TweetFollow Us on Twitter

AppleTalk Fundamentals
Volume Number:3
Issue Number:4
Column Tag:Connections

AppleTalk: Fundamentals

By Dave Kosiur, Editor & Publisher, Connections Newsletter

Connections is a new industry newsletter in the MacInTouch style that covers issues related to networking on the Mac. Connections and MacTutor have agreed to promote each other as "sister" publications and we will be re-printing articles from Connections in MacTutor from time to time. This article is re-printed from the first issue. Connections is edited and published by Dave Kosiur. Bob Denny of Alisa Systems serves on it's advisory board, as he does for MacTutor. Subscriptions are $60 per year for the bimonthly publication, available from Connections, PO Box 5894, Fullerton, CA 92635. This is a valuable source of network specific information and we encourage your support.

AppleTalk is Apple's design of a simple, inexpensive and flexible network for connecting computers, peripheral devices, and servers. AppleTalk's flexibility allows it to be used to connect peripherals such as the LaserWriter, or act as a stand-alone local-area network for up to 32 nodes, or form portions of a larger network by using bridges and gateway devices.

Fig. 1 The Physical Connection

What is AppleTalk? At a purely physical level, AppleTalk is a network with a bus topology that uses a trunk cable between connection modules. Interfacing with the network is handled by the Serial Communications Control chip found in every Mac. Any device (computer, peripheral, etc.) attaches to a connection box via a short cable (called a drop cable), as shown in figure 1. This type of network is known as a multidrop line or a multipoint link. AppleTalk is capable of supporting up to 32 nodes (devices) per network and can transmit data at a rate of 230,400 bits per second. Nodes can be separated by a maximum cable length of 1000 feet.

AppleTalk, as specified by Apple, is wired using relatively inexpensive shielded, twisted-pair cable and Apple's connection boxes. One box is required per device; in the case of the Mac, the box plugs into the serial printer port in the back of the Mac using an attached drop cable. A trunk cable segment from one node on the network plugs into one port on the connection box, and another cable segment leading to the next node in the network plugs into the other port on the box.

One of the advantages of AppleTalk relates to the design of these connection boxes. The boxes are designed so that the continuity of the trunk cable and the network is maintained even if a device is disconnected from the network by unplugging it from the connection box. (Unplugging the trunk from the connection box does disrupt the integrity of the network, however.) The physical layout of an AppleTalk network can therefore be designed by locating the connection boxes where desired without worrying if a device will be initially connected to each one of the boxes. Additional devices can be added to the network at any time simply by plugging them into the boxes.

There are alternatives to using Apple's connection boxes. Farallon Computing markets their PhoneNET system, which fully supports the AppleTalk protocols. In the case of PhoneNET, the physical transmission medium is ordinary telephone wire, allowing the user to use the in-house telephone wiring for his network. PhoneNET uses the two of the unused wires found in a normal telephone installation, supporting both a telephone and a Mac connected to the same telephone wall box. In addition, PhoneNET links are capable of supporting 3000-foot distances between nodes. Farallon has a series of devices (repeaters, Star Controller) for extending the network.

With the recent announcement of DuPont's system for AppleTalk, users can also use fiber optic connections for an AppleTalk network. A concentrator is also available for constructing star networks. Two advantages of the fiber optics system are its immunity to EMI-RFI interference and improved data security; nodes may be a maximum of 4900 feet apart.

AppleTalk Protocols and the OSI Model

The Physical Layer has the responsibility of bit encoding/decoding, synchronization, signal transmission/ reception and carrier sensing. As mentioned previously, the Serial Communications Control chip in the Mac takes care of the AppleTalk port, which happens to be the printer port on current Macs. As long as connection modules conform to the signal descriptions of the Physical Layer, any transmission medium can be used for the actual network.

The AppleTalk Link Access Protocol (ALAP) must be common to all systems on the network bus and handles the node-to-node delivery of data between devices connected to a single AppleTalk network. ALAP determines when the bus is free, encapsulates the data in frames, sends its data, and recognizes when data should be received. ALAP is also responsible for assigning node numbers to each station on a network. The ALAP software assigns a random node number when the Mac is booted and keeps that number as long as it does not conflict with a previously assigned node number (if it does conflict, ALAP tries again).

The Link Access Protocol uses a method called CSMA/CA, or carrier-sense multiple access with collision avoidance, for access control. Carrier sense means that a sending node first listens to the network to hear if any other node is using the bus and defers to the ongoing transmission. Collision avoidance means that the protocol attempts to minimize collisions between transmitted data packets. In AppleTalk CSMA/CA, all transmitters wait until the bus is idle for a minimum time plus a random amount of added time before transmitting (or retransmitting after a collision).

While the ALAP protocol provides delivery of data over a single AppleTalk network, the Datagram Delivery Protocol (DDP) extends this mechanism to include a group of interconnected AppleTalk networks, known as an internet. An internet can be formed, for example, by using a bridge between two, or more, AppleTalk networks.

