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What is a computer network?

The IBM Cloud network delivers global support for an ever-expanding set of applications, particularly for those with the most demanding availability requirements, and does so without compromising security. Manage your resources with greater ease. The IBM Cloud catalog lists starters and services that you can choose to implement in your web or mobile apps. A starter is a template that includes predefined services and application code. IBM divested itself of low margin industries (DRAM, IBM Network, personal printers, and hard drives). The company regained the business initiative by building upon the decision to keep the company whole – it unleashed a global services business that rapidly rose to become a leading technology integrator. The IBM PC Network Baseband Adapter (#1221) is a feature card for connecting the new IBM Personal System/2 (TM) Model 30 and IBM Personal Computers to the baseband IBM PC Network.

A computer network comprises two or more computers that are connected—either by cables (wired) or WiFi (wireless)—with the purpose of transmitting, exchanging, or sharing data and resources. You build a computer network using hardware (e.g., routers, switches, access points, and cables) and software (e.g., operating systems or business applications).

Geographic location often defines a computer network. For example, a LAN (local area network) connects computers in a defined physical space, like an office building, whereas a WAN (wide area network) can connect computers across continents. The internet is the largest example of a WAN, connecting billions of computers worldwide.

You can further define a computer network by the protocols it uses to communicate, the physical arrangement of its components, how it controls traffic, and its purpose.

Computer networks enable communication for every business, entertainment, and research purpose. The internet, online search, email, audio and video sharing, online commerce, live-streaming, and social networks all exist because of computer networks.

Computer network types

As networking needs evolved, so did the computer network types that serve those needs. Here are the most common and widely used computer network types:

• LAN (local area network): A LAN connects computers over a relatively short distance, allowing them to share data, files, and resources. For example, a LAN may connect all the computers in an office building, school, or hospital. Typically, LANs are privately owned and managed.
• WLAN (wireless local area network): A WLAN is just like a LAN but connections between devices on the network are made wirelessly.
• WAN (wide area network): As the name implies, a WAN connects computers over a wide area, such as from region to region or even continent to continent. The internet is the largest WAN, connecting billions of computers worldwide. You will typically see collective or distributed ownership models for WAN management.
• MAN (metropolitan area network): MANs are typically larger than LANs but smaller than WANs. Cities and government entities typically own and manage MANs.
• PAN (personal area network): A PAN serves one person. For example, if you have an iPhone and a Mac, it’s very likely you’ve set up a PAN that shares and syncs content—text messages, emails, photos, and more—across both devices.
• SAN (storage area network): A SAN is a specialized network that provides access to block-level storage—shared network or cloud storage that, to the user, looks and works like a storage drive that’s physically attached to a computer. For more information on how a SAN works with block storage, see our video 'Block Storage vs. File Storage' and 'Block Storage: A Complete Guide.'
• CAN (campus area network): A CAN is also known as a corporate area network. A CAN is larger than a LAN but smaller than a WAN. CANs serve sites such as colleges, universities, and business campuses.
• VPN (virtual private network): A VPN is a secure, point-to-point connection between two network end points (see ‘Nodes’ below). A VPN establishes an encrypted channel that keeps a user’s identity and access credentials, as well as any data transferred, inaccessible to hackers.

Important terms and concepts

The following are some common terms to know when discussing computer networking:

• IP address: An IP address is a unique number assigned to every device connected to a network that uses the Internet Protocol for communication. Each IP address identifies the device’s host network and the location of the device on the host network. When one device sends data to another, the data includes a ‘header’ that includes the IP address of the sending device and the IP address of the destination device.

• Nodes: A node is a connection point inside a network that can receive, send, create, or store data. Each node requires you to provide some form of identification to receive access, like an IP address. A few examples of nodes include computers, printers, modems, bridges, and switches. A node is essentially any network device that can recognize, process, and transmit information to any other network node.

• Routers: A router is a physical or virtual device that sends information contained in data packets between networks. Routers analyze data within the packets to determine the best way for the information to reach its ultimate destination. Routers forward data packets until they reach their destination node.

• Switches: A switch is a device that connects other devices and manages node-to-node communication within a network, ensuring data packets reach their ultimate destination. While a router sends information between networks, a switch sends information between nodes in a single network. When discussing computer networks, ‘switching’ refers to how data is transferred between devices in a network. The three main types of switching are as follows:

  • Circuit switching, which establishes a dedicated communication path between nodes in a network. This dedicated path assures the full bandwidth is available during the transmission, meaning no other traffic can travel along that path.

