Devices on a network can only understand each other if they use the same
protocols. A specific network can use up to seven layers of protocol at once — the
lowest layer dealing with network hardware whilst higher levels accommodate
file transfers and the user interface.
These layers allow data can be moved easily from between networks, irrespective of the computer platform — for example, PC files can be carried over a Mac network. The layers make the network appear transparent to the data.
The OSI Protocol Model
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The International Standards Organisation (ISO) has devised a general-purpose seven layer protocol mode for Open System Interconnection (OSI). Each layer is built upon those beneath as follows:-
Layer Function
7 Application Allows recipient to use data using with applications
6 Presentation Modifies files to suit recipient’s computer
5 Session Co-ordinates actions of sender and recipient
4 Transport Confirms data has been sent correctly (not always used)
3 Network Directs data to the correct recipient (only for a network)
2 Data Link Basic control of data flow and error detection
1 Physical Defines connection system and speed
Layers 1 to 5 are concerned with internetworking whilst 6 and 7 are for inter-operation, Although the model isn’t fully implemented, various standards for Layers 1 to 4 are well established. The remaining layers are more sophisticated and are being developed above the existing layers. Of these the Presentation Layer is important since it includes file interpretation, decryption and decompression.
It’s often difficult to know where one protocol ends and another begins! These protocols can appear in various of forms — often as add-on components to the system or as part of an integrated application.
OSI and Existing Standards
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Not all existing standards fit exactly into the OSI model. For example, the IEEE specifications for Ethernet (802.3), Token Bus (802.4), Token Ring (802.5) and Metropolitan Area Network (MAN) (802.6) use their own set of protocols that encompass the Physical and Data Link layers. Fortunately this causes no problems for the layers above!
These standards use the following layers in place of layers 1 and 2 of the OSI model:-
Layer
Logical Link Control (LLC)
Media Access Control (MAC)
Physical
This Physical Layer is specific to the 802.x standards and doesn’t correspond to the Physical Layer in the OSI model. Station Management Protocol (SMT) for a Fibre Distributed Data Interface (FDDI) encompasses both the MAC and 802.x Physical layers shown above.
Physical Layer / Network Hardware
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The Physical Layer is concerned with the transfer of individual data bits between two devices. The simplest form of communication involves asynchronous data transfer in which data is sent in byte-length bursts to minimise timing problems, usually by means of RS-232 (V.24) protocol. The link can be made directly or with a modem over a telephone or radio circuit. Many networks include servers to give access to such a link.
In a real network there must be a common understanding between several devices at once. This is easier if synchronous data transfer is employed, in which a code pattern is sent at regular intervals to maintain timing accuracy.
√π See the Modems chapter for more about using modems
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A Local Area Network (LAN) joins together a number of workstations. These are known as client workstations if a dedicated file server is used to supply centralised data.
Topography
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The physical layout of wiring used in a network, known as its topography, isn’t always obvious. For example, a Token Ring with each terminal wired to a Multi-station Access Unit (MAU) looks like a star network, even though it’s actually a ring!
Also the logical layout of a network needn’t match the physical arrangement. For example a Token Bus (see below) is wired in a linear form but the data passes between terminals as if it were a ring!
The following characteristics should be noted:-
z Linear
Common circuit shared by every device, each with a unique hardware address.
z Star
Individual circuits connected to a central switching hub — no circuits are shared!
z Ring
Each device extracts required data, regenerates the signal and passes it on.
Cable Types
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Cables come in the following forms:-
z Unshielded Twisted-pair (UTP)
For rates of up to 10 Mbit/s, although higher rates have been used.
Since there’s no shielding this cable may cause interference problems.
z Shielded Twisted-pair (STP)
For up to 16 Mbit/s. Distance of between 100 m and 1.5 km may be covered
without signal regeneration.
z Co-axial
For up to 50 Mbit/s. It’s possible to cover up to 500 m without regeneration.
z Fibre-optic
For 100 Mbit/s or higher without regeneration over 2 km or more.
The Bayonet ST connector is more efficient than the 9 mm SMA version.
