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"R" Networking Definitions & Concepts...

Redirector .. to .. Routing Table Maintenance Protocol (RTMP)

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Rate Distortion Theory:

Rate distortion theory is the branch of information theory addressing the problem of determining the minimal amount of entropy (or information) "R" that should be communicated over a channel such that the source (input signal) can be reconstructed at the receiver (output signal) with given distortion "D". As such, rate distortion theory gives the theoretical bounds for how much compression can be achieved using lossy data compression methods. Many of the existing audio, speech, image, and video compression techniques have transforms, quantization, and bit rate allocation procedures that capitalize on the general shape of rate-distortion functions.

Rate distortion theory was created by Claude Shannon in his founding work on information theory.

In rate distortion theory, the rate is usually understood as the number of bits per data sample to be stored or transmitted. The notion of distortion is a subject of on-going discussion. In the most simple case (which is actually used in most cases), the distortion is defined as the variance of the difference between input and output signal (i.e., the mean-squared error of the difference). However, since we know that most lossy compression techniques operate on data that will be perceived by humans (listen to music, watch pictures and video) the distortion measure preferebly should include some aspects of human perception. In audio compression perceptual models, and therefore perceptual distortion measures, are well developed and routinely used in compression techniques such as MP3 or Vorbis, but often not easy to include in rate-distortion theory. In image and video compression, the human perception models are less well developed and inclusion is mostly limited to the JPEG and MPEG weighing (quantization, normalization) matrix.


A redirector is a program running in a workstation that is attached to a network. It interceptes requests for network resources and services and redirects them over the network to file servers or peer workstations. For example, if a workstation user makes a request for local files, the redirector forwards the request to the local operating system. If the request is for files on a network server, the redirector forwards the request out over the network to the appropriate server. The request is placed in a packet with the address of the server.

On NetWare and other PC networks, redirector software is individually installed at each workstation along with the driver software for the network interface adapter installed in the computer.

Reduced Instruction Set Computer (RISC):

Microprocessors have instruction sets called microcode that programmers use to create low-level computer programs. The instruction sets perform various tasks, such as moving values into registers or executing instructions to add the values in registers. Microcode can be either simple or complex, depending on the microprocessor manufacturer's preference and the intended use of the chip. There are two categories:

  • Complex Instruction Set Computer (CISC) designs include a rich set of microcode that simplifies the creaton of programs that run on the processor.
  • Reduced Instruction Set Computer (RISC) designs, as the name implies, have a reduced set of instructions that improve the efficiency of the processor, but require more complex external programming.

RISC designs are based on work performed at IBM by John Cocke. He found that about 20 percent of a computer's instructions did about 80 percent of the work. His 80/20 rule spawned the development of RISC architecture, which reduces the number of instructions to only those that are used most. The other insturctions must be implemented in external software.

CISC designed microprocessors dominated the microcomputer market in the 1980's, but RISC designs are becoming more prominent as users require more speed. In addition, modular programming techniques and advanced programming interfaces have made it easier for developing programs in general, thus hiding some of the complexities of programming associated with RISC design. RISC based processors include the MIPS chips, the DEC Alpha, and the IBM RS family of chips. Current and future processor designs seem to favor RISC over CISC.

Redundant Array of Independent Disks (RAID):

Raid was develop at the University of California at Berkeley in the mid-1980s. It was designed to provide greater practical performance, capacity and reliability by properly coordinating the read/write activities among a series of linked drives. It addressed the topic of how to organize several small disks to make them behave and appear as on large drive. RAID was built upon existing technology rather than developed as a radical new product.

RAID originally stood for redundant array of inexpensive drives because the drives used were not that expensive. Later it changed to independent disks. Although these systems were inexpensive considering their capacity and data secuity features, they were more expensive than single drive solutions because they contained more advanced software and hardware technology.

Relational Database:

A database in which the only data structures are relations. A database whose logical structure is made up of nothing but a collection of relations (tables). The relational data model is the result of the work of one man -- Edgar (E.F.) Codd. During the 1960s, Dr Codd, although trained as a mathematician, was working with existing data models. His experience led him to believe that they were clumsy and unnatural ways of representing data relationships. He therefore went back to mathematical set theory and focused on the construct known as a relation (or set). He extended that concept to produce the relational database model, which he introduced in a seminal paper in 1970. Codd, E.F. "A Relational Model of Data for Large Shared Databanks." Communications of the ACM, Vol. 13, No. 6, June, 1970

A relational database consists of tables that are appropriately structured. Appropriateness is referred to as normalization, which is the process or technique for producing a set of tables with desirable properties that supports the requirements of a an end user or entity.