AppleTalk's address header (a part of each data packet) is used for identification of a process on the network and consists of a socket number, node number, and network number. A socket is a communication endpoint within a node on the network. Sockets belong to processes or functions that are implemented within software in the node. One Mac may have several AppleTalk connections open at one time, so the node number is not enough to identify a network address. In addition, node numbers are unique only within a single physical network, so DDP requires that each network be assigned a network number. The Datagram Delivery Protocol takes care of assigning socket numbers, as well as node numbers and network numbers, to provide a unique identification for every process occurring on the AppleTalk network.

As we move on to the Transport Layer, several protocols exist to add different types of functionality to the underlying services. The Routing Table Maintenance Protocol (RTMP) allows bridges and internet routers to dynamically discover routes to the different AppleTalk networks in an internet. The routing tables pair network numbers with the local node number of the bridge through which the shortest path to that net exists.

The AppleTalk Transaction Protocol, or ATP, is part of the Transport Layer and is responsible for controlling the transactions (flow of data) between requestor and responder sockets. This transaction-oriented protocol can be contrasted to other types of transport layers which support a two-way link between clients that can act as though they had an error-free hardwired link between them.

The basic function of the Name Binding Protocol (NBP) is the translation of a character string name into the internet address of the corresponding client. A key feature of the network is that most objects are accessible by name rather than by address (better for the user). NBP also introduces the concept of a zone, which is an arbitrary subset of networks in an internet where each network is in one and only one zone. The concept of zones is provided to assist the establishment of departmental or other user-understandable grouping of the entities of the internet. AppleTalk names consist of three fields: the object name (e.g., Dave), the type name (e.g., printer), and the zone name (e.g., Bldg. 1).

The Echo Protocol (EP) is a simple protocol that allows any node to send data to any other node on an AppleTalk internet and receive an echoed copy of that data in return. The Echo Protocol is mainly meant for network maintenance functions.

The specifications for the AppleTalk Data Stream Protocol (ADSP) have not yet been published (Inside AppleTalk, current version dated July 14, 1986). ADSP is designed to provide byte-stream data transmission in a full duplex mode between any two sockets on an AppleTalk internet. The Zone Information Protocol (ZIP) is used to maintain an internet-wide mapping of networks to zone names. Most of ZIP's services are transparent to the normal (non-bridge) node; the majority of ZIP is implemented in the bridges of an internet. ZIP is used by the Name Binding Protocol to determine which networks belong to a given zone.

In the Session Layer, the AppleTalk Session Protocol (ASP) is a general protocol designed to interact with ATP to provide for establishing, maintaining and closing sessions. Central to ASP is the concept of a session; two network entities, one in a workstations and the other in a server, can set up an ASP session between themselves (identified by a unique sessions identifier). ASP is an asymetric protocol in that the workstation initiates the session connection and issues sequences of commands, to which the server responds; the server may not send commands to the workstation.

The specifications for the AppleTalk Filing Protocol (AFP) have not been generally publicized. However, AFP has been finalized with the introduction of the AppleShare file server software from Apple, which uses AFP. AFP is a presentation layer protocol designed to control access to remote file systems.

At this time, third-party products are available for connecting Macintoshes and IBM PCs to AppleTalk, as well as using gateways for access to EtherNet as a backbone network to larger computers. Future articles in Connections will deal with these products and the diversity of products (file servers, spoolers, etc.) that can take advantage of such connections. For additional information, see MacTutor, Vol. 1, Numbers 10 and 11 for articles by Bob Denny & Alan Wootton.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Latest Forum Discussions

See All

Avatar Legends: Realms Collide pre-regis...
Despite the staying power that the series has, Avatar: The Last Airbender surprisingly fumbled in the mobile gaming sphere, with Generations shuttering about a mere year after launch. Now, Tilting Point is giving it a go, with the upcoming Avatar... | Read more »
Return to the glory days of tactical RPG...
Back on the old Sega Mega Drive, or Genesis depending on where you are, there was a little series of tactical role-playing games called Shining Force, and I adored them. It started a love for this grid-based genre that persisted through the years... | Read more »
Take on the grandest beasts of all as th...
It has been hyped for a while but now it is finally here, the new Dancing in the Tempest season has arrived in Monster Hunter Now. Kicking off a banner summer for the game, it is time to strap on your boots and face the greatest threat yet, as the... | Read more »
The indie hit Vault of the Void will lan...
In a market where a lot of AAA games are starting to feel like reboots, remakes, or the same thing we’ve seen ten times with a different name slapped on it, indie developers are a boon. Especially ones who build a successful game singlehandedly,... | Read more »
Stumble Guys tries to catch up to the hy...
Do you remember when the Fallout TV series launched on Amazon and everyone nearly lost their minds? Well, it appears that Scopely missed that particular craze, and only now are we going to get a Fallout crossover, and by now, I mean in version 0.... | Read more »
Bid farewell to Penacony as Honkai: Star...
Penacony has been a story of twists, exciting new characters, and strong allies, and soon Honkai: Star Rail will be finishing it with a bang. Version 2.3, fittingly titled Farewell Penacony, will be launching June 19th and will feature updates to... | Read more »
HoYoverse roll out their plans for Anime...
For those who are looking to book a getaway in July, you might give some thought to Los Angeles between the 4th and 7th, which just so happens to coincide with the Anime Expo 2024. Amongst all the storied attendees is HoYoverse, who will be... | Read more »
The first rule of Brok the InvestiGator...
Mobile gamers were recently able to get their hands on BROK the InvestiGator, a point-and-click following the adventures of the titular reptile, a detective who can solve crimes through wit or brawn. If you were one that chose the latter then... | Read more »
Diablo Immortal celebrates second annive...
It has been two years since Diablo Immortal launched and despite some very valid criticism of its business model, it has done pretty well for itself. The Tempest class also gives it a lot of grace. To celebrate this anniversary, the March of the... | Read more »
Pokemon GO pulls on its jersey for a foo...
There have been a lot of jokes about this, some by me, but Pokemon Go has genuinely done a lot of good by getting people out and about.Pokemon GO Fest 2024: Madrid is fast approaching, and Niantic has set up a new area in a bit to get people to... | Read more »