  • Packet switching involves breaking down data into independent components called packets which, because of their small size, make fewer demands on the network. The packets travel through the network to their end destination.

  • Message switching sends a message in its entirety from the source node, traveling from switch to switch until it reaches its destination node.

• Ports: A port identifies a specific connection between network devices. Each port is identified by a number. If you think of an IP address as comparable to the address of a hotel, then ports are the suites or room numbers within that hotel. Computers use port numbers to determine which application, service, or process should receive specific messages.

• Network cable types: The most common network cable types are Ethernet twisted pair, coaxial, and fiber optic. The choice of cable type depends on the size of the network, the arrangement of network elements, and the physical distance between devices.

Examples of computer networks

The wired or wireless connection of two or more computers for the purpose of sharing data and resources form a computer network. Today, nearly every digital device belongs to a computer network.

In an office setting, you and your colleagues may share access to a printer or to a group messaging system. The computing network that allows this is likely a LAN or local area network that permits your department to share resources.

A city government might manage a city-wide network of surveillance cameras that monitor traffic flow and incidents. This network would be part of a MAN or metropolitan area network that allows city emergency personnel to respond to traffic accidents, advise drivers of alternate travel routes, and even send traffic tickets to drivers who run red lights.

The Weather Company worked to create a peer-to-peer mesh network that allows mobile devices to communicate directly with other mobile devices without requiring WiFi or cellular connectivity. The Mesh Network Alerts project will allow the delivery of life-saving weather information to billions of people, even without an internet connection.

Computer networks and the internet

The internet is actually a network of networks that connects billions of digital devices worldwide. Standard protocols allow communication between these devices. Those protocols include hypertext transfer protocol (the ‘http’ in front of all website addresses). Internet protocol (or IP addresses) are the unique identifying numbers required of every device that accesses the internet. IP addresses are comparable to your mailing address, providing unique location information so that information can be delivered correctly.

Internet Service Providers (ISPs) and Network Service Providers (NSPs) provide the infrastructure that allows the transmission of packets of data or information over the internet. Every bit of information sent over the internet doesn’t go to every device connected to the internet. It’s the combination of protocols and infrastructure that tells information exactly where to go.

How do they work?

Computer networks connect nodes like computers, routers, and switches using cables, fiber optics, or wireless signals. These connections allow devices in a network to communicate and share information and resources.

Networks follow protocols, which define how communications are sent and received. These protocols allow devices to communicate. Each device on a network uses an Internet Protocol or IP address, a string of numbers that uniquely identifies a device and allows other devices to recognize it.

Routers are virtual or physical devices that facilitate communications between different networks. Routers analyze information to determine the best way for data to reach its ultimate destination. Switches connect devices and manage node-to-node communication inside a network, ensuring that bundles of information traveling across the network reach their ultimate destination.

Architecture

Computer network architecture defines the physical and logical framework of a computer network. It outlines how computers are organized in the network and what tasks are assigned to those computers. Network architecture components include hardware, software, transmission media (wired or wireless), network topology, and communications protocols.

Main types of network architecture

There are two types of network architecture: peer-to-peer (P2P) and client/server. In P2P architecture, two or more computers are connected as “peers,” meaning they have equal power and privileges on the network. A P2P network does not require a central server for coordination. Instead, each computer on the network acts as both a client (a computer that needs to access a service) and a server (a computer that serves the needs of the client accessing a service). Each peer makes some of its resources available to the network, sharing storage, memory, bandwidth, and processing power.

In a client/server network, a central server or group of servers manage resources and deliver services to client devices in the network. The clients in the network communicate with other clients through the server. Unlike the P2P model, clients in a client/server architecture don’t share their resources. This architecture type is sometimes called a tiered model because it's designed with multiple levels or tiers.

Network topology

Network topology refers to how the nodes and links in a network are arranged. A network node is a device that can send, receive, store, or forward data. A network link connects nodes and may be either cabled or wireless links.

