Linux

Linux is a generic term referring to Unix-like computer operating systems based on the Linux kernel. Their development is one of the most prominent examples of free and open source software collaboration; typically all the underlying source code can be used, freely modified, and redistributed, both commercially and non-commercially, by anyone under the terms of the GNU GPL and other free software licences.

Linux is predominantly known for its use in servers, although can be installed on a wide variety of computer hardware, ranging from embedded devices , mobile phones and even some watches to supercomputers. Linux distributions, installed on both desktop and laptop computers, have become increasingly commonplace in recent years, partly owing to the popular Ubuntu distribution and the emergence of netbooks.

The name “Linux” comes from the Linux kernel, originally written in 1991 by Linus Torvalds.The rest of the system usually comprises components such as the Apache HTTP Server, the X Window System, the GNOME and KDE desktop environments, and utilities and libraries from the GNU Project (announced in 1983 by Richard Stallman). Commonly-used applications with desktop Linux systems include the Mozilla Firefox web-browser and the OpenOffice.org office application suite. The GNU contribution is the basis for the Free Software Foundation‘s preferred name GNU/Linux.

UNIX

The Unix operating system was conceived and implemented in 1969 at AT&T’s Bell Laboratories in America by Ken Thomson, Dennis Ritchie, Douglas McIlroy, and Joe Ossanna, and first released in 1971, it was written in assembly language and later re-written in C in 1973 by Dennis Ritchie. Its wide availability and portability due to being written in C meant that it was widely adopted, copied and modified by academic institutions and businesses, with its design being influential on authors of other systems.

Security-Enhanced Linux (SELinux) is a Linux feature that provides a variety of security policies, including U.S. Department of defence style mandatory access controls, through the use of Linux Security Modules (LSM) in the Linux kernel. It is not a Linux distribution, but rather a set of modifications that can be applied to Unix-like operating systems, such as Linux and BSD. Its architecture strives to streamline the volume of software charged with security policy enforcement, which is closely aligned with the Trusted Computer System Evaluation Criteria (TCSEC, referred to as Orange Book) requirement for trusted computing base (TCB) minimization (applicable to evaluation classes B3 and A1) but is quite unrelated to the least privilege requirement (B2, B3, A1) as is often claimed.^{[citation needed]} The germinal concepts underlying SELinux can be traced to several earlier projects by the U.S. National Security Agency.

Netware  Features

NetWare is a network operating system developed by Novell, Inc. It initially used cooperative multitasking to run various services on a personal computer, and the network protocols were based on the archetypal Xerox Network Services stack.

NetWare has been superseded by Open Enterprise Server (OES). The latest version of NetWare is v6.5 Support Pack 8, which is identical to OES 2 SP1, NetWare Kernel.

There were several reasons for NetWare’s performance.

File service instead of disk service

At the time NetWare was first developed, nearly all LAN storage was based on the disk server model. This meant that if a client computer wanted to read a particular block from a particular file it would have to issue the following requests across the relatively slow LAN:

1. Read first block of directory
2. Continue reading subsequent directory blocks until the directory block containing the information on the desired file was found, could be many directory blocks
3. Read through multiple file entry blocks until the block containing the location of the desired file block was found, could be many directory blocks
4. Read the desired data block

NetWare, since it was based on a file service model, interacted with the client at the file API level:

1. Send file open request (if this hadn’t already been done)
2. Send a request for the desired data from the file

All of the work of searching the directory to figure out where the desired data was physically located on the disk was performed at high speed locally on the server. By the mid-1980s, most NOS products had shifted from the disk service to the file service model. Today, the disk service model is making a comeback, see SAN.

Aggressive caching

From the start, NetWare was designed to be used on servers with copious amounts of RAM. The entire file allocation table (FAT) was read into RAM when a volume was mounted, thereby requiring a minimum amount of RAM proportional to online disk space; adding a disk to a server would often require a RAM upgrade as well. Unlike most competing network operating systems prior to Windows NT, NetWare automatically used all otherwise unused RAM for caching active files, employing delayed write-backs to facilitate re-ordering of disk requests (elevator seeks). An unexpected shutdown could therefore corrupt data, making an uninterruptible power supply practically a mandatory part of a server installation.