Data normalization is a set of rules and techniques concerned with:

  • Identifying relationships among attributes,
  • Combining attributes to form relations, and
  • Combining relations to form a database.

An attribute (also known or called a field) is a database data element. A relation is a group of attributes.

For example, a relation called PERSON may contain the attributes NAME, ADDRESS, DATE_OF_BIRTH, HEIGHT, WEIGHT, SALARY, etc. A Human Resources database may contain many relations such as PERSON, DEPENDENTS and DEPARTMENTS.

Remote Copy (rcp):

Remote copy allows files to be copied from or to remote systems. Its syntax is similar to copy (cp) except that the file path can include the name of a remote host. rcp is often compared to ftp. They both move files between hosts on a network, but ftp runs as an interactive program with many subcommands, while rcp runs with a simple command-line interface.

Remote Login (rlogin):

Remote Login (rlogin) allows a user sitting at one computer to connect to a remote machine and establish an interactive login session. rlogin has functions similar to telnet or TELNET.

Remote Shell (rsh):

Remote shell passes a command to a remote host for execution. Standard output and standard error from the remote execution are returned to the local host. There is no parallel to rsh in the standard TCP/IP protocols.


A device that extends the maximum length of cable in a single network, so that the network can be expanded. Repeaters are used to link two network cables of the same type. AppleTalk networks use two primary cable types: LocalTalk and Ethernet, which are both considered the hardware links associated with AppleTalk. Basically, a repeater can link two LocalTalk cables to make a longer LocalTalk cable, or two Ethernet cables to make a longer Ethernet cable.

The primary purpose of a repeater is to amplify and reclock the electrical signal. A signal traveling along a cable is gradually degraded (weakened) by the resistance of the path, making the voltage of the wave too low to be detected (heard) by the receiver. A repeater solves this problem by reissuing the signal at full strength across to the second cable. Obviously, the placement of the repeater is important. The repeater must be located at some place along the cable where it can still detect the incoming, somewhat weak, signal.

A repeater has two primary limitations. The first is that a repeater strengthens a signal, but does so only for two passes. Once a signal has gone through the second repeater in a row, the signal is no longer strong enough to be repeated a third time. Secondly, a repeater does not solve the problem of too may collisions. The number of collisions goes up with the number of nodes (devices) in use on a segment. When the number of collisions causes the network to fail or to slow to a crawl, you need to move up a step and install a device called a bridge.

Replication (in Databases):

Replication is a technique that copies all or a part of a database to another location for backup and/or to provide the latest updates to the database. Replication is only one technique for keeping distributed databases in syncronization. The other popular method is two-phase commit. Both are briefly described next for comparison.

  • Two-phase commit is used in online transaction processing where information is written to two or more separate databases simultaneously. A bank transaction that updates branch office databases is an example. In a two-phase commit, all the databases involved must first acknowledge to a monitoring program that they are ready to write data, then acknowledge that they have in fact made the write. If any system fails to acknowledge, all systems abort the write. See "Transaction Processing" for more information.
  • Replication involves copying all or part of a database at regular intervals, or after major changes, from a remote site to the master database, or from the master database to remote sites.

Not all database updates are time-critical, and this is where replication is useful. For example, a directory service contains the names of users and resources on the network. This database (or partitions of it) is copied to remote locations so users at those locations can access the data locally, rather than using a wide area network (WAN) connection to access data at a master site. Changes to the database such as the addition of a new user are usually not critical. Remote users probably won't need this new information right away, so the remote databases are replacated at periodic intervals, or during evening hours so as not to tie up expensive WAN links.

Reverse Address Resolution Protocol (RARP):

The Reverse Address Resolution Protocol (RARP), defined in RFC 903, is a variant of the address resolution protocol. RARP also translates addresses, but in the opposite direction. It converts Ethernet addresses to IP addresses, instead of IP addresses to Ethernet addresses. The RARP protocol really has nothing to do with routing data from one system to another. It is here discussed because of its close relationship to ARP, and because it is sometimes confused with ARP. ARP is the protocol that maps IP addresses to physical Ethernet addresses, so datagrams can be delivered from one host to another.

