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"E" Networking Definitions & Concepts...

E1 Carrier .. to .. EtherTalk Link Access Protocol (ELAP)

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E1 Carrier:

In digital telecommunications, E1 is a carrier channel configuration defined by the CCITT (Consultative Committee for International Telegraphy and Telephony) and used in Europe, Mexico, and South America. Like the T carrier channels (T1, T2, and so on) defined in North America, the E1 carrier channel is built up of 64 kilobit per second (kbps) voice channels. See the DS (Digital Service) article for a discussion of how the T-carrier channels are defined. The E1 carrier is defined as thirty 64 kbps voice channels and two 64 kbps signaling channels. In ISDN B and D channel terminology, this type of carrier is known as 30B+2D. The E1 carrier has a bandwidth of 2.048 megabits per second (Mbps). E1 links can be multiplexed into higher capacity carriers. Because the hierarchy also allocates channels for link management and signaling, the data rates are higher than the number of 64 kbps channels indicates. Also see Digital Communication, and compare with T1 carrier.

EARN (European Academic and Research Network):

A European network that provides file transfer and e-mail (electronic mail) services for universites and research institutions.

Earth Station:

The ground-based portion of a satellite communications system is called an earth station or a ground station. The station consists of an antenna and receiver (or transceiver) that are in communication with a satellite in geosynchronous orbit.

Signals can be beamed from an earth station to the satellite and from there to the destination node (another earth station). These communications services can be leased from various companies. For long distances, the prices are competitive with earth based connections (such as leased or public lines).

The size of the antenna required to receive signals at an earth station depends on the transmission frequency. For 19.2 kilobit per second (kbps) lines, an antenna of about 1.2 to 3 meters (4 to 10 feet) in diameter is sufficient. For faster speeds (such as 1.544 megabit per second speed of T1 lines), larger antennas are required. These are harder to install and maintain, and may require special permits.

EBCDIC (Extended Binary Coded Decimal Interchange Code):

An 8-bit code for representing letters, numerals, punctuation marks, and other symbols. Primarily used in IBM equipment, EBCDIC defines 256 character codes. The other, more commonly used code for representing characters is ASCII (American Standard Code for Information Interchange).

Eclipse (Open-Source):

Eclipse is a new open-source, Java-based, extensible development platform designed for nothing in particular but everything in general. Because of its roots, it is currently most popular as a Java integrated development environment (IDE). Eclipse ships with plugins for writing and debugging Java code. Additional plugins for more advanced Java development, such as JSP/servlets, are available from third parties.

Eclipse has its own GUI framework called the Standard Widget Toolkit (SWT), which is portable across all major platforms, runs fast, and looks native. You can use SWT to develop your own applications, the same way you might use AWT/Swing. A smaller framework named JFace, built on top of SWT, adds dialogs, wizards, models, and other essentials to the basic SWT widgets.

EJBs: (Enterprise Java Beans)

( E nterprise J ava B eans) A software component in Sun's J2EE platform, which provides a pure Java environment for developing and running distributed applications. EJBs are written as software modules that contain the business logic of the application. They reside in and are executed in a runtime environment called an "EJB Container," which provides a host of common interfaces and services to the EJB, including security and transaction support. At the wire level, EJBs look like CORBA components.

There are three types of EJBs. (1) Session beans are used to perform processing, (2) entity beans are used to represent data, which can be a row or a table in a database, and (3) message driven beans are generated to process Java Messaging Service (JMS) messages.

EJBs inherently provide future scalability and also allow multiple user interfaces to be used. For example, both a Web browser and a Java application could be used to access EJBs, or one could be switched for the other at a later date. However, if these are not important issues, servlets, JSPs and regular Java applications can be used for business logic rather than EJBs. See J2EE, EJB local interface, JavaBeans, distributed objects and component software .


Electronic mail (e-mail) provides the ability to transmit written messages over short or long distances instantaneously through the use of a microcomputer or terminal attached to a communication network. The users that communicate through electronic mail do not have to be on-line at the same time. Each can leave messages to the other and retrieve the replies at later times.