Understanding topology types provides the basis for building a successful network. There are a number of topologies but the most common are bus, ring, star, and mesh:

• A bus network topology is when every network node is directly connected to a main cable.
• In a ring topology, nodes are connected in a loop, so each device has exactly two neighbors. Adjacent pairs are connected directly; non-adjacent pairs are connected indirectly through multiple nodes.
• In a star network topology, all nodes are connected to a single, central hub and each node is indirectly connected through that hub.
• A mesh topology is defined by overlapping connections between nodes. You can create a full mesh topology, where every node in the network is connected to every other node. You can also create partial mesh topology in which only some nodes are connected to each other and some are connected to the nodes with which they exchange the most data. Full mesh topology can be expensive and time-consuming to execute, which is why it's often reserved for networks that require high redundancy. Partial mesh provides less redundancy but is more cost effective and simpler to execute.

Security

Computer network security protects the integrity of information contained by a network and controls who access that information. Network security policies balance the need to provide service to users with the need to control access to information.

There are many entry points to a network. These entry points include the hardware and software that comprise the network itself as well as the devices used to access the network, like computers, smartphones, and tablets. Because of these entry points, network security requires using several defense methods. Defenses may include firewalls—devices that monitor network traffic and prevent access to parts of the network based on security rules.

Processes for authenticating users with user IDs and passwords provide another layer of security. Security includes isolating network data so that proprietary or personal information is harder to access than less critical information. Other network security measures include ensuring hardware and software updates and patches are performed regularly, educating network users about their role in security processes, and staying aware of external threats executed by hackers and other malicious actors. Network threats constantly evolve, which makes network security a never-ending process.

The use of public cloud also requires updates to security procedures to ensure continued safety and access. A secure cloud demands a secure underlying network.

Read about the top five considerations for securing the public cloud.

Mesh networks

As noted above, a mesh network is a topology type in which the nodes of a computer network connect to as many other nodes as possible. In this topology, nodes cooperate to efficiently route data to its destination. This topology provides greater fault tolerance because if one node fails, there are many other nodes that can transmit data. Mesh networks self-configure and self-organize, searching for the fastest, most reliable path on which to send information.

Type of mesh networks

There are two types of mesh networks—full mesh and partial mesh:

• In a full mesh topology, every network node connects to every other network node, providing the highest level of fault tolerance. However, it costs more to execute. In a partial mesh topology, only some nodes connect, typically those that exchange data most frequently.
• A wireless mesh network may consist of tens to hundreds of nodes. This type of network connects to users over access points spread across a large area. Watch the video below to learn how The Weather Channel created a mesh network that disseminates severe weather alerts even when other communications networks are congested.

Load balancers and networks

Load balancers efficiently distribute tasks, workloads, and network traffic across available servers. Think of load balancers like air traffic control at an airport. The load balancer observes all traffic coming into a network and directs it toward the router or server best equipped to manage it. The objectives of load balancing are to avoid resource overload, optimize available resources, improve response times, and maximize throughput.

For a complete overview of load balancers, see 'Load Balancing: A Complete Guide.'

Content delivery networks

A content delivery network (CDN) is a distributed server network that delivers temporarily stored, or cached, copies of website content to users based on the user’s geographic location. A CDN stores this content in distributed locations and serves it to users as a way to reduce the distance between your website visitors and your website server. Having cached content closer to your end users allows you to serve content faster and helps websites better reach a global audience. CDNs protect against traffic surges, reduce latency, decrease bandwidth consumption, accelerate load times, and lessen the impact of hacks and attacks by introducing a layer between the end user and your website infrastructure.

Live-streaming media, on-demand media, gaming companies, application creators, e-commerce sites—as digital consumption increases, more content owners turn to CDNs to better serve content consumers.

Chief Network Architect Ryan Sumner gives a further explanation in the video 'What is a Content Delivery Network?':

Read about How IBM Cloud CDN delivers improved customer experience.

Computer networking solutions and IBM

Computer networking solutions help businesses enhance traffic, keep users happy, secure the network, and easily provision services. The best computer networking solution is typically a unique configuration based on your specific business type and needs.