The default dirty cache delay time was fixed at 2.2 seconds in NetWare 286 versions 2.x. Starting with NetWare 386 3.x, the dirty disk cache delay time and dirty directory cache delay time settings controlled the amount of time the server would cache changed (“dirty”) data before saving (flushing) the data to a hard drive. The default setting of 3.3 seconds could be decreased to 0.5 seconds but not reduced to zero, while the maximum delay was 10 seconds. The option to increase the cache delay to 10 seconds provided a significant performance boost. Windows 2000 and 2003 server do not allow adjustment to the cache delay time. Instead, they use an algorithm that adjusts cache delay.

Efficiency of NetWare Core Protocol (NCP)

Most network protocols in use at the time NetWare was developed didn’t trust the network to deliver messages. A typical client file read would work something like this:

1. Client sends read request to server
2. Server acknowledges request
3. Client acknowledges acknowledgement
4. Server sends requested data to client
5. Client acknowledges data
6. Server acknowledges acknowledgement

In contrast, NCP was based on the idea that networks worked perfectly most of the time, so the reply to a request served as the acknowledgement. Here is an example of a client read request using this model:

1. Client sends read request to server
2. Server sends requested data to client

All requests contained a sequence number, so if the client didn’t receive a response within an appropriate amount of time it would re-send the request with the same sequence number. If the server had already processed the request it would resend the cached response, if it had not yet had time to process the request it would only send a “positive acknowledgement”. The bottom line to this ‘trust the network’ approach was a 2/3 reduction in network transactions and the associated latency.

Client Support

Support can include the following:

• Making a system known to the network (host name and Ethernet address information)
• Providing installation services to remotely boot and install a system
• Providing Solaris OS services and application services to a system with limited disk space or no disk space

Client Support Services can also provide prompt on-site service if you need assistance at any facility. Ongoing support through maintenance programs ensures you stay up-to-speed with solution developments and optimize your resources for maximum performance.

Packet  Switching/Circuit Switching

Circuit switching dominates the public switched telephone network or PSTN. Network resources set up calls over the most efficient route. That might mean a call from New York to San Francisco goes through switching centers in San Diego, Chicago, and Saint Louis. But no matter how convoluted the route, that path or circuit stays the same throughout the call. Got it? One call, one circuit. It’s like having a dedicated railroad track with only one train, your call, permitted on the track at a time.

Before contrasting circuit switching with packet switching, let’s talk about some digital basics. Voice and data from the local loop goes digital once it hits a telephone switch. Traffic between American telephone offices is nearly all digital, you know, 1s and 0s. Bits. That includes most circuit switched traffic, like we just discussed. All these bits get packaged into small groups called packets, frames, blocks, or cells. TCP/IP, X.25, ATM, frame relay, pick your packet switched technology, all traffic gets put into one form of packet or another. But simply packetizing data does not mean a call is packet switched.

TDMA and CDMA in wireless, T-Carrier and SONET in wireline networks, are transmission methods, transport mechanisms that carry information from one point to another across the telephone network. They packetize data but do not in general switch that data. According to their own protocol or standard, they package up data sent to them in the most efficient way possible, without interfering in switching. If my laptop is connected to the internet over a cellular modem, for example, then I am using TCP/IP to surf the net, while the modem may be using TDMA or CDMA to actually transport the call. Read more in the Ericsson quote below, where voice over the internet is sent using wideband CDMA. I don’t mean to confuse anyone here, I just want to point out the difference between packets in switching and packets in certain transmission technologies. If you really want to get confused, know that some packet technologies like TCP/IP combine elements of both transmission and switching. But stay with the discussion.

Packet switching dominates data networks like the internet. A data call or communication from San Francisco to New York is handled much differently than with circuit switching. With circuit, all packets go directly to the receiver in an orderly fashion, one after another on a single track. Like the train we mentioned before, hauling one boxcar after another. With packet switching routers determine a path for each packet or boxcar on the fly, dynamically, ordering them about to use any railroad track available to get to the destination. Other packets from other calls race upon these circuits as well, making the most use of each track or path, quite unlike the circuit switched calls that occupy a single single path to the exclusion of all others.

ATM

ATM stands for Asynchronous Transfer Mode. Pretty catchy, eh? ATM, sometimes called cell relay, is a high speed multiplexing and switching technology for data. An advanced packet switching scheme, ATM makes all its packets one length. These uniformly small packets or cells let data flow smoothly, like a collection of images on movie film, all moving through the projector at a constant frame rate. ATM also boasts improved error control compared to conventional packet switching, as well as numerous other features that now make it a core technology of data networks world wide. ATM can run over SONET or T-Carrier.