Electronic mail has the capability to forward messages to different locations, send word processed or spreadsheet documents to any user of the network, and transmit the same message to more than one user by using a mail list.

A mail list contains the names and electronic mailbox addresses of people that the message must be sent to. The electronic mail system reads the names and addresses from the list and sends the message to all users on the mail list. Electronic mail improves corporate and individual communications significantly.

Electromagnetic Interference (EMI):

Random or periodic energy from external sources that can interfere with transmissions over copper cable, or copper circuit boards. EMI sources can be artifacts (such as motors or lighting -- particularly fluorescent lighting) or natural phenomena (such as atmospheric or solar activity). Compare this with RFI (radio frequency interference).


A complete functional duplication of one machine or device by another. For example, a PC may emulate a 3270 terminal in order to communicate with an IBM mainframe. A hardware device or a software package that provides emulation is called an emulator.


The technique used by layered protocols in which a lower level protocol accepts a message from a higher level protocol and places it in the data portion of the low level frame. Encapsulation often means that packets traveling across a physical network have a sequence of headers in which the first head comes from the physical network frame, the next from the Internet Protocol (IP), the next from the transport protocol, and so on up through the OSI (Open Systems Interconnection) model.

For example, messages from a terminal are encapsulated in a frame that contains a header and the user information. The header provides the address information necessary to identify the terminal. Additional framing information is usually required to allow the demultiplexer to delineate the beginning and end of each message.

End Node:

In a network, a station that serves as a source or a destination for a packet. An end node should be able to communicate through all the layers in the OSI (Open Systems Interconnection) Reference Model or an equivalent layered model. Also see node.

End of Content (EOC):

In telecommunications, a special character used to indicate the end of a message or page.

End Office (EO):

In telephony, a central office, which is where a subscriber's lines are terminated and connected to other exchanges.

End System (ES):

In the OSI (Open Systems Interconnection) Reference Model, an end system (ES) is a network entity, such as a node, that uses or provides network services or resources. An end system is known as a host in Internet terminology.

Architecturally, and end system uses all seven layers of the OSI Reference Model. This is in contrast to an intermediate system (IS), or router, which uses only the bottom three layers (the subnet layers) of the model. The figure below "Communications Involving Intermediate and End Systems" shows the relationship between intermediate and end sytems.


U.S. Geological Survey's Earth Resources Observation Systems.

Error-Control Techniques:

Error-control techniques were developed to ensure virtually error-free communication of user data information. In addition to the header, each block of information would have a number of "check-bits" appended prior to transmission. These check-bits would enable the receiver to detect whether the received blocks contained errors and, if so, to request re-transmissions. In this manner high levels of reliability in data transmission could be achieved.


Is the physical (cable) layout of a LAN system based on the IEEE 802.3 standard. It is a set of protocols for only the two lower most layers of the OSI Reference Model: the physical and the data link layers. The protocols of a network operating system, such as AppleTalk or TCP/IP, are then layered atop Ethernet to form a complete network.

Ethernet is called a multiple protocol medium. This means that Ethernet can support protocols from different network operating systems at the same time, a feature that adds to its versatility.

Ethernet is the standard for a 1 - persistent best effort delivery system that uses CSMA/CD technology. Basically the idea is this, when a station wants to transmit, it listens to the cable. If the cable is busy, the station waits until it goes idle, otherwise it transmits immediately. If two or more stations simultaneously begin transmitting on an idle cable, they will collide. All colliding stations then terminate their transmission, wait a random time, and repeat the whole process again.

Ethernet is a popular local area network technology which was invented at the Xerox Corporation Palo Alto Research Center. Ethernet itself is a passive coaxial cable; the interconnections contain all active components. Xerox Corporation, Digital Equipment Corporation, and Intel Corporation (known in networking circles collectively as "DIX"), developed and published the standard for 10Mbps Ethernet. Originally, the coaxial cable specified for Ethernet was 1/2 inch diameter heavily shielded cable, up to 500 meters in length, known as 10Base-5, which is called thicknet. Resistance, in the form of 50W terminators, is added between the center wire and shield at each end to prevent reflection of electrical signals.