Transmission Techniques
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Most networks are baseband systems that provide only one data channel. This necessitates a mechanism to prevent two devices using the channel at once. Different network types use different approaches.
Ethernet and LocalTalk use Carrier Sense Multiple Access with Collision Detection (CSMA/CD). Any device already using the network has priority — other devices sense the presence of its carrier and have to wait. When the channel becomes vacant it’s possible that two devices will begin transmission at the same time. If this happens they both stop sending signals, wait a random period of time, and then try again.
Token Bus uses token passing to effectively provide Time Division Multiplexing (TDM) in which each device has a time slot — but only if it wants to send a message. In a Token Ring one device acts as a Monitor Station, issuing tokens allowing other devices to transmit, whilst all the other devices wait to see if they can become the Monitor Station.
Repeaters and Other Devices
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A repeater operates purely in the Physical Layer by cleaning up the data from one segment of a network and passing it on to the next — a multi-port repeater provides this for several segments. Some devices introduce retiming to compensate for long cables or reconstruct the preamble in a data packet should it be damaged.
A bridge operates in the OSI Physical and Data Link Layers. In IEEE 802.x systems it works across the MAC and 802.x Physical layers. A bridge acts as a store and forward device, selectively passing data, often between networks of a different type.
A router also connects different types of networks, but directs the data to its destination, usually by means of Internet Protocol (IP) in the Network Layer. IP uses addresses that are independent of any hardware addresses that may be in use. A brouter acts as a bridge until it receives data of a specific type that causes it to use IP in the same way as a router.
A gateway operates at any of the higher Layers, perhaps offering some inter-operation between different types of network.
The LocalTalk box has a plug-actuated switch on both network sockets. These provide termination in the event of a network plug being removed. The box can be disconnected from the computer’s serial port without disrupting the network as a whole.
A single LocalTalk segment can accommodate up to 32 nodes. This can be expanded to up to 254 nodes by adding additional segments by means of a bridge device. A gateway can be used for access to Ethernet which in turn can reach other types of networks.
LocalTalk cards can be fitted into a PC or other computers to enable file transfers or for printing on a PostScript LaserWriter. Any other functions, apart from opening text files, require additional protocols at higher layers.
Variations such as Farallon’s PhoneNet use RJ11 modular connectors and a slightly different electrical arrangement. Up to 30 Macs can be connected over distances of 150 m, increasing to 450 m with screened cable. Terminators must be fitted in unused sockets. Although the hardware is different it’s still LocalTalk as far as the Mac’s concerned!
Ethernet
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Ethernet operates as a baseband network at 10 Mbit/s — or at 100 Mbit/s for Fast Ethernet. In a Fast Ethernet network you should ensure that all Macs are fitted with interfaces or Ethernet cards that are suitable for the higher speed.
Each Ethernet card or built-in interface has a 48-bit address built into its hardware. This MAC address identifies each device on a world-wide basis — if used, changing a board will also change the station’s address! In self-contained networks a fixed 16-bit address can be assigned to each station instead. The lower address bits are set to ‘on’ to broadcast messages to all stations or to multicast to selected stations.
Apple computers use the following sets of addresses, as expressed in hex notation:-
00:00:07:xx:xx:xx
00:A0:47:xx:xx:xx
where xx is defined in the individual Ethernet interface.
The transmitted data includes a two-byte length field that isn’t required when a fixed-length LLC message is sent. In these circumstances it can be used to indicate which of the higher layers of protocols are to be used — for example, TCP/IP.
The term EtherTalk simply refers to Apple’s implementation of Ethernet. EtherTalk supports the framing arrangement used in the original 802.3 standard and the alternative framing employed in Ethernet version 2.
Baseband (Base) forms of Ethernet comes in the following variations:-
Each tap is attached to a transceiver, also known as a Media Access Unit (MAU) or AU, fitted with an Access Unit Interface (AUI) connector, often a 15 way D plug (DB15). The AU may include LED indicators to show activity on the bus.