Price Scanner via MacPrices.net

Apple Watch Ultra Watch 2 on sale for $719, s...
Amazon is offering an $80 discount on every Apple Watch Ultra 2 model this week. Their price is now $719. Shipping is free. For the latest prices & deals, keep an eye on our Apple Watch Price... Read more
New sale at Amazon: 16-inch M3 Pro and M3 Max...
Amazon is offering instant discounts on 16″ M3 Pro and 16″ M3 Max MacBook Pros ranging up to $350 off MSRP. Shipping is free. These are the lowest prices currently available for new 16″ Apple MacBook... Read more
Get a 13-inch M2 MacBook Air today at Apple f...
Apple has 13″ M2 MacBook Airs available for only $849 today in their Certified Refurbished store. These are the cheapest M2-powered MacBooks for sale at Apple. Apple’s one-year warranty is included,... Read more
Clearance Mac Studio with M1 Max CPU availabl...
Apple has clearance M1 Max Mac Studios available in their Certified Refurbished store for $270 off original MSRP. Each Mac Studio comes with Apple’s one-year warranty, and shipping is free: – Mac... Read more
Apple has 24-inch M3 iMacs on sale for $200-$...
Apple has a full line of 24-inch M3 iMacs available in their Certified Refurbished store starting at $1099 and ranging up to $260 off original MSRP. Each iMac is in like-new condition and comes with... Read more
24-inch M1 iMacs are available at Apple start...
Apple has clearance M1 iMacs available in their Certified Refurbished store starting at $1049 and ranging up to $300 off original MSRP. Each iMac is in like-new condition and comes with Apple’s... Read more
Back to School savings: Take $50-$100 off new...
Apple will take $50-$100 off new 11″ and 13″ M2 iPad Airs for all teachers, students, and staff of any educational institution with a .edu email address as part of their Apple Education discount,... Read more
Could A Smarter Siri Infused With AI (‘Apple...
FEATURE – The iPhone is already smart, but it’s about to become more intelligent. AI — short for artificial intelligence — is widely expected to be the main topic of discussion at this year’s WWDC (... Read more
Update: For WWDC, Amazon has lowered prices o...
Amazon has every configuration and color of Apple’s M3 MacBook Airs now on sale for $170-$210 off MSRP, starting at only $899 shipped, as Apple holds their annual WWDC conference this week. Their... Read more
Deal Alert! 2nd-generation Apple AirPods on s...
Amazon has 2nd generation Apple AirPods on sale right now for only $79.99 shipped. That’s $50 (38%) off Apple’s MSRP. Their price is the lowest currently available for a new set of AirPods from any... Read more

Jobs Board

Beauty Consultant - *Apple* Blossom Mall -...
Beauty Consultant - Apple Blossom Mall Location:Winchester, VA, United States (https://jobs.jcp.com/jobs/location/191170/winchester-va-united-states) - Apple Read more
Senior Software Engineer - *Apple* Fundamen...
…center of Microsoft's efforts to empower our users to do more. The Apple Fundamentals team focused on defining and improving the end-to-end developer experience in Read more
Sublease Associate Optometrist- *Apple* Val...
Sublease Associate Optometrist- Apple Valley, CA- Target Optical Date: Jun 17, 2024 Brand: Target Optical Location: Apple Valley, CA, US, 92307 **Requisition Read more
Rehabilitation Medicine Technician - *Apple*...
Rehabilitation Medicine Technician - Apple Hill (Outpatient Clinic) - Day/Evening Location: York Hospital, York, PA Schedule: Part Time Sign-On Bonus Eligible Read more
Operations Associate - *Apple* Blossom Mall...
Operations Associate - Apple Blossom Mall Location:Winchester, VA, United States (https://jobs.jcp.com/jobs/location/191170/winchester-va-united-states) - Apple Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.