Network Fault Tolerance

Redundant Array of Inexpensive disks (RAID)

RAID is a fault tolerant method of storing data, meaning that a failure can occur and the system will still function. The various RAID categories are:

• 0 - Disk striping - Data is written across multiple drives in parallel. Different parts of the data is written at the same time to more than one drive. If there are two drives, half the data is written to one drive, while the rest of the data is written to the other drive. All partitions on striped drives must be the same size. No fault tolerance is provided with RAID-0.

• 1 - Disk mirroring - All the data is written to two drives so each drive has a complete of all stored data. If one drive fails, the other can be used to get a copy of the data. To be more fault tolerant, more than one controller card may be used to control the mirrored hard drives. This is called disk duplexing and will allow the system to keep functioning if one controller card fails.

• 2 - Disk striping with error correction codes (ECC).

• 3 - Disk striping with ECC parity information stored on a separate drive.

• 4 - Disk striping with blocks with parity information stored on a separate drive.

• 5 - Disk striping with blocks with parity information stored using multiple drives. Uses five
disks with one fifth of each one to store parity information.

Sector Sparing

Sector sparing will detect when data is going to be read from or written to a bad sector on the hard drive and will move the data to a good sector. The bad sector is marked as not available so it is not used again.

Windows NT support

Supports RAID-0,1, and 5 along with sector sparing.

Terms:

• DAT - Digital Audio Tape

• Sector Sparing - A method of fault tolerance that automatically identifies and marks bad sectors as not available. It is also called hot-fixing.

• SLED - Single Large Inexpensive disk - The concept that a large disk costs less per amount of storage than several smaller ones. Somehow this concept is used as a means of fault tolerance.

Network Drivers

Driver interfaces allow multiple protocol stacks to use one network interface card. The two in use today are listed below. They are not compatible with each other.

Open Driver Interface (ODI)

ODI is normally found on NetWare networks and was developed by Novell and Apple. It consists of:

• Multiple Protocol Interface - Provides connectivity from the data link layer to the network layer.

• Link Support Layer - It includes functions for managing protocol stack assignments and coordinating numbers assigned to MLIDs.

• Multiple-Link Interface Driver (MLID) - Passes data between the data link layer and the hardware or the network media. The drivers are protocol-independent.

Allows multiple drivers to be used on one card and lets one protocol use multiple cards.

Network Driver Interface Specification (NDIS)

NDIS, from Microsoft, is used on Microsoft networks. It allows multiple protocols to be used on a network card and supports the data link layer of the network model.

Transport Driver Interface (TDI)

This is a standard for passing messages between the drivers at the data link layer and the protocols working at the network layer such as IP or NetBEUI. It was produced by Microsoft.

AppleTalk Network

AppleTalk Topology is bus. Cable type is STP. The connectors are specialized. The media access method is CSMA/CA . Maximum segment and network length is 300 meters. The maximum number of connected segments is 8. There are 32 maximum nodes per segment with 254 maximum number of nodes per network. Speed is 230.4Kbps. The cabling system used with AppleTalk is called LocalTalk.

Addressing
Addressing is dynamic with each computer, when powered on, choosing its last used address or a random address. The computer broadcasts that address to determine if the address is used. If it is used, it will broadcast another random address until it finds an unused address.

EtherTalk and TokenTalk provide for use of AppleTalk network protocols on top of ethernet and token ring architectures respectively.

LocalTalk
LocalTalk uses STP cable and bus topology. Using CSMA/CA for media access, computers will first determine if any other computers are transmitting, before they transmit. A packet is transmitted prior to transmitting that alerts other computers that a transmission will be sent. Usually LocalTalk is only used in small environments.