -a. This normally works in Microsoft Windows and Unix Operating Systems.

There are instances wherein the ARP cache would become corrupt. A common indication of this can be seen when the connections to Web pages time out and eventually fail. When this happens, the ARP cache needs to be cleared.

Clearing the ARP cache can be done using the netsh command. Below is an example of how to use the netsh command:

C:\>netsh interface ip delete arpcache

After entering the command, hit the Ok button. The system would then clear the ARP cache. The previous problems encountered will most likely be solved. To ensure that the ARP cache has been cleared, the arp command can be used to view the table.

C:\>arp -a

Information regarding the interface and the Internet address and physical address type can be seen.

There are cases where an error is encountered while clearing the ARP cache. The system may display an error message that says that the operation could not be completed. To solve this problem, Routing and Remote Services should be disabled. However, this should be done only when this feature is not being used.

SPDIF

SPDIF is a standard that was jointly developed by Sony and Philips, two media device companies. The acronym stands for Sony Philips Digital Interface or Sony/Philips Interconnect Format. The exact definition of the standard is defined under the IEC 958 type II, and S/P-DIF.

SPDIF refers to both the physical layer needed to transfer digital data between a media player device and speakers, and the Data Link Layer protocol specifications. The physical layer refers to all the physical and electrical specifications for devices, as well as the relationship between a physical medium and a device. On the other hand, the data link layer presents the procedural and functional means to transfer data. It can also detect and correct errors that may occur in the physical layer.

The data format and the physical layer are used to transport digital audio signals from CD players, DVD players, PC audio cards, car audio systems and such to speakers that will interpret them as sound.

SPDIF Audio Data Format

The SPDIF Audio Data Format is a component of the IEC-60958 standards. It is derived from the AES/EBU standard, with the IEC-958 type II as its designation.

It is a derived version of the original AES/EBU consumer standard. It is similar to AES/EBU at the protocol level, but it can function with cheaper hardware. The only difference between the S/PDIF and AES/EBU protocol is the Channel Status Bit. The Channel Status Bit is the designated portion of the digital audio data transmitted over IEC958 between hardware devices.

Both S/PDIF and AES/EBU include two 192 data words from the left and right channel data. SPDIF is divided into 12 words of 16 bits each, while AES/EBU is divided into 8 words of 24 bits each.

SPDIF Audio Data Rate

There is no given rate or resolution in the SPDIF protocol. The actual equipment containing the SPDIF connectors will verify the data rate from the SPDIF signal received by both pieces of audio hardware.

The SPDIF protocol uses the Bi-phase mark code that contains either 1 or 2 transitions for each bit. It allows the primary word clock to be extracted directly from the base signal.

Usually, the SPDIF data rates are 48 kHz in a Digital Audio Tape and 44.1 kHz in a stereo CD audio.

The transmission rate is limited to only 16-bit audio because of audio CD restrictions. However, there are some SPDIF protocols that support audio with over 20 bits.

Additional Reading on SPDIF