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Introduction to NAS & SAN

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    Terms Used in NAS and SAN

    Note: This is an organic list of terms. If you discover a term that you believe belongs in this list, please tell us! Same goes if you hear a term you can't define. Send it to us and we'll look it up and post it here.

    Class 1 Service: This service level guarantees bandwidth and ordering of packets. It also returns confirmations of transmission.

    Class 2 Service: This service level is connectionless and can deliver packets out-of-order. Delivery of packets is however guaranteed and confirmations are sent.

    Class 3 Service: This is the lowest service level and does not guarantee either ordering or delivery.

    Connection: A connection is established when one host contacts another host with the desire to send/receive information via the transport layer. First, the hosts establish connection, identifying one another and where they each sit on the network or Internet, then data transfer occurs. The hosts send IP packets to one another in a mutually organized manner. Finally, the connection is released, completing the transaction. These three steps ensure that the data arrives in order, without corruption.

    Connection-Less Protocol: Data transmission occurs between two hosts that have not previously set up a connection. This method can save bandwidth, but is more prone to errors than connectioned protocols. See Packet Switching.

    Direct-attached storage: a torage device, either RAID, disk, or tape, attached directly to the application server that uses the data on the storage unit. Most often SCSI.

    Distributed Network: A network with nodes in multiple locations, such as an ISP with a node in Tokyo as well as one in New York City.

    DNS: A Domain Name Server (DNS) resolves domain names to their equivalent IP addresses so that IP traffic can be transported to the correct destination. Each Domain Name (i.e., is associated, at a minimum, with a Primary and a Secondary DNS. Domain Name Servers are located throughout the Internet. Many ISP's maintain their own DNS servers to reduce their use of bandwidth. Occasionally, these local DNS become out of date and lead to browser errors.

    Domain Name: See DNS

    Failover: If a network is equipped with redundant resources, such as mirrored servers or tandem load balancers, the secondary device can assume the duties of the primary should the primary fail. This can be done manually or automatically depending on the setup.

    Fabric Switch: In this category of switch, any port on any switch can provide (subject to bandwidth availability) full speed access to any other port on the network. The network consists of a fabric of linked switches.

    FC-0: This is the Physical layer of the Fibre Channel protocol stack. This layer includes the definition of all physical components used in Fibre Channel.

    Fault Tolerance: A network that is fully redundant and has maximum up-time is considered fault tolerant. In other words, should a component of the network fail, the network will continue processing requests and the failure will be transparent to clients.

    FC-1 This is the Encode/Decode layer in Fibre Channel specification. It covers the byte encoding and character-level error control.

    FC-2: This is what is called the Framing Protocol Layer. It incorporates the management of frames, flow control and CRC generation. It also manages sequences of frames comprising a transmission, and exchanges between nodes on the Fibre Channel to accomplish commands akin to the SCSI I/O sequence. This layer also provides the management of the three service classes: Class 1, Class 2, and Class 3.

    FC-3: This layer is called the Common Services Layer and is currently not used.

    FC-4: This layer is the Protocol Mappings Layer and is the layer that maps protocols such as SCSI and IP to the underlying layer protocols.

    Fibre Channel Arbitrated Loop (FC/AL): This is one of the possible physical topologies of Fibre Channel. In this topology, the Fibre Channel is connected in a loop with devices all connecting to the loop. It can be thought of as a similar structure to a token ring network. Up to 126 nodes can be connected to the loop.

    Fibre Channel Fabric: This is a structure where addressing of ports on a network of Fibre Channel is made independently of the physical location or address of the target port. Switches are responsable for passing Fibre Channel packets to the target port regardless of which Fibre Channel loop or switch the port physically resides on. Multiple switches can be connected to create large networks with up to 224 addressable ports.

    Fibre Channel Point-to-Point: This topology for Fibre Channel provides a simple direct connection between just two nodes. This is the closest approximation to the traditional SCSI topology.

    Fibre Channel Topology: A number of possible topologies exist for the connection of Fibre Channel. One is point-to-point, where a direction connection is made between nodes on the Fibre Channel. Another is Arbitrated Loop where multiple nodes can be connected together. Lastly there is a Fibre Channel Fabric topology which allows for multiple loops and point-to-point connections to be combined into networks using switches.

    FTP: File Transfer Protocol (FTP) is based on TCP/IP and is used to transfer files from one site to another via the Internet or through a LAN or WAN.

    Host: A fancy word for a computer on a network. This includes a user or server across the Internet or in the same office. The hostname of a server on the Internet also includes the domain name to make a complete Internet address. For instance, if a machine is known locally as server1 and is part of the domain at, the full hostname is

    HTTP: Hypertext Transfer Protocol is the medium by which web pages (HTML documents) are distributed via the Internet. Some load balancers do an "HTTP poll" to determine server health on a network. In this case, the load balancer requests a specific HTTP page; if the page is okay, the server is deemed healthy. If, for some reason, the page does not respond properly, the server is deemed inactive and no more traffic is sent to it.

    Hub: This is a simple connectivity device that allows for devices to be connected to a fibre channel loop by being attached to a hub port. The advantage of this is that failures of a single device on the loop can be isolated from the other ports on the loop. The aggregate bandwidth of the hub is still that of a single fibre channel loop.