An AUI cable, up to 50 m in length, links the plug on the AU to a matching socket at the rear of a computer’s Ethernet card. An AU designed for direct connection into a Mac computer has an AAUI-15 plug instead of a 15 way D plug.
The limits on tap positioning can cause problems if you need several devices in close proximity — in any event they’ll all be connected via a single AU. For several terminals, hosts or modems with slow asynchronous ports you can use a terminal server, also known as a Network Interface Unit (NIU). Alternatively you can use a fan-out box with AUI cables for connection to each device — since this can work as a self-contained network without a final AU connection it’s also known as Ethernet-in-a-Box. The best option is probably a multi-port transceiver — you can cascade these to provide even more circuits.
A network can be expanded into extra segments by means of repeaters. A multi-port repeater can add several extra segments — in 10Base-2 format if desired. Long distances can be accommodated by interposing a fibre-optic link between two or more segments.
10Base-2 doesn’t usually need an AU — it uses a BNC ‘T’ junction adaptor that plugs directly into the back of an Ethernet card. Providing the adaptor isn’t dismantled the integrity of the network is unharmed if a plug’s removed from the back of a computer.
Some older cards don’t have an internal transceiver — there’s no BNC connector! In this situation you’ll need to plug an external 10Base-2 transceiver, also known as a Media Access Unit (MAU or AU) to the multi-way connector on the card.
A version of Ethernet operating over UTP cable containing two twisted pairs — four wires in all. It requires a central concentrator of hub — often connected with RJ45
modular plugs. The total cable length from device to hub shouldn’t exceed 110 m.
A number of local concentrators can be used in conjunction with a central concentrator. If the distances between concentrators is too large for conventional wiring a fibre-optic link can be used instead.
Some Macs include connectors for this format. Many Ethernet cards don’t have an internal transceiver for a 10Base-T connection. In this situation you’ll need to plug an external 10Base-T transceiver, also known as a Media Access Unit (MAU or AU) to the multi-way connector on the card.
If your PC doesn’t support Ethernet you can connect a special type of twisted-pair AU (TPAU) to its parallel port. The cable between AU and the PC shouldn’t exceed 15 m.
100Base-TX
A Fast Ethernet version of 10Base-T. The concentrator and Ethernet cards or interfaces must be designed for use with Fast Ethernet. Some devices can automatically switch themselves to accept 100Base-TX or 10Base-T data.
100Base-FX
A form of Fast Ethernet carried over fibre-optic cable. This format can be useful for connecting local concentrators to a central concentrator. Fibre-optic links don’t suffer from any interference caused by differences in the electrical potential between buildings.
Many Ethernet cards don’t have an internal transceiver for a 100Base-FX connection. In this situation you’ll need to plug an external Fibre-optic transceiver (FOT) form of Media Access Unit (MAU or AU) to the multi-way connector on the card.
Ethernet Derivatives
A number of other variants are based on the IEEE 802.3 standard that defines Ethernet.
Starlan conforms to the 1Base-5 format, operating at 1 Mbit/s over two twisted pairs. It uses RJ45 connectors and a central hub that accommodates up to 12 devices, each with a cable up to 244 m in length. Up to 10 devices can also be daisy-chained over a distance of 122 m on one arm of the star — providing it ends in a star-term terminator.
DECNet, a well-established system, is also based on the 802.3 standard. Both DECNet and Starlan can use any network operating system, although Novell is the most common.
Token Bus
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Token Bus is less common than other networks. Although the physical wiring may be in linear or bus form the data travels in a logical ring using broadband or carrierband techniques. It operates at 1 to 10 Mbit/s over co-axial cable.
Arcnet, also known as Tandy’s Vianet, is a star -configured form of Token Bus that operates at 2.5 Mbit/s with up to 255 nodes, each less than 600 m from a central active hub. Additional passive hubs, each accommodating up to three devices with cables up to 30 m in length, can be provided at distances of up to 30 m from the active hub.
Token Ring
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Token Ring networks run at 1, 4 or 16 Mbit/s, with some Token Ring cards accepting dual-speed operation. This benefit is easily lost since the whole system slows down to the rate of the slowest card! Each station can be connected to a Type 1 STP cable via a wall plate and an adaptor cable or to a Type 3 UTP cable via a UTP transceiver.
