In computer networks, a proxy server is a server (a computer system or an application program) that acts as an intermediary for requests from clients seeking resources from other servers. A client connects to the proxy server, requesting some service, such as a file, connection, web page, or other resource, available from a different server. The proxy server evaluates the request according to its filtering rules. For example, it may filter traffic by IP address or protocol. If the request is validated by the filter, the proxy provides the resource by connecting to the relevant server and requesting the service on behalf of the client. A proxy server may optionally alter the client's request or the server's response, and sometimes it may serve the request without contacting the specified server. In this case, it 'caches' responses from the remote server, and returns subsequent requests for the same content directly.
A proxy server has many potential purposes, including:
* To keep machines behind it anonymous (mainly for security).
* To speed up access to resources (using caching). Web proxies are commonly used to cache web pages from a web server.
* To apply access policy to network services or content, e.g. to block undesired sites.
* To log / audit usage, i.e. to provide company employee Internet usage reporting.
* To bypass security/ parental controls.
* To scan transmitted content for malware before delivery.
* To scan outbound content, e.g., for data leak protection.
* To circumvent regional restrictions.
A proxy server that passes requests and replies unmodified is usually called a gateway or sometimes tunneling proxy.
A proxy server can be placed in the user's local computer or at various points between the user and the destination servers on the Internet.
A reverse proxy is (usually) an Internet-facing proxy used as a front-end to control and protect access to a server on a private network, commonly also performing tasks such as load-balancing, authentication, decryption or caching.
Wednesday, January 13, 2010
STM-1
The STM-1 frame is the basic transmission format for SDH. A STM-1 signal has a byte-oriented structure with 9 rows and 270 columns of bytes with a total of 2430 bytes (9 rows * 270 columns = 2430 bytes). Each byte corresponds to a 64kbit/s channel.
Rate (Frame capacity): : 2430 x 8 x 8000 = 155.520 Mbit/s.
Rate (Frame capacity): : 2430 x 8 x 8000 = 155.520 Mbit/s.
Tuesday, January 12, 2010
Asynchronous Transfer Mode (ATM)
Asynchronous Transfer Mode (ATM) is a standardized digital data transmission technology. ATM is implemented as a network protocol and was first developed in the mid 1980s.
Asynchronous Transfer Mode is a cell-based switching technique that uses asynchronous time division multiplexing.
It encodes data into small fixed-sized cells (cell relay) and provides data link layer services that run over OSI Layer 1 physical links. This differs from other technologies based on packet-switched networks (such as the Internet Protocol or Ethernet), in which variable sized packets (known as frames when referencing Layer 2) are used. ATM exposes properties from both circuit switched and small packet switched networking, making it suitable for wide area data networking as well as real-time media transport.
ATM uses a connection-oriented model and establishes a virtual circuit between two endpoints before the actual data exchange begins.
ATM has proven very successful in the WAN scenario and numerous telecommunication providers have implemented ATM in their wide-area network cores. Many ADSL implementations also use ATM. However, ATM has failed to gain wide use as a LAN technology, and lack of development has held back its full deployment as the single integrating network technology in the way that its inventors originally intended. Since there will always be both brand-new and obsolescent link-layer technologies, particularly in the LAN area, not all of them will fit neatly into the synchronous optical networking model for which ATM was designed. Therefore, a protocol is needed to provide a unifying layer over both ATM and non-ATM link layers, as ATM itself cannot fill that role. IP already does that; therefore, there is often no point in implementing ATM at the network layer.
Asynchronous Transfer Mode is a cell-based switching technique that uses asynchronous time division multiplexing.
It encodes data into small fixed-sized cells (cell relay) and provides data link layer services that run over OSI Layer 1 physical links. This differs from other technologies based on packet-switched networks (such as the Internet Protocol or Ethernet), in which variable sized packets (known as frames when referencing Layer 2) are used. ATM exposes properties from both circuit switched and small packet switched networking, making it suitable for wide area data networking as well as real-time media transport.
ATM uses a connection-oriented model and establishes a virtual circuit between two endpoints before the actual data exchange begins.
ATM has proven very successful in the WAN scenario and numerous telecommunication providers have implemented ATM in their wide-area network cores. Many ADSL implementations also use ATM. However, ATM has failed to gain wide use as a LAN technology, and lack of development has held back its full deployment as the single integrating network technology in the way that its inventors originally intended. Since there will always be both brand-new and obsolescent link-layer technologies, particularly in the LAN area, not all of them will fit neatly into the synchronous optical networking model for which ATM was designed. Therefore, a protocol is needed to provide a unifying layer over both ATM and non-ATM link layers, as ATM itself cannot fill that role. IP already does that; therefore, there is often no point in implementing ATM at the network layer.