A definition by the F.C.C.: Cell relay is a high-bandwidth, low-delay, switching and multiplexing packet technology. Its combination of simplified error and flow control, fixed-length cells which allow high-speed switching, and procedures for allocating network bandwidth enable it to support voice, data, image, and video traffic. Asynchronous transfer mode (ATM) is the international standard implementation of cell relay. It is defined to work over different physical media and at speeds ranging from 45-622 Mbps, with extensions to lower and higher speeds possible. Vendors are beginning to produce ATM network equipment and carriers are beginning to assemble ATM networks. Current service offerings are developmental in nature, however, and it is expected to take several years for the technology to mature. Significant infrastructure investments by carriers will be required to make ATM widely available.

Frame Relay

Frame relay is another fast packet switching scheme, this one with much less overhead than the earlier X.25 networks. Errors or bad frames are discarded, not retransmitted or checked, so things move along quickly. Packets vary in length and are called frames for this protocol. Commercial service began in 1992 and is now widely deployed. An internet service provider might use this technology to connect to the internet over a T-1 or T-3 line. Meant mostly for clean metallic lines or preferably fiber optic cable. Runs over SONET or T-Carrier.

Frame relay is essentially an enhancement of X.25, and takes advantage of the widespread implementation of fiber optic communication links by long-distance carriers. Fiber is much less prone to introducing errors in a data stream, so frame relay does not use most of X.25′s extensive checking at switching nodes; these processes are instead completed by the sending and receiving devices.

Frame relay is designed to operate at speeds up to 1.5 Mbps, but may be enhanced to operate at higher speeds in the future. It is particularly well-suited to the interconnection of LANs which generate bursty traffic consisting of variable-length frames of data. Frame relay accepts this traffic as is, adding only a wide area network address at the front and its own check sequence at the end of each frame. Frame relay interfaces for customer premise equipment such as routers, bridges, and hubs are available from a number of vendors.

SONET

Sonet stands for Synchronous Optical Network, a high speed transmission technology designed to send traffic over fiber optic cable, but now also used over coax. [More here] That compares to the older and more well known T-Carrier system, which started in 1963, an inherently slower technique since it was built for copper cables, noisy, error producing, bandwidth limited metallic lines. Like T-1, SONET is a transmission standard, one that carries other protocols to their destination. So circuit switched voice traffic, X.25 network data packets, ATM cells, and TCP/IP based internet traffic all moves over the road that SONET provides. SONET also sports modern network features including bandwidth management, real time monitoring of the system, survivable networking, and universal connectivity. Sound like so much mush? Not really. Nathan Muller says SONET will provide communications’ transport mechanism for the next three to four decades; all modern telephone companies are building their new networks with it. Read the files below for more information.

ISDN

Integrated services digital network is a set of communications standards enabling traditional telephone lines to carry voice, digital network services, and video. Prior to ISDN, the phone system was viewed as a way to transport voice, with some special services available for data. The key feature of the ISDN is that it integrates speech and data on the same lines, adding features that were not available in the classic telephone system. There are several kinds of access interfaces to ISDN defined as basic rate interface (BRI), primary rate interface (PRI) and broadband ISDN (B-ISDN).

ISDN is a circuit-switched telephone network system, that also provides access to packet switched networks, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in better voice quality than an analog phone. It offers circuit-switched connections (for either voice or data), and packet-switched connections (for data), in increments of 64 kbit/s. Another major market application is Internet access, where ISDN typically provides a maximum of 128 kbit/s in both upstream and downstream directions (which can be considered to be broadband speed, since it exceeds the narrowband speeds of standard analog 56k telephone lines). ISDN B-channels can be bonded to achieve a greater data rate, typically 3 or 4 BRIs (6 to 8 64 kbit/s channels) are bonded.

SDH

SDH (Synchronous Digital Hierarchy) is an international standard for high speed
telecommunication  over optical/electical networks which can transport digital signals
in variable capacities. It is a synchronous system which intend to provide a more flexible , yet simple
network infrastructure.

SDH (and its American variant- SONET) emerged from standard bodies somewhere around 1990.

T1/E1

T1 circuit.  [T1 circuits, also known as a DS1 circuits, are] high-speed digital connections, which can be used for voice or data. The T1 transmits data at a speed of 1.544 Mbps, which is faster than most modems.  T1s are engineered circuits that are used for businesses. The T1 consists of 24 channels, which can be allotted to voice, data or both. The T1 transmission occurs over two-pair fiber wiring however it is sometimes brought to the building on regular copper wires. T-1 circuit definition.  T1 circuit definition. 