At 4 Mbit/s a ring can cover up to 385 m with Type 1 cable or 145 m with Type 3 cable. A typical Multi-station Access Units (MAU) can connect up to 8 stations — MAUs at different sites are linked with Type 1 cable. A network made up of Type 1 cable can have up to 260 nodes with 33 MAUs. With Type 3 cable this falls to 72 nodes and 9 MAUs.
One station acts as a Monitor Station, providing tokens that allow other stations to send data — the other stations wait to see when they can become the Monitor. Each station has a 32-bit address; the first two bytes for the LAN ring number, the rest for the station number. Ideally this address should match the Internet Protocol (IP) address used in the Network Layer to provide each station with a unique worldwide identification.
Wide Area Networks
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A Wide Area Network (WAN) is similar to a LAN but operates over a much larger area — although the dividing line between a LAN and WAN isn’t accurately defined! Long distances are covered by joining together LANs at separate sites.
For intermittent links a dial-up telephone line and a modem (via a LAN modem server ) is adequate. More intensive links require a dedicated circuit — either analogue twisted-pair circuit and modem or all (or part) of a digital trunk route. The latter, operated by the communications authority (PTT), can be used to create a private data network (PDN). They often use packet switching (PS) or Asynchronous Transfer Mode (ATM) techniques.
The following systems are used for WANs:-
Integrated Services Digital Network
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ISDN is the most common form of packet switching trunk route. In Europe the coaxial cable or fibre-optic version runs at 2.048 Mbit/s and offers customers up to 30 Bearer (B) channels at 64 kbit/s each, plus extra Data (D) channels at 16 kbit/s.
The ISDN2 service provides two B channels via a twisted-pair cable, effectively giving 128 kbit/s, plus a single D channel. By using two twisted-pair cables ISDN4 doubles this to give a transfer rate as high as 12.5 M per minute.
These services can be connected to any Mac via a suitable PCI card or interface. Networks can be connected to the ISDN via a router.
Asynchronous Transfer Mode
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ATM supports multimedia voice and video information for desktop conferencing and high-resolution video broadcasting. It combines packet switching and circuit switching techniques (as used in telephone exchanges) and runs at up to 622 Mbit/s.
ATM cards can be fitted into a PowerMac.
Fibre Data Distributed Interface
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FDDI runs at 100 Mbit/s using duplex fibre-optic cable in a ring configuration. It can support 500 nodes over a distance of 100 km — with up to 2 km between active nodes. It’s often used as a backbone or spine PDN that links together several LANs. It doesn’t suffer from any interference caused by differences in the electrical potential between buildings.
An FDDI concentrator may be used to create a partially star-like configuration. In critical systems the duplex cable can form a complete route via several devices and back again This effectively creates two rings — a primary ring and a secondary ring. The latter can maintain operation in the event of a failure in the primary ring.
Suitable PCI cards are available for a PowerMac.
X.25
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A public network for linking LANs at 64 kbit/s (or 8 kbit/s at peak periods). Devices designed for X.25 operation need only a modem to connect to the system. Other networks and devices must be connected with a Packet Assembler Disassembler (PAD), plus a modem if there’s not one in the PAD itself. The X.28 standard specifies how a PAD should operate when used in conjunction with a device fitted with an RS-232C port.
The X.25 specification defines the Physical Layer (X.21), the Data Link Layer (Link Access Protocol B (LAPB), a simplified form of HDLC) and the Network Layer. The latter makes connections using a virtual call or permanent virtual circuit. End-to-end checking is accommodated in the Transport layer.
Frame Relay
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Similar to X.25, but operating at any speed between 64 kbit/s and 2.048 Mbit/s with end-to-end checking in the Data Link Layer. It employs Link Access Protocol F (LAPF), a derivative of LAPD as used in ISDN, with permanent virtual circuits.
Metropolitan Area Network
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A MAN is a dual-ring system that runs at 155 Mbit/s. It’s operated by the PTT and can be shared by several organisations. Data flows in opposite directions in each ring and every device is given a time slot in which it can send data.