Content delivery networks (CDNs), load balancers, and network security—all mentioned above—are examples of technologies that can help businesses craft optimal computer networking solutions. IBM offers additional networking solutions, including:

• Gateway appliances are devices that give you enhanced control over network traffic, let you accelerate your network’s performance, and give your network a security boost. Manage your physical and virtual networks for routing multiple VLANs, for firewalls, VPN, traffic shaping and more.
• Direct Link secures and accelerates data transfer between private infrastructure, multiclouds, and IBM Cloud.
• Cloud Internet Services are security and performance capabilities designed to protect public-facing web content and applications before they reach the cloud. Get DDoS protection, global load balancing and a suite of security, reliability and performance capabilities designed to protect public-facing web content and applications before they reach the cloud. Watch the following video for more info on DDoS attacks and how they occur:

Networking services in IBM Cloud provide you with networking solutions to enhance your traffic, keep your users happy, and easily provision resources as you need them.

Sign up for an IBMid and create your IBM Cloud account.

Systems Network Architecture (SNA) is IBM's proprietary networking architecture, created in 1974.^[1] It is a complete protocol stack for interconnecting computers and their resources. SNA describes formats and protocols and is, in itself, not a piece of software. The implementation of SNA takes the form of various communications packages, most notably Virtual Telecommunications Access Method (VTAM), the mainframe software package for SNA communications.

History[edit]

SNA was made public as part of IBM's 'Advanced Function for Communications' announcement in September, 1974,^[2] which included the implementation of the SNA/SDLC (Synchronous Data Link Control) protocols on new communications products:

• IBM 3767 communication terminal (printer)
• IBM 3770 data communication system

They were supported by IBM 3704/3705 communication controllers and their Network Control Program (NCP), and by System/370 and their VTAM and other software such as CICS and IMS. This announcement was followed by another announcement in July, 1975, which introduced the IBM 3760 data entry station, the IBM 3790 communication system, and the new models of the IBM 3270 display system.

SNA was mainly designed by the IBM Systems Development Division laboratory in Research Triangle Park, North Carolina, USA, helped by other laboratories that implemented SNA/SDLC. The details were later made public by IBM's System Reference Library manuals and IBM Systems Journal.

SNA is still used extensively in banks and other financial transaction networks, as well as in many government agencies. While IBM is still providing support for SNA, one of the primary pieces of hardware, the 3745/3746 communications controller, has been withdrawn from the market by IBM. There are an estimated 20,000^[when?] of these controllers installed however, and IBM continues to provide hardware maintenance service and microcode features to support users. A robust market of smaller companies continues to provide the 3745/3746, features, parts and service. VTAM is also supported by IBM, as is the IBM Network Control Program (NCP) required by the 3745/3746 controllers.

In 2008 an IBM publication said:

with the popularity and growth of TCP/IP, SNA is changing from being a true network architecture to being what could be termed an 'application and application access architecture.' In other words, there are many applications that still need to communicate in SNA, but the required SNA protocols are carried over the network by IP.^[3]

Objectives of SNA[edit]

IBM in the mid-1970s saw itself mainly as a hardware vendor and hence all its innovations in that period aimed to increase hardware sales. SNA's objective was to reduce the costs of operating large numbers of terminals and thus induce customers to develop or expand interactive terminal-based systems as opposed to batch systems. An expansion of interactive terminal-based systems would increase sales of terminals and more importantly of mainframe computers and peripherals - partly because of the simple increase in the volume of work done by the systems and partly because interactive processing requires more computing power per transaction than batch processing.

Hence SNA aimed to reduce the main non-computer costs and other difficulties in operating large networks using earlier communications protocols. The difficulties included:

• Often a communications line could not be shared by terminals of different types, as they used different 'dialects' of the existing communications protocols. Up to the early 1970s, computer components were so expensive and bulky that it was not feasible to include all-purpose communications interface cards in terminals. Every type of terminal had a hard-wired communications card which supported only the operation of one type of terminal without compatibility with other types of terminals on the same line.
• The protocols which the primitive communications cards could handle were not efficient. Each communications line used more time transmitting data than modern lines do.
• Telecommunications lines at the time were of much lower quality. For example, it was almost impossible to run a dial-up line at more than 19,200 bits per second because of the overwhelming error rate, as comparing with 56,000 bits per second today on dial-up lines; and in the early 1970s few leased lines were run at more than 2400 bits per second (these low speeds are a consequence of Shannon's Law in a relatively low-technology environment).

As a result, running a large number of terminals required a lot more communications lines than the number required today, especially if different types of terminals needed to be supported, or the users wanted to use different types of applications (.e.g. under CICS or TSO) from the same location. In purely financial terms SNA's objectives were to increase customers' spending on terminal-based systems and at the same time to increase IBM's share of that spending, mainly at the expense of the telecommunications companies.