However, many office environments now use a lighter coaxial cable sometimes called thinnet or cheapernet, also known as 10Base-2. It is also possible to run Ethernet over shielded twisted pair cable, known as 10Base-T.

Called the ether, the cable itself is completely passive; all the active electronic components that make the network function are associated with devices that are attached to the network.

Ethernet Address:

This is low level address (unique) i.e., the physical address. These address's are assigned by the manufacture (after they purchase several blocks of the physical addresses from the IEEE organization) to devices such as host's, 4511A's, PC's, ethernet cards or NIC's (Network Interface Cards), etc. See hardware address.

Ethernet Frame Format:

The Ethernet should be thought of as a link-level connection among devices. Thus, it makes sense to view the data transmitted as a frame. Ethernet frames are of variable length, with no frame larger than 1536 bytes. As in all packet-switched networks, a frame must identify its destination. The figure below shows the Ethernet frame format that contains both the sender's physical address as well as the destination physical address.

In addition to identifying the source and destination, each frame transmitted across the Ethernet contains a preamble, type field, data field, and Cyclic Redundancy Code (CRC). The preamble consists of 64 bits of alternation 0s and 1s to help receiving nodes synchronize. The 32-bit CRC helps the interface detect transmission errors: the sender computes the CRC as a function of the data in the frame, and the receiver recomputes CRC to verify that the packet has been received intact. Finally, higher levels of protocol use the 16-bit type field to identify frame contents and data format.

Explaination of the Ethernet Transmission Frame:

  • Preamble. The preamble is used to provide synchronization and to mark the start of a frame. The same bit pattern is used for the Ethernet preamble field as in the IEEE 802.3 preamble and start frame delimiter fields. The preamble is actually generated and removed as part of the data encoding/decoding function in the physical layer and is not always shown as part of the Ethernet frame format.
  • Address Fields. The frame includes both destination and source address fields. Ethernet specifies the use of 48-bit addresses, whereas IEEE 802.3 allows either 16-bit or 48-bit addresses.
  • Packet Type Field. Ethernet does not support the use of a length field and padding. Instead, the 2 bytes are used to contain a type field. The value specified in the type field is meaningful to the higher network layers and is not defined as part of the Ethernet specification.
  • Data Field. The data portion of the frame is passed to the data link layer by the client layer. It must be a multiple of 8 bits. Ethernet defines a minimum frame size of 72 bytes and a maximum frame size of 1526 bytes, including the preamble. If the data to be sent is smaller or larger than these sizes, it is the responsibility of the higher layers to pad it or break it into individual packets. The reason for specifying a minimum frame size is to reduce problems in collision handling. The maximum packet size reflects practical considerations related to buffer sizes in the adapter card and a need for a limit on the length of time the transmission medium is tied up while transmitting a single frame.
  • Frame Check Sequence. Ethernet uses a frame check sequence field as a way of providing error checking. The field contains a cyclic redundancy check (CRC) value that is calculated from the other fields in the frame. When the frame is received, the value is calculated again using the received data, and the calculated value is compared to the value in the frame. If the two do not match, an error has occurred, and this is reported to the client layer.
Ethernet Goals:
  • Simplicity. Features that would complicate the design without substantially contributing to meeting the other goals have been excluded.
  • Low Cost. Since technological improvements will continue to reduce the overall cost of stations wishing to connect to Ethernet, the cost of the connection itself should be minimized.
  • Compatibility. All implementations of the Ethernet should be capable of exchanging data at the data link level. For this reason, the specification avoids optional features, to eliminate the possibility of incopatible variants of Ethernet.
  • Addressing Flexibility. The addressing mechanisms should provide the capability to target frames to a single station, a group of stations, or all stations on the network.
  • Fairness. All stations should have equal access to network when averaged over time.
  • Progress. No single station operating in accordance with the protocol should be able to prevent the progress of other stations.
  • High Speed. The network should operate efficiently at a data rate of 10 megabits per second.
  • Low Delay. At any given level of offered traffic, the network should introduce as little delay as possible in the transfer of a frame.
  • Stability. The network should be stable under all load conditions, in the sense that the delivered traffic should be a monotonically nondecreasing function of the total offered traffic.
  • Maintainability. The Ethernet design should allow for network maintenance, operation, and planning.
  • Layered Architerture. The Ethernet design should be specified in layered terms to separate the logical aspects of the data link protocol from the physical details of the communication medium.
Ethernet Meltdown:

An event that causes saturation or near saturation on an Ethernet. It usually results from illegal or misrouted packets and typically lasts only a short time. As an example, consider an IP datagram directed to a nonexistent host and delivered via hardware broadcast to all machines on the network. Gateways receiving the broadcast will send out ARP packets in an attempt find the host and deliver the datagram.