Data Link Layer
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The Data Link Layer enables blocks of data, called frames or packets, to be transported over the Physical Layer with error detection, error correction and flow control. Protocols such as CCITT V.32 are only for an asynchronous link via a modem and telephone line.
In a synchronous network each data frame is separated by timing information. Synchronous Data Link Control (SDLC), a derivative of High Level Data Link Control (HDLC), is used in systems that use SNA (see below) and other protocols.
Packets
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The data is divided into packets, each identifying itself with an established connection (a virtual circuit) or fully describing its destination as a datagram. The latter type may include a packet ID number to ensure packets are reassembled in the correct sequence at the destination. Although packets are sent in the correct order the data links can change this!
Errors and Flow Control
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Error detection and correction uses a cyclic redundancy check (CRC) or a software checksum. If a packet is correctly received the receiver responds with an acknowledge (ACK) signal.
If not, it sends a not-acknowledge (NAK) instead. The sender responds to an ACK by sending the next packet and to a NAK by sending the original packet(s) again.
If the sender doesn’t get any response from the receiver it’ll try sending the packet for a specified number of times — it there’s still no response it abandons the transfer and informs the sender. Sometimes a NAK can be received after others packets have already been sent — to avoid complication most systems use the Go-Back-N method in which a specified number of previous packets are retransmitted following any NAK message.
The window size defines the number of packets that can be sent without any ACK or NAK being received. This avoids the possibility of packet IDs restarting from zero
— resulting in an incorrect and confused sequence of packets at the receiver.
Network Layer
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The Network Layer is concerned with directing packets of data to the correct device or node within a network.
AppleTalk
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AppleTalk incorporates both Network and Transport Layers of the OSI model. It comes as part of the Mac’s system and lets any number of Macs communicate via LocalTalk, EtherTalk or TokenTalk hardware. AppleTalk can be used on its own or in conjunction with other networking software, such as TCP and IP (see below). It also works with a Mac running A/UX — Apple’s version of the UNIX operating system.
The Open Transport form of AppleTalk also supports Datalink Provider Interface (DLPI), X/Open Transport Interface (XTI) and UNIX System V STREAMS environments. A future version should also accommodate NetWare (TCP/IPX), Windows (SMB/TCP/NetBIOS), DECNet and LAT — and should replace the Serial Tool, Connection Manager and Communications Resource Manager in the Communications Toolbox. The Shared Library Manager extension must be enabled to use Open Transport.
AppleTalk can be used on a non-Mac computer fitted with an AppleTalk card — but such a machine won’t be able to read any Mac files, apart from pure text files, without special software. Most Mac applications can handle recognised files over any network, providing the network employs the AppleTalk Filing Protocol (AFP).
Internetworking Protocol
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IP is often used with TCP (see below). It uses a unique 32-bit Internet address that identifies each specific Internet or Intranet site. The least significant 16 bits identify a particular device on a network (perhaps using a number related to its hardware address) and the next 14 bits give the network’s location. An Internet address is expressed in the form:-
195.79.171.10
with each byte shown in decimal and separated from the next byte by a decimal point. Digits at the left-hand act as an area code for your network whilst those to the right specify your subnetwork or terminal more exactly. The division of these numbers across an entire network will depend on the network itself.
To send data to all other stations requires an all networks (all nets) broadcast. This uses the following address:-
255.255.255.255
A broadcast to part of a network (a subnetwork) is known as a subnet broadcast or unicast. This uses an address of the form:-
xxx.xxx.xxx.255
where xxx.xxx.xxx is the address of the subnet.
Domain Name System
The Domain Name System (DNS) identifies each Internet site in a more recognisable form that a 32-bit address. An example domain name could be:-
mycomputer.topcompany.comuk
where
mycomputer = a computer on the network run by 'topcompany'
topcompany = the name of the company
comuk = indication of a commercial business in the United Kingdom
Transport Layer
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The Transport Layer divides a message between two devices into packets as required by the Network Layer. For perfect transmission this requires host-to-host flow control, error detection and error correction, although error detection can occur in the Data Link Layer.