    IP Address: A 32 bit designation for a host, that consists of a network address, host number, and subnet mask. These are written in dot notation (i.e., that actually corresponds to numbers of bits. IP addresses can be in one of three classes: A, B, and C. They can be "legitimate" or "non-legitimate". Legitimate addresses are public, such as those used for web sites, whereas non-legitimate addresses are given to devices behind the scenes, such as web servers reached through a proxy device such as a firewall.

    IP Packet: A segment of information that maintains its integrity as it travels through the Internet or across a network. Each packet contains header information that includes the type of traffic it is (FTP or UDP, for example) and where it originated from. SNMP can be used to track packets to gather a variety of statistics on network usage and load. The header information is also used by the responding server/load-balancing device to ensure that the answering packet goes to the correct location.

    Load Balancing: The process by which load (number of requests, number of users, etc.) is spread throughout a network so that no individual device becomes overwhelmed by too much traffic, causing it to fail. Load balancing also involves redirection in the case of server or device failure to allow for Failover and promote Fault tolerance.

    Network Attached Storage(NAS): In general, NAS is storage in a form that is readily accessible on a network, as opposed to direct-attached storage. Traditionally this meant having a storage array device that incorporated a file system and presented either a networked file system protocol such as NFS or CIFS, or else emulated a disk device so that the array could be connected to a storage I/O interconnect to the host.

    NAT: Network Address Translation (NAT) is used on devices such as firewalls and certain load balancers to allow clients to request information of a public IP address (assigned to a web site, for example) for which the information is actually located on servers with non-legitimate addresses (such as a group of web servers in a farm located behind the firewall or load balancer.

    Petabyte: A unit of storage equal to one quadrillion bytes, or 1024 TeraBytes. SimplStor by Zerowait offers a great competitive and compact open source NAS containing a petabyte of storage at an affordable price.

    Packet Switching: During transport from one host to another, packets may be routed out of order and across a variety of paths to get to the desired end point. UDP uses this method of transport. It is up to the computer at the destination to reassemble the packets into the original order.

    Persistent Connection: Also known as a "sticky connection". For instance, when a user accesses, the browser downloads the index.html file, plus all the images and buttons referenced in the HTML file. The retrieval of all of these individual page elements in HTTP may be performance enhanced by having the browser/server communicate with a single long session for several page elements, instead of a series of small sessions as each element downloads.

    Session: A lasting connection, usually involving the exchange of many packets between a host and a server.

    Ping: The Packet Internet Groper is used to test the availability of a host on a network or on the Internet. A UNIX command, ping also works in Windows through TCP/IP. In DOS/WINDOWS, at the DOS prompt, enter PING and the IP address or the domain name of the server you want to test. If you get replies the server is up. If not, it may be down or there may be a problem between you and the server (such as a downed phone line). To test this, perform the command TRACERT with the same IP address or domain name.

    Port: Because most network interfaces have only one or two physical ports (the means by which data comes into the computer from outside), you need to designate port numbers for different kinds of IP traffic. For example, port 80 is commonly used for HTTP traffic and port 21 is used for FTP. Fibre Channel ports come in a number of flavours depending on the topology of the Fibre Channel. N_Ports are simple equipment node ports in a point-to-point connection topology. NL_Ports are node ports connected to an Arbitrated loop. F_Ports are point-to-point ports connected to a fabric. Generally this means that the F_Port is a port on a switch. FL_Ports are ports connecting from one loop to a switch and thus to a fabric. E_Ports are expansion ports used to interconnect switches together. G_Ports are classified by some switch companies as ports that can be either E_Ports or F_Ports depending on usage.

    Redirection: The process by which traffic that is destined for one location is sent to another. This can be from server to server or, across a distributed network, from one WAN or LAN to another. Redirection is frequently used as a method of Failover.

    Server: A process that runs on a host that relays information to a client upon the client sending it a request. Servers come in many forms: application servers, web servers, database servers, and so forth. All IP-based servers can be load balanced. See Web Server.

    Server farm or cluster: A group of servers (usually four or more) that mirror one another or otherwise act as backup for one another or another farm.

    Spoofing: When a device, such as a load balancer, answers a request in the name of another device, such as a web server in a farm located behind it.

    Storage Area Network (SAN): A network of storage devices. Most often, Fibre Channel based networking, using a Fibre Channel switch between host and storage.

    SuperFarm: A server farm containing multiple server farms.

    SNMP Management: Simple Network Management Protocol (SNMP) is a method by which you can generate reports and monitor your network and the equipment on it. Generally, SNMP works on the TCP/IP level.

    TCP/IP: Transmission Control Protocol over Internet Protocol. These are two transmission protocols that work together to help the servers, clients and devices on the network talk to one another. FTP, HTTP, UDP, SNMP, and telnet run on top of TCP/IP.

    UDP: User Datagram Protocol transports data as a connectionless protocol, using packet switching.

    Web Server: Although it might seem that a web server is a machine, it is really a process running on a machine that serves HTTP content to web browsers on client machines. This is important because you can actually put several web servers on a single machine as long as you designate specific ports through which traffic will travel. Along the same lines, there are also FTP servers, mail servers, and so on, each of which handles a specific type of traffic.

    Zoning: This is the term used by some switch companies to denote the division of a SAN into subnets that provide different levels of connectivity or addressability between specific hosts and devices on the network. In effect routing tables are used to control access of hosts to devices. This zoning can be performed by cooperative consent of the hosts or can be enforced at the switch level. In the former case, hosts are responsable for communicating with the switch to determine if they have the right to access a device.

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