SNA also aimed to overcome a limitation of the architecture which IBM's System/370 mainframes inherited from System/360. Each CPU could connect to at most 16 I/O channels^[4] and each channel could handle up to 256 peripherals - i.e. there was a maximum of 4096 peripherals per CPU. At the time when SNA was designed, each communications line counted as a peripheral. Thus the number of terminals with which powerful mainframes could otherwise communicate was limited.

Principal components and technologies[edit]

Improvements in computer component technology made it feasible to build terminals that included more powerful communications cards which could operate a single standard communications protocol rather than a very stripped-down protocol which suited only a specific type of terminal. As a result, several multi-layer communications protocols were proposed in the 1970s, of which IBM's SNA and ITU-T's X.25 became dominant later.

The most important elements of SNA include:

• IBM Network Control Program (NCP) is a communications program running on the 3705 and subsequent 37xx communications processors that, among other things, implements the packet switching protocol defined by SNA. The protocol performed two main functions:
  • It is a packet forwarding protocol, acting like modern switch - forwarding data packages to the next node, which might be a mainframe, a terminal or another 3705. The communications processors supported only hierarchical networks with a mainframe at the center, unlike modern routers which support peer-to-peer networks in which a machine at the end of the line can be both a client and a server at the same time.
  • It is a multiplexer that connected multiple terminals into one communication line to the CPU, thus relieved the constraints on the maximum number of communication lines per CPU. A 3705 could support a larger number of lines (352 initially) but only counted as one peripheral by the CPUs and channels. Since the launch of SNA IBM has introduced improved communications processors, of which the latest is the 3745.
• Synchronous Data Link Control^[5] (SDLC), a protocol which greatly improved the efficiency of data transfer over a single link:^[6]
  • It is a Sliding window protocol, which enables terminals and 3705 communications processors to send frames of data one after the other without waiting for an acknowledgement of the previous frame - the communications cards had sufficient memory and processing capacity to remember the last 7 frames sent or received, request re-transmission of only those frames which contained errors, and slot the re-transmitted frames into the right place in the sequence before forwarding them to the next stage.
  • These frames all had the same type of envelope (frame header and trailer)^[7] which contained enough information for data packages from different types of terminal to be sent along the same communications line, leaving the mainframe to deal with any differences in the formatting of the content or in the rules governing dialogs with different types of terminal.

Remote terminals (e.g., those connected to the mainframe by telephone lines) and 3705 communications processors would have SDLC-capable communications cards.

This is the precursor of the packet communication that eventually evolved into today's TCP/IP technology^{[citation needed]}. SDLC itself evolved into HDLC,^[8] one of the base technologies for dedicated telecommunication circuits.

• VTAM, a software package to provide log-in, session keeping and routing services within the mainframe. A terminal user would log-in via VTAM to a specific application or application environment (e.g. CICS, IMS, DB2 or TSO/ISPF). A VTAM device would then route data from that terminal to the appropriate application or application environment until the user logged out and possibly logged into another application. The original versions of IBM hardware could only keep one session per terminal. In the 1980s further software (mainly from third-party vendors) made it possible for a terminal to have simultaneous sessions with different applications or application environments.

Advantages and disadvantages[edit]

SNA removed link control from the application program and placed it in the NCP. This had the following advantages and disadvantages:

Advantages[edit]

• Localization of problems in the telecommunications network was easier because a relatively small amount of software actually dealt with communication links. There was a single error reporting system.
• Adding communication capability to an application program was much easier because the formidable area of link control software that typically requires interrupt processors and software timers was relegated to system software and NCP.
• With the advent of Advanced Peer-to-Peer Networking (APPN), routing functionality was the responsibility of the computer as opposed to the router (as with TCP/IP networks). Each computer maintained a list of Nodes that defined the forwarding mechanisms. A centralized node type known as a Network Node maintained Global tables of all other node types. APPN stopped the need to maintain Advanced Program-to-Program Communication (APPC) routing tables that explicitly defined endpoint to endpoint connectivity. APPN sessions would route to endpoints through other allowed node types until it found the destination. This is similar to the way that routers for the Internet Protocol and the Netware Internetwork Packet Exchange protocol function. (APPN is also sometimes referred to PU2.1 or Physical Unit 2.1. APPC, also sometime referred to LU6.2 or Logical Unit 6.2, was the only protocol defined to APPN networks, but was originally one of many protocols supported by VTAM/NCP, along with LU0, LU1, LU2 (3270 Terminal), and LU3. APPC was primarily used between CICS environments, as well as database services, because it contact protocols for 2-phase commit processing). Physical Units were PU5 (VTAM), PU4 (37xx), PU2 (Cluster Controller). A PU5 was the most capable and considered the primary on all communication. Other PU devices requested a connection from the PU5 and the PU5 could establish the connection or not. The other PU types could only be secondary to the PU5. A PU2.1 added the ability to a PU2.1 to connect to another PU2.1 in a peer-to-peer environment.^[9])

