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SANs (Storage Area Networks)

SANs are dedicated networks that access to an array of storage devices such as RAID arrays, optical disks, and tape backups. One way to think of a SAN is as a high-speed network within a data center. In contrast, LANs extend outward from the data center. SANs are a radical shift from the traditional server-attached storage because storage is offloaded from servers, freeing up server resources to handle data processing and other tasks. The SAN concept is to make any storage device available to any user or device on the enterprise network. Any server attached to the SAN can access any disk, which means that any user on the network has access to any disk. This is because the SAN sub network provides any-to-any connections between servers and disk.

SAN (System Area Network)

A SAN (system area network) is a relatively local network designed for high-speed interconnection in cluster environments (server to server), multiprocessing systems (processor to processor), and SANs (storage area networks). The architecture is now almost exclusively switched fabric. Fibre Channel is an example of a SAN technology. It provides a high-speed switched environment in which any device on the network can connect with any other device and communicate over a dedicated high-speed link. InfiniBand is another.

Satellite Communication Systems

Satellite Communication Systems Satellite communication systems consist of Earth-orbiting communications platforms that receive and retransmit signals from earth-based stations. A typical television satellite receives a signal from a base station and broadcasts it to a large number of terrestrial receivers. Signals to satellites are called “uplinks,” and signals from satellites are called “downlinks.” Uplinks have also been

called “shooting the bird.” The downlink covers an area called the “footprint,” which may be very large or cover a focused area. Satellites use microwave frequencies. Since they are overhead, the transmissions are line of sight to the receiver.

SDH (Synchronous Digital Hierarchy)

SDH is the ITU standard for a synchronous optical network that supports multiple gigabit-per-second transmission speeds over fiber-optic cable. Long-distance and regional telecommunication companies outside of North America implement SDH on their fiber-optic trunk lines. SONET, which is basically a subset of SDH, is used in the United States.

SDLC (Synchronous Data Link Control)

SDLC is an IBM-defined Data Link Control protocol developed in the 1970s for communication over wide area links to IBM host systems in SNA (Systems Network Architecture ) environments. SDLC is based on synchronous, bit-oriented operations as compared to byte-oriented protocols Such as BISYNC(Binary Synchronous Communications)

Data and communications security is critical on today’s networks. Hackers, Internet intruders, eavesdroppers, forgers, and other attackers are everywhere. Few people have not heard of some sort of computer and network attack. Many are victims. An entire volume could be written on security issues.

A network segment is a data link layer and physical network that is shared by one or more Attached nodes. Ethernet and token ring are shared data link layer networks. One segment may Be attached to another segment with a bridge to form an extended broadcast domain ;but the Bridge filters traffic and, in the case of Ethernet, keeps collisions contained with in each segment.

Serial Communication and Interfaces

Serial communication equipment and interfaces transmit signals across point -to- point data links. Bits are sent one after another in a serial stream. Serial connections are commonly used to connect computers to modems. The bit stream traverses the serial link and enters the modem, where it is modulated onto an analog signal for transmission across telephone lines. Serial lines are normally bidirectional and use one wire for transmit and another for receive.

A server is either a hardware-based processing device or a software-based process that provides services to network users. This topic describes the former: servers, processors ,server components, racks, and related components. A server is a network-connected computer system that provides services to network users or Web services to Internet users. Servers may be located anywhere on the network, but for

large companies, they are usually located in secure centrally managed data centers. Collections of servers are often called “server farms.” Servers may be configured in clusters, which are interconnected sets of servers that provide fault tolerance and load balancing (sending user requests to the available or appropriate server).

SLA (Service-Level Agreement)

An SLA is a contract between a service provider and a customer. The service provider may be a telecommunications carrier, an Internet service provider, or any company that provides outsourcing services. The services provided may include dedicated leased lines, shared packet-oriented services, Web hosting services, off-site application management (i.e., ASPs), and off-site network management (i.e., MSPs). The SLA specifies the terms of the agreement and how much the customer will pay for those services.