“T1 Lines have been the primary source of mission critical bandwidth for companies of all sizes for nearly 20 years…  ‘Full T1 Line’ is a term typically used to describe a circuit that provides 1.5 megabits per second of high speed Internet access, which is the most common type of T1. A Full T1 can also be used to carry telephone lines and/or VoIP calls…

A T1 Line consists of 24 channels that transmit data at 64Kbps each, therefore giving a Full T1 Line the capacity to transmit 1.54Mbps of data synchronously (upstream and downstream). Using today’s standard email applications and common Internet searching, this amount of bandwidth could support anywhere from 1 to approximately 75 users depending on their needs, preferences, and the company’s budget. Most commonly Full T1 Lines are used in offices with 5 to 50 employees. A T1 circuit provides the most reliable bandwidth available when leased from a high quality T1 Provider, which is why businesses are willing to pay more for a T1 than for other services like DSL or Cable.”

E1

A 2.048 Mbps point-to-point dedicated, digital circuit provided by the telephone companies in Europe. E1 is the European counterpart of the North American T1 line, which transmits at 1.544 Mbps, and E1 and T1 lines can be interconnected for international use. E2 through E5 lines provide multiple E1 channels. An E1 line uses two wire pairs (one for transmit, one for receive) and time division multiplexing (TDM) to interleave 32 64-Kbps voice or data channels.

T3 and E3 Technologies

A T3 is a dedicated phone connection supporting data rates of about 43 Mbps. A T-3 line actually consists of 672 individual channels, each of which supports 64 Kbps. T-3 lines are used mainly by Internet Service Providers (ISPs) connecting to the Internet backbone and for the backbone itself.  T-3 lines are sometimes referred to as DS3 lines. 

An E3 is the European equivalent of a T3 circuit. It is a term for a digital facility used for transmitting data over a telephone network at 34 Mbps.  It has smaller bandwidth and fewer subchannels than a T3. E3′s are found in all countries other than the United States, Singapore, and Japan.

Print Service

Print service does the following;

• Discover and select print services based on their capabilities
• Specify the format of print data
• Submit print jobs to services that support the document type to be printed.
• Provides classes and interfaces that describe the types of Print Service attributes and how they can be collected into attribute sets.

System Security

Safeguard systems against web and software vulnerabilities and email threats

Today’s complex threats—phishing, rootkits, zero-day exploits, and drive-by downloads—need more than basic anti-virus. Our integrated protections and controls cut costs, simplify compliance, and reduce the frequency and urgency of patching.

Network switch

A network switch is a computer networking device that connects network segments.
The term commonly refers to a Network bridge that processes and routes data at the Data link layer (layer 2) of the OSI model. Switches that additionally process data at the Network layer (layer 3 and above) are often referred to as Layer 3 switches or Multilayer switches.
The term network switch does not generally encompass unintelligent or passive network devices such as hubs and repeaters.
The first Ethernet switch was introduced by Kalpana in 1990.
The network switch, packet switch (or just switch) plays an integral part in most Ethernet local area networks or LANs. Mid-to-large sized LANs contain a number of linked managed switches. Small office, home office (SOHO) applications typically use a single switch, or an all-purpose converged device such as gateway access to small office/home office broadband services such as DSL router or cable Wi-Fi router. In most of these cases, the end user device contains a router and components that interface to the particular physical broadband technology, as in the Linksys 8-port and 48-port devices. User devices may also include a telephone interface to VoIP.

In the context of a standard 10/100 Ethernet switch, a switch operates at the data-link layer of the OSI model to create a different collision domain per switch port. If you have 4 computers A/B/C/D on 4 switch ports, then A and B can transfer data between them as well as C and D at the same time, and they will never interfere with each others’ conversations. In the case of a “hub” then they would all have to share the bandwidth, run in Half duplex and there would be collisions and retransmissions. Using a switch is called micro-segmentation. It allows you to have dedicated bandwidth on point to point connections with every computer and to therefore run in Full duplex with no collisions.