Disadvantages[edit]

• Connection to non-SNA networks was difficult. An application which needed access to some communication scheme, which was not supported in the current version of SNA, faced obstacles. Before IBM included X.25 support (NPSI) in SNA, connecting to an X.25 network would have been awkward. Conversion between X.25 and SNA protocols could have been provided either by NCP software modifications or by an external protocol converter.
• A sheaf of alternate pathways between every pair of nodes in a network had to be predesigned and stored centrally. Choice among these pathways by SNA was rigid and did not take advantage of current link loads for optimum speed.
• SNA network installation and maintenance are complicated and SNA network products are (or were) expensive. Attempts to reduce SNA network complexity by adding IBM Advanced Peer-to-Peer Networking functionality were not really successful, if only because the migration from traditional SNA to SNA/APPN was very complex, without providing much additional value, at least initially. SNA software licences (VTAM) cost as much as $10,000 a month for high-end systems. And SNA IBM 3745 Communications Controllers typically cost over $100K. TCP/IP was still seen as unfit for commercial applications e.g. in the finance industry until the late 1980s, but rapidly took over in the 1990s due to its peer-to-peer networking and packet communication technology.
• The design of SNA was in the era when the concept of layered communication was not fully adopted by the computer industry. Applications, databases and communication functions were mingled into the same protocol or product, which made it difficult to maintain and manage. That was very common for the products created in that time. Even after TCP/IP was fully developed, the X Window System was designed with the same model where communication protocols were embedded into graphic display application.
• SNA's connection based architecture invoked huge state machine logic to keep track of everything. APPN added a new dimension to state logic with its concept of differing node types. While it was solid when everything was running correctly, there was still a need for manual intervention. Simple things like watching the Control Point sessions had to be done manually. APPN wasn't without issues; in the early days many shops abandoned it due to issues found in APPN support. Over time, however, many of the issues were worked out but not before TCP/IP became increasingly popular in the early 1990s, which marked the beginning of the end for SNA.

Security[edit]

SNA at its core was designed with the ability to wrap different layers of connections with a blanket of security. To communicate within an SNA environment you would first have to connect to a node and establish and maintain a link connection into the network. You then have to negotiate a proper session and then handle the flows within the session itself. At each level there are different security controls that can govern the connections and protect the session information.^[10]

Network Addressable Units[edit]

Network Addressable Units in a SNA network are any components that can be assigned an address and can send and receive information. They are distinguished further as follows:

• a System Services Control Point (SSCP) provides resource management and other session services (such as directory services) for users in a subarea network;^[11]
• a Physical Unit is a combination of hardware and software components that control the links to other nodes.^[12]
• a Logical Unit acts as the intermediary between the user and the network.^[13]

Logical Unit (LU)[edit]

SNA essentially offers transparent communication: equipment specifics that do not impose any constraints onto LU-LU communication. But eventually it serves a purpose to make a distinction between LU types, as the application must take the functionality of the terminal equipment into account (e.g. screen sizes and layout).

Within SNA there are three types of data stream to connect local display terminals and printers; there is SNA Character String (SCS), used for LU1 terminals and for logging on to an SNA network with Unformatted System Services (USS), there is the 3270data stream mainly used by mainframes such as the System/370 and successors, including the zSeries family, and the 5250 data stream mainly used by minicomputers/servers such as the System/34, System/36, System/38, and AS/400 and its successors, including System i and IBM Power Systems running IBM i.