SLIP (Serial Line Internet Protocol)

When one TCP/IP system connects with another TCP/IP system over a serial point-to-point communication line (e.g., a dial-up modem), some way is needed to transport IP packets (a network layer activity) across the serial link (a data link layer activity). Basically, IP packets must be encapsulated into data link layer frames to make the trip across the serial link. Two schemes have been adopted by the Internet community to provide these links: SLIP and PPP (Point-to-Point SLIP in nearly all installations. PPP transports other protocols such as DECnet, IPX, and

AppleTalk, and supports synchronous transfers, frame error detection, and controls to automatically configure addresses and links. SLIP is described in RFC 1055 (A Nonstandard for Transmission of IP Datagrams Over Serial Lines: Slip, June 1988)

SNMP (Simple Network Management Protocol)

Short for Simple Network Management Protocol, a set of protocols for managing complex networks. The first versions of SNMP were developed in the early 80s. SNMP works by sending messages, called protocol data units (PDUs), to different parts of a network. SNMP-compliant devices, called agents, store data about themselves in Management Information Bases (MIBs) and return this data to the SNMP requesters.

A socket is the endpoint of a connection over a TCP/IP network, much

like a phone is the endpoint of a connection in the telephone network. A socket is the combination of a TCP port and an IP address. Ports are logical interfaces for applications, and they are assigned numbers. For example, The Web’s HTTP protocol is located at port 80. Using the telephone analogy, the TCP port is then like a telephone number and the IP address is the location of the phone. A socket is like a phone that has been assigned a phone number. However, a typical host can have many sockets active at the same time (multiplexing).A socket is created during a connection setup between two systems when a process makes a request to a remote machine to open a connection at a specific port. Once the connection is set up, data transfers can begin.

Softswitch refers to an architecture for a device that supports the integration of IP telephony and the PSTN. In the NPN, the traditional circuit-switched voice network will slowly give way to a packet-oriented voice and data network based on Internet technology. Soft switches are an alternative form of Class 5 switch. A Class 5 switch is a big expensive telephony switch, located in central offices all over the world. It accepts dial-up telephone calls from users and creates circuits across a hierarchy of telephone switches, some local, and some regional, national, or international. Call setup and management is handled by SS7 (Signaling System 7), which runs as an out-of-band signaling protocol to control PSTN switching equipment.

Spanning Tree )802.1D Bridging(

The 802.1D Spanning Tree Protocol provides path redundancy while eliminating loops in bridged networks. Path Redundancy ensures that when the main link fails, there is another link acting as backup.Loops appear when there is more than one path available in a network. When the loop appears, the switches see stations appear in both directions. The switch becomes confused and multiple frames are sent.As a solution to this problem, the Spanning-Tree Protocol defines a tree that spans all switches in an extended network. Spanning-Tree Protocol forces certain redundant data paths into a standby (blocked) state. If one network segment in the Spanning-Tree Protocol becomes unreachable, or if Spanning-Tree Protocol costs changes, the spanning-tree algorithm reconfigures the spanning-tree structure and reestablishes the link by activating the standby path.

Spread Spectrum Signaling

Wireless mobile communications and wireless LANs can use a variety of schemes to transmit analog or digital information between base stations and users. One method is to transmit at a specific frequency, much like a radio station transmits at a frequency that you dial on your radio. For example, the AMPS mobile cellular telephone system operates in the 824-MHz to 894-MHz frequency range. This range is divided into a pool of 832 full-duplex channel pairs (one sends, one receives). Calls are made over the channels. The only problem with this scheme is that anyone with an appropriate radio receiver can listen in on a target frequency. The other problem is that the frequency occupies a narrow band that is susceptible to interference, either accidental or malicious. Spread spectrum is a technique of spreading a signal out over a very wide bandwidth, often over 200 times the bandwidth of the original signal.

SPX (Sequenced Packet Exchange)

SeeIPX/SPX (Internetwork Packet Exchange/Sequenced Packet Exchange).

SS7 (Signaling System 7)

SS7 is an out-of-band signaling system used by the carriers to set up

telephone calls. It is a protocol standard defined by the ITU. Network elements in the public-switched telephone network use SS7 to exchange information used not only to set up calls but to control the network. Part of SS7’s call setup process is to create a circuit for the call through the telephone network and then place the call on the circuit. SS7 is a message-based system that operates on a network that is separate from the digital lines that carry calls. This differs from early telephone signaling systems in which signals were transmitted as multifrequency tones in the same channels as calls. In addition, SS7 enables advanced services that were not possible in the older system, such as toll-free numbers, caller ID, call forwarding, call waiting, and local number portability.