This is Apple's data-link product (hardware) that allows an AppleTalk network to be connected by Ethernet cables/media. It is also a name given to protocols transmittedover Ethernet media.

The Apple EtherTalk product provides high-speed connection of computing devices in the AppleTalk network system. It uses standard Ethernet technology including thick or thin coaxial and twisted-pair cabling with data transmission at 10 Mbits/second. This high-bandwidth medium is desirable for network segments that carry heavy traffic or require very agile response characteristics.

When used in an AppleTalk system, EtherTalk's faster transmission speed results in better performance. Furthermore, EtherTalk can support as many concurrently active AppleTalk devices as can be connected to an Ethernet network.

EtherTalk relies on an extension of the Ethernet data-link protocol that uses AARP (AppleTalk Address Resolution Protocol). This extended protocol, is known as the EtherTalk Link Access Protocol (ELAP).

EtherTalk Link Access Protocol (ELAP):

Is the link-access protocol used in an EtherTalk network. ELAP is built on top of the standard Ethernet data-link layer.

Event (in temporal databases):

An event is an isolated instant in time. An event is said to occur at time t if it occurs at any time during the chronon represented by t.

Alternative names are instant, or moment. Both "instant" and "moment" may be confused with the distinct term "chronon" (-E5 -- homographs [a word that is spelled the same as another word but differs in meaning and orgin and may differ in pronunciation and syllabication] and homonyms [one of two or more words that have the same sound and often the same spelling but differ in meaning] should be avoided. Names with an already accepted meaning, such as an informal meaning, shoud not be given an additional meaning, and -E7 -- new names should be consistent with related and already existing and accepted names.)

Extreme Programming (XP):

Extreme Programming (XP) is a new lightweight software developing methodology. XP is the most popular of the "agile" development methodologies. Agile development is an integral part of technology movements in the software development world. XP has quickly gained in popularity over the last few years and is now becoming recognized by development shops from large consulting firms through small software houses. Some of the companies that are using XP include DaimlerChrysler, First Union National Bank, and Ford Motor Company.

XP has a rock-solid, software engineering base where integrating tests, real-time code review, and quick release cycles all ensure quality of code. The good news is that most of the software development tools used in XP are free! You'll discover how to use these tools to both test and build your applications.

XP can be used across development languages and platforms. XP works equally well on both Java and .NET platforms. Common approaches and tools mean that developers can switch easily between either platform; that is, as long as you know the development language!

Extreme Programming (XP) is a new, vibrant way of developing software. But with dozens of software development methodologies in existence; RUP, Waterfall, and RAD, just to name a few; why do we need a new one? To answer that question, we first need to understand a little about what software development is, how it has evolved, and what problems it faces.

Software development has evolved from its early days in the academic and military fields into a number of distinct methodologies. All methodologies have one underlying characteristic: They are designed to enable developers to solve customer problems. Some of these problems have nothing to do with computers at all and can be traced to other factors such as poor communication or unexpected change.

Software developers realized that closing the gap between themselves and customers would be key to success. The ultimate success is keeping the customer happy! The problem of how to obtain flexibility remained even when using techniques such as rapid development. A new breed of highly flexible, customer focused development methodologies have been emerging over the last decade that are beginning to turn the tide back on project failures. XP is the most popular of these Agile methodologies and combines adaptability with high-quality software results.

Extreme Programming uses an object-oriented approach as its preferred development paradigm. XP encompasses a set of rules and practices that occur within the context of four framework activities planning, design, coding, and testing.

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