SNA defines several kinds of devices, called Logical Unit types:^[14]

• LU0 provides for undefined devices, or build your own protocol. This is also used for non-SNA 3270 devices supported by TCAM or VTAM.
• LU1 devices are printers or combinations of keyboards and printers.
• LU2 devices are IBM 3270 display terminals.
• LU3 devices are printers using 3270 protocols.
• LU4 devices are batch terminals.
• LU5 has never been defined.
• LU6 provides for protocols between two applications.
• LU7 provides for sessions with IBM 5250 terminals.

The primary ones in use are LU1, LU2, and LU6.2 (an advanced protocol for application to application conversations).

Physical Unit (PU)[edit]

• PU1 nodes are terminal controllers such as IBM 6670 or IBM 3767
• PU2 nodes are cluster controllers running configuration support programs such as IBM 3174, IBM 3274, or the IBM 4701 or IBM 4702 Branch Controller
• PU2.1 nodes are peer-to-peer(APPN) nodes
• PU3 was never defined
• PU4 nodes are front-end processors running the Network Control Program (NCP) such as the IBM 37xx series
• PU5 nodes are host computer systems^[15]

The term 37xx refers to IBM's family of SNA communications controllers. The 3745 supports up to eight high-speed T1 circuits, the 3725 is a large-scale node and front-end processor for a host, and the 3720 is a remote node that functions as a concentrator and router.

SNA over Token-Ring[edit]

VTAM/NCP PU4 nodes attached to IBM Token Ring networks can share the same Local Area Network infrastructure with workstations and servers. NCP encapsulates SNA packets into Token-Ring frames, allowing sessions to flow over a Token-Ring network. The actual encapsulation and decapsulation takes place in the 3745.

SNA over IP[edit]

As mainframe-based entities looked for alternatives to their 37XX-based networks, IBM partnered with Cisco in the mid-1990s and together they developed Data Link Switching, or DLSw. DLSw encapsulates SNA packets into IP datagrams, allowing sessions to flow over an IP network. The actual encapsulation and decapsulation takes place in Cisco routers at each end of a DLSw peer connection. At the local, or mainframe site, the router uses Token Ring topology to connect natively to VTAM. At the remote (user) end of the connection, a PU type 2 emulator (such as an SNA gateway server) connects to the peer router via the router's LAN interface. End user terminals are typically PCs with 3270 emulation software that is defined to the SNA gateway. The VTAM/NCP PU type 2 definition becomes a Switched Major Node that can be local to VTAM (without an NCP), and a 'Line' connection can be defined using various possible solutions (such as a Token Ring interface on the 3745, a 3172 Lan Channel Station, or a Cisco ESCON-compatible Channel Interface Processor).

See also[edit]

Notes[edit]

1. ^(Schatt 1991, p. 227).
2. ^IBM Corporation. 'IBM Highlights, 1970-1984'(PDF). Retrieved Apr 19, 2019.
3. ^IBM Corporation (2008). Networking on z/OS(PDF). p. 31.
4. ^devices that acted as DMA controllers for control units, which in turn attached peripherals such as tape and disk drives, printers, card-readers
5. ^Synchronous Data Link Control Concepts(PDF) (Fifth ed.). IBM. May 1992. GA27-3093-4.
6. ^(Pooch, Greene & Moss 1983, p. 310).
7. ^(Pooch, Greene & Moss 1983, p. 313).
8. ^(Friend et al. 1988, p. 191).
9. ^IBM Systems Network Architecture and APPN PU2.1 References Guides
10. ^Buecker, Axel; et al. (2015). Reduce Risk and Improve Security on IBM Mainframes: Volume 2 Mainframe Communication and Networking Security. IBM Corporation. p. 132. ISBN0738440949. Retrieved April 23, 2019.
11. ^IBM Corporation. 'z/OS Communications Server: SNA Network Implementation Guide'. IBM Knowledge Center.Retrieved on 3 October 2015.
12. ^IBM Corporation. 'z/OS Communications Server: SNA Network Implementation Guide'. IBM Knowledge Center.Retrieved on 3 October 2015.
13. ^IBM Corporation. 'z/OS Communications Server: SNA Network Implementation Guide'. IBM Knowledge Center.Retrieved on 3 October 2015.
14. ^(Schatt 1991, p. 229).
15. ^Microsoft. 'Physical Unit (PU)'. Retrieved September 7, 2012.

References[edit]

Ibm Network Printer 17

• Friend, George E.; Fike, John L.; Baker, H. Charles; Bellamy, John C. (1988). Understanding Data Communications (2nd