E1
An E1 link operates over two separate sets of wires, usually twisted pair cable. A nominal 3 Volt peak signal is encoded with pulses using a method that avoids long periods without polarity changes. The line data rate is 2.048 Mbit/s (full duplex, i.e. 2.048 Mbit/s downstream and 2.048 Mbit/s upstream) which is split into 32 timeslots, each being allocated 8 bits in turn. Thus each timeslot sends and receives an 8-bit sample 8000 times per second (8 x 8000 x 32 = 2,048,000). This is ideal for voice telephone calls where the voice is sampled into an 8 bit number at that data rate and reconstructed at the other end. The timeslots are numbered from 0 to 31.
T1
T1 is a high speed digital network (1.544 mbps) developed by AT&T in 1957 and implemented in the early 1960's to support long-haul pulse-code modulation (PCM) voice transmission. The primary innovation of T1 was to introduce "digitized" voice and to create a network fully capable of digitally representing what was up until then, a fully analog telephone system.
Perhaps the way to really begin this discussion is to discuss the AT&T Digital Carrier System referred to as "ACCUNET T1.5". It is described as a "two-point, dedicated, high capacity, digital service provided on terrestrial digital facilities capable of transmitting 1.544 Mb/s. The interface to the customer can be either a T1 carrier or a higher order multiplexed facility such as those used to provide access from (fiber optic) and radio systems."
Perhaps the way to really begin this discussion is to discuss the AT&T Digital Carrier System referred to as "ACCUNET T1.5". It is described as a "two-point, dedicated, high capacity, digital service provided on terrestrial digital facilities capable of transmitting 1.544 Mb/s. The interface to the customer can be either a T1 carrier or a higher order multiplexed facility such as those used to provide access from (fiber optic) and radio systems."
Digital Signal 3 (DS3)
Digital Signal 3 rate 1.544Mbps.
A Digital Signal 3 (DS3) is a digital signal level 3 T-carrier. It may also be referred to as a T3 line.
DS3 interconnect cables must be made with true 75 ohm cable and connectors. Cables or connectors which are 50 ohm or which significantly deviate from 75 ohms will result in reflections which will lower the performance of the connection, possibly to the point of it not working. Bellcore standard GR-139-CORE defines type 734 and 735 cables for this application. Due to losses, there are differing distance limitations for each type of cable. 734 has a larger center conductor and insulator for lower losses for a given distance. The BNC connectors are also very important as are the crimping and cable stripping tools used to install them. Trompeter, Cannon, Amphenol, Kings, and Canare are some of the true 7.5 x 10 ohm connectors known to work. RG-6 or even inexpensive RG-59 cable will work in a pinch when properly connectorized, though it does not meet telephony technical standards.
A Digital Signal 3 (DS3) is a digital signal level 3 T-carrier. It may also be referred to as a T3 line.
DS3 interconnect cables must be made with true 75 ohm cable and connectors. Cables or connectors which are 50 ohm or which significantly deviate from 75 ohms will result in reflections which will lower the performance of the connection, possibly to the point of it not working. Bellcore standard GR-139-CORE defines type 734 and 735 cables for this application. Due to losses, there are differing distance limitations for each type of cable. 734 has a larger center conductor and insulator for lower losses for a given distance. The BNC connectors are also very important as are the crimping and cable stripping tools used to install them. Trompeter, Cannon, Amphenol, Kings, and Canare are some of the true 7.5 x 10 ohm connectors known to work. RG-6 or even inexpensive RG-59 cable will work in a pinch when properly connectorized, though it does not meet telephony technical standards.
Digital Signal 0 (DS0)
The DS0 rate was introduced to carry a single digitized voice call. For a typical phone call, the audio sound is digitized at an 8 kHz sample rate using 8-bit pulse-code modulation for each of the 8000 samples per second. This resulted in a data rate of 64 kbit/s.
The DS0 rate, and its equivalents E0 and J0, form the basis for the digital multiplex transmission hierarchy in telecommunications systems.
The Digital Signaling rate of 64 kbit/s, corresponding to the capacity of one voice-frequency-equivalent channel.
The DS0 rate, and its equivalents E0 and J0, form the basis for the digital multiplex transmission hierarchy in telecommunications systems.
The Digital Signaling rate of 64 kbit/s, corresponding to the capacity of one voice-frequency-equivalent channel.
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