Gateway
A gateway is a network point that acts as an entrance to another network. On the Internet, a node or stopping point can be either a gateway node or a host (end-point) node. Both the computers of Internet users and the computers that serve pages to users are host nodes, while the nodes that connect the networks in between are gateways. For example, the computers that control traffic between company networks or the computers used by internet service providers (ISPs) to connect users to the internet are gateway nodes.
In the network for an enterprise, a computer server acting as a gateway node is often also acting as a proxy server and a firewall server. A gateway is often associated with both a router, which knows where to direct a given packet of data that arrives at the gateway, and a switch, which furnishes the actual path in and out of the gateway for a given packet.
On an IP network, clients should automatically send IP packets with a destination outside a given subnet mask to a network gateway. A subnet mask defines the IP range of a network. For example, if a network has a base IP address of 192.168.0.0 and has a subnet mask of 255.255.255.0, then any data going to an IP address outside of 192.168.0.X will be sent to that network’s gateway. While forwarding an IP packet to another network, the gateway might or might not perform Network Address Translation.
A gateway is an essential feature of most routers, although other devices (such as any PC or server) can function as a gateway.
Most computer operating systems use the terms described above. A computer running Microsoft Windows however describes this standard networking feature as Internet Connection Sharing; which will act as a gateway, offering a connection between the Internet and an internal network. Such a system might also act as a DHCP server. Dynamic Host Configuration Protocol (DHCP) is a protocol used by networked devices (clients) to obtain various parameters necessary for the clients to operate in an Internet Protocol (IP) network. By using this protocol, system administration workload greatly decreases, and devices can be added to the network with minimal or no manual configurations.

Router
A router is a networking device whose software and hardware are usually tailored to the tasks of routing and forwarding information. For example, on the Internet, information is directed to various paths by routers.
Routers connect two or more logical subnets, which do not necessarily map one-to-one to the physical interfaces of the router. The term “layer 3 switch” often is used interchangeably with router, but switch is a general term without a rigorous technical definition. In marketing usage, it is generally optimized for Ethernet LAN interfaces and may not have other physical interface types. In comparison, a network hub does not do any routing, instead every packet it receives on one network line gets forwarded to all the other network lines.
Routers operate in two different planes:
• Control plane, in which the router learns the outgoing interface that is most appropriate for forwarding specific packets to specific destinations,
• Forwarding plane, which is responsible for the actual process of sending a packet received on a logical interface to an outbound logical interface.

Network Interface Card
A network interface controller (NIC) is a hardware device that handles an interface to a computer network and allows a network-capable device to access that network. The NIC has a ROM chip that contains a unique number, the multiple access control (MAC) Address burned into it. The MAC address identifies the device uniquely on the LAN. The NIC exists on both the ‘Physical Layer’ (Layer 1) and the ‘Data Link Layer’ (Layer 2) of the OSI model.
Sometimes the words ‘controller’ and ‘card’ are used interchangeably when talking about networking because the most common NIC is the network interface card. Although ‘card’ is more commonly used, it is less encompassing. The ‘controller’ may take the form of a network card that is installed inside a computer, or it may refer to an embedded component as part of a computer motherboard, a router, expansion card, printer interface or a USB device.
Although other network technologies exist, Ethernet has achieved near-ubiquity since the mid-1990s. Every Ethernet network card has a unique 48-bit serial number called a MAC address, which is stored in ROM carried on the card. Every computer on an Ethernet network must have a card with a unique MAC address. Normally it is safe to assume that no two network cards will share the same address, because card vendors purchase blocks of addresses from the Institute of Electrical and Electronics Engineers (IEEE) and assign a unique address to each card at the time of manufacture.
Whereas network cards used to be expansion cards that plug into a computer bus, the low cost and ubiquity of the Ethernet standard means that most newer computers have a network interface built into the motherboard. These either have Ethernet capabilities integrated into the motherboard chipset or implemented via a low cost dedicated Ethernet chip, connected through the PCI (or the newest PCI Express) bus. A separate network card is not required unless multiple interfaces are needed or some other type of network is used. Newer motherboards may even have dual network (Ethernet) interfaces built-in.
The card implements the electronic circuitry required to communicate using a specific physical layer and data link layer standard such as Ethernet or token ring. This provides a base for a full network protocol stack, allowing communication among small groups of computers on the same LAN and large-scale network communications through routable protocols, such as IP.
There are four techniques used to transfer data, the NIC may use one or more of these techniques.
• Polling is where the microprocessor examines the status of the peripheral under program control.
• Programmed I/O is where the microprocessor alerts the designated peripheral by applying its address to the system’s address bus.
• Interrupt-driven I/O is where the peripheral alerts the microprocessor that it’s ready to transfer data.
• DMA is where an intelligent peripheral assumes control of the system bus to access memory directly. This removes load from the CPU but requires a separate processor on the card.
A network card typically has a twisted pair, BNC, or AUI socket where the network cable is connected, and a few LEDs to inform the user of whether the network is active, and whether or not there is data being transmitted on it. Network cards are typically available in 10/100/1000 Mbit/s varieties. This means they can support a notional maximum transfer rate of 10, 100 or 1000 Megabits per second.