Data can be sent as a binary file or as an ASCII file. A binary file, using all possible digital values, can contain applications and graphics documents as well as text. Most e-mail services use ASCII files to represent text, using only the values from 0 to 127. Before sending a binary file over e-mail it must be converted into ASCII form.
In systems that can convey binary files all information is sent in fixed-size blocks known as Protocol Data Units (PDUs). Since the receiver knows where each PDU begins and ends it disregards those codes inside the PDU that might otherwise be used for instructions.
AppleTalk
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AppleTalk (see above) occupies both the Network and Transport Layers with ATP (Apple Transport Protocol) in the Transport Layer. Both TCP (see below) and IP can be used with AppleTalk on a Mac running A/UX — Apple’s version of the UNIX operating system.
Transmission Control Protocol
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TCP is used on the Internet or for creating an Intranet over any other kind of network. It easily transports files as PDUs between two computers of any kind. Several classes of service are available, including flow control with error correction and reordering of packets into their correct sequence at the receiver. TCP also supports transfers with File Transfer Protocol (FTP), Simple Mail Transfer Protocol (SMTP) and Telnet.
TCP is invariably used with IP (see above ) and Point-to-Point Protocol (PPP), The latter ensures a successful transfer over a series of interconnected networks.
√πSee the Internet chapter for more about TCP
Protocols for Asynchronous Links
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Several different protocols are used in an asynchronous link, as used via a modem and a telephone circuit. Networks obtain access to such a link via a modem server.
Microcom Networking Protocol Level 4 (MNP4) includes error correction and is recognised by the Apple Modem Tool communication tool. Modems that use V.42 error correction also support MNP4 whilst those with V.42bis data compression also support MNP5.
Other protocols such as XMODEM, YMODEM, ZMODEM, Crosstalk and Kermit include error correction but offer varying standards of performance.
√πSee the Communications and Modems chapters for error correction and compression
Session Layer
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The Session Layer allows different computers to open files from a different type of machine. MacLinkPlus is one application that makes this possible.
Structured Query Language
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A shared database is one of the most useful features a network can provide. The database server application must be running on the network’s file server computer and the client workstations must be running appropriate software.
Various applications can use the database if there’s a common query language. IBM’s Structured Query Language (SQL) is used for their own database server on mainframe or mini-computers and for servers on other operating systems such as UNIX and VMS.
It also works with A/UX, a version of UNIX that runs on Mac computers. Apple’s Data Access Language (DAL) is an improved form of SQL that supports the original version and can be used with MS-DOS, OS/2, Windows, UNIX and A/UX operating systems.
DAL supports many database servers, including DEC Rdb, Informix, Ingres, Oracle and Sybase on a VAX network using the VMS operating system, DB2 on an IBM system using Multiple Virtual Storage with Time Sharing Option (MVS/TSO) and SQL/DS on an IBM system using a Virtual Machine and Conversational Monitor System (VM/CMS). The letters DS stand for Directory Services — it maintains information about the network’s resources.
If you want to use DAL on a Mac you should enable the DAL extension.
Other Network Software
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System Network Architecture
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IBM’s System Network Architecture (SNA) is a networking system that works on all kinds of computer, using seven protocol layers in a similar form to the OSI model — the layers are Physical Control, Data Link Control, Path Control, Transmission Control, Data Flow Control, Presentation Services and Transaction Services.
In the upper layers System Application Architecture (SAA) makes each application look similar on all types of computer whilst Common User Access (CUA) ensures a consistent response to keyboard commands. Advanced Program-to-Program Communication (APPC) allows data and instructions to be passed between applications and between computers over the network.
DECNet and Novell
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Other network protocols, such as DECNet and Novell NetWare (used in UNIX-based networks and as an extension to MS-DOS or with Mac computers) are outside the scope of this Guide. Some inter-operation can be obtained using protocols such as OSI, TCP/IP, SNA/APPC (see above), ATP (Apple), IPX/SPX (Novell) and XNS (Xerox).