SSH is a “secure shell,” which means it provides encrypted “virtual” terminal (Telnet) and file transfer sessions. It also supports authenticated remote logon. It is designed to replace UNIX commands such as rlogin, rsh, and rcp, which have been found to be vulnerable to attacks. SSH provides the mechanisms that allow clients and servers to negotiate secure connections. SSH can create secure remote X sessions for X Window System users.

SSL (Secure Sockets Layer)

SSL is a Web protocol for establishing authenticated and encrypted sessions between Web servers and Web clients. SSL starts with a handshake routine that first establishes a TCP/IP connection. Next, the server is authenticated to the client by verifying its public key. Once authenticated, the server selects the strongest cryp to graphic algorithm supported by both the client and server and within the restrictions enforced by a particular country. Next, a shared secret key is generated that is used to encrypt all data flowing between the client and server. Finally, an encrypted SSL connection is established.

An internetwork is a collection of networks joined by routers. Paths through the network must be found and made available to the router so it knows the best path on which to forward a packet to its destination. Static routing is the process a network administrator does to manually configure network routes. The alternative is dynamic routing. If the

internetwork is small, static routing may be the best approach; but if it is large, dynamic routing is preferred. Note that in a dynamic routing environment, some paths may be manually configured to control the routing environment. Dynamic routing requires routing algorithms. Dynamic routing protocols assist in the automatic creation of routing tables.

Store-and-Forward Networking

“Store and forward” is a phrase used in many networking technologies. Packets are stored and forwarded through routers, as discussed under “Routers” and “Routing Protocols and Algorithms.” Network applications use store-and-forward messaging when real-time connections are not essential, as discussed under “MOM (Message-Oriented Middleware).Finally, electronic mail systems use store-and-forward techniques to move e-mail from one user to another across messaging servers, as discussed under “Electronic Mail.”

Structured Cabling Standards

Structured wiring or cabling is a preplanned cabling system that is designed with growth and reconfiguration in mind. Structured wiring forms an infrastructure that is usually hierarchical in design with high-speed backbones or interconnects. The backbone or interconnects must be high speed because periphery networks connect to it. Enterprise hubs and switches are at the top of the hierarchy. Departmental or workgroup hubs and switches connect to these. Then entire systems may connect to a data center.

See IP (Internet Protocol).

S/WAN was originally an initiative of RSA Security, in conjunction with

leading firewall and TCP/IP stack vendors. S/WAN’s goal is to help companies build secure firewall-to-firewall connections over the Internet between their company sites or between business partners .Basically, S/WAN creates secure VPNs (virtual private networks). All data that is transmitted between sites is encrypted to hide it from wiretappers. Connections mimic a private leased line over the Internet.

S/WAN uses the IETF’s IPSec specification as the basis for implementing interoperability among different firewall and TCP/IP products. S/WAN devices sit at the edge of a network where it connects with the Internet.

Switching, Multilayer

See Multilayer Switching.

Switching and Switched Networks

Network switches are multipoint connection devices that provide a point of attachment for a single computer or another device (hub or switch) that has multiple computers attached to it. The most important feature is that any device attached to one port can directly communicate with a device on another port over what is essentially a private link. The significance of this technology can be seen when compared to older Ethernet shared LAN technologies. The traditional coaxial cable Ethernet LAN implements a linear cable topology that is shared by all the computers attached to it. Only one device can transmit at a time, so some computers will need to wait while another is transmitting. Computers “listen” for signals on the cable to see if it is being used.

Synchronous Communications

When devices exchange data, there is a flow or stream of information between the two. In any data transmission, the sender and receiver must have a way to extract individual characters or blocks (frames) of information. Imagine standing at the end of a data pipe. Characters arrive in a continuous stream of bits, so you need a way to separate one block of bits from another. In asynchronous communications, each character is separated by the equivalent of a flag so you know exactly where characters are located. In synchronous communications, both the sender and receiver are synchronized with a clock or a signal encoded into the data stream.