Wireless access point
In computer networking, a wireless access point (WAP) is a device that allows wireless communication devices to connect to a wireless network using Wi-Fi, Bluetooth or related standards. The WAP usually connects to a wired network, and can relay data between the wireless devices (such as computers or printers) and wired devices on the network.
In industrial wireless networking, the design is rugged with a metal cover, a Din-Rail mount, and a wider temperature range during operations, high humidity and exposure to water, dust, and oil. Wireless security includes: WPA-PSK, WPA2, IEEE 802.1x/RADIUS, WDS, WEP, TKIP, and CCMP (AES) encryption.
Different from the computer consumer market, industrial wireless access point can also be used as a bridge, router, or a client.

Prior to wireless networks, setting up a computer network in a business, home, or school often required running many cables through walls and ceilings in order to deliver network access to all of the network-enabled devices in the building. With the advent of the Wireless Access Point, network users are now able to add devices that access the network with few or no cables. Today’s WAPs are built to support a standard for sending and receiving data using radio frequencies rather than cabling. Those standards, and the frequencies they use are defined by the IEEE. Most WAPs use IEEE 802.11 standards.
Modem
Modem (from modulator-demodulator) is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. Modems can be used over any means of transmitting analog signals, from driven diodes to radio.
The most familiar example is a voiceband modem that turns the digital 1s and 0s of a personal computer into sounds that can be transmitted over the telephone lines of Plain Old Telephone Systems (POTS), and once received on the other side, converts those 1s and 0s back into a form used by a USB, Ethernet, serial, or network connection.
Modems are generally classified by the amount of data they can send in a given time, normally measured in bits per second, or “bps”. They can also be classified by Baud, the number of times the modem changes its signal state per second. For example, the ITU V.21 standard used audio frequency-shift keying, aka tones, to carry 300 bit/s using 300 baud, whereas the original ITU V.22 standard allowed 1200 bit/s with 600 baud using phase-shift keying.

System area network cards
System area network cards are interfaces installed for the purpose of connecting computer systems in a cluster. Clustering is a principle by which server systems are configured, and operate, as a single unit. In addition to providing increased processing and storage capabilities, it is also possible to configure servers to be fault-tolerant in either a compensatory or fail-over capacity.
In order for the systems in a cluster to communicate, network interfaces are installed that allow them to communicate directly without the need to use standard network links. These cards are normally high-performance units that utilize either twisted-pair or fiber-optic cabling. If there are only two systems in the cluster, a special cable can connect the two systems directly. If there are more than two, a hub is used to facilitate the connection.

Network Bridge
A network bridge connects multiple network segments at the data link layer (Layer 2) of the OSI model, and the term Layer 2 switch is very often used interchangeably with bridge. Bridges are similar to repeaters or network hubs, devices that connect network segments at the physical layer; however, with bridging, traffic from one network is managed rather than simply rebroadcast to adjacent network segments. In Ethernet networks, the term “bridge” formally means a device that behaves according to the IEEE 802.1D standard—this is most often referred to as a network switch in marketing literature. Bridges tend to be more complex than hubs or repeaters. Bridges can analyze incoming data packets to determine if the bridge is able to send the given packet to another segment of the network.

CSU/DSU
A CSU/DSU (Channel Service Unit/Data Service Unit) is a digital-interface device used to connect a Data Terminal Equipment device or DTE, such as a router, to a digital circuit (for example a T1 or T3 line).
A CSU/DSU operates at the physical layer (layer 1) of the OSI model. CSU/DSUs are also made as separate physical products; CSUs and DSUs. The DSU or both functions may be included as part of an interface card inserted into a DTE. If the CSU/DSU is external, the DTE interface is usually compatible with the V.xx or RS-232C or similar serial interface.
Digital lines require both a channel service unit (CSU) and a data service unit (DSU). The CSU provides termination for the digital signal and ensures connection integrity through error correction and line monitoring. The DSU converts the data encoded in the digital circuit into synchronous serial data for connection to a DTE device.