RARP helps configure diskless systems by allowing diskless workstations to learn their IP addresses. A diskless workstation has no disk to read its TCP/IP configuration from--not even its IP address. However, every system knows its Ethernet address because it is encoded in the Ethernet interface hardware. The diskless workstation uses the Ethernet broadcast facility to ask which IP address maps to its Ethernet address. When a server on the network sees the request, it looks up the Ethernet address in the table /etc/ethers. If it finds a match, the server replies with the workstation's IP address.

RG-58 Coaxial Cable:

A two-conductor shielded cable similar to television coaxial cable used primarily in Ethernet networks. RG-58 is often called "thin coax." It has a 50-ohm impedance characteristic.

RG-62 Coaxial Cable:

A two-conductor shielded cable similar to television coaxial cable used for ARCNET network topologies. The cable has a 93-ohm impedance characteristic.

RJ-11 and RJ-45 Connections:

Telephone-type jacks used in Ethernet 10-BaseT and other twisted-pair cable network systmes. There are two styles:

  • RJ-11 is a four-wire modular connector for telephones.
  • RJ-45 is an eight-wire modular connector for networks and some phone systems.

A router is usually defined to be a node in the network which forwards packets in a lower layer without a protocol change occurring in that layer. Routers operate at the network level, which in most cases is the IP level. These internet routers are capable of taking routing decisions based on information in the IP protocol header and in their routing tables.

Basically routing means that a packet must initially be sent through one or more nodes in order to reach another computer. It can also be considered a device that connects networks together, isolating traffic within each network. The networks can be of the same type (such as two Ethernet networks, or two AppleTalk networks) or of different types (such as AppleTalk, to Token Ring or Ethernet. Or Token Ring to Ethernet). A router receives data transmitted from other networks and re transmits it to its proper destination over the most efficient route; this route may include several routers, each forwarding the data to the next.


Routing is the process of determining an end-to-end path between the sender and the receiver for a packet. This is one of the major functions of the network layer in the OSI (Open Systems Interconnection) Reference Model. Another function is relaying, which is actually passing packets along the path.

Routing Table Maintenance Protocol (RTMP):

In the AppleTalk protocol suite, a transport layer protocol for tracking and updating the information in the routing table for an internetwork. RTMP is similar to the RIP (Routing Information Rrotocol) in the TCP/IP protocol suite and the RIP (Router Information Protocol) used in Novell's NetWare.

rsync (Open Source):

rsync is an open source utility that provides fast incremental file transfer. rsync is freely available under the GNU General Public License. rsync - faster, flexible replacement for rcp

rsync is a program that behaves in much the same way that rcp (remote file copy) does, but has many more options and uses the rsync remote-update protocol to greatly speed up file transfers when the destination file already exists.

The rsync remote-update protocol allows rsync to transfer just the differences between two sets of files across the network link, using an efficient checksum-search algorithm described in the technical report that accompanies this package.

Some of the additional features of rsync are:

  • support for copying links, devices, owners, groups and permissions
  • exclude and exclude-from options similar to GNU tar
  • a CVS exclude mode for ignoring the same files that CVS would ignore
  • can use any transparent remote shell, including rsh or ssh
  • does not require root privileges
  • pipelining of file transfers to minimize latency costs
  • support for anonymous or authenticated rsync servers (ideal for mirroring)

There are eight different ways of using rsync. They are:

  • for copying local files. This is invoked when neither source nor destination path contains a : separator
  • for copying from the local machine to a remote machine using a remote shell program as the transport (such as rsh or ssh). This is invoked when the destination path contains a single : separator.
  • for copying from a remote machine to the local machine using a remote shell program. This is invoked when the source contains a : separator.
  • for copying from a remote rsync server to the local machine. This is invoked when the source path contains a :: separator or a rsync:// URL.
  • for copying from the local machine to a remote rsync server. This is invoked when the destination path contains a :: separator or a rsync:// URL.
  • for copying from a remote machine using a remote shell program as the transport, using rsync server on the remote machine. This is invoked when the source path contains a :: separator and the --rsh=COMMAND (aka "-e COMMAND") option is also provided.
  • for copying from the local machine to a remote machine using a remote shell program as the transport, using rsync server on the remote machine. This is invoked when the destination path contains a :: separator and the --rsh=COMMMAND option is also provided.
  • for listing files on a remote machine. This is done the same way as rsync transfers except that you leave off the local destination.

Note that in all cases (other than listing) at least one of the source and destination paths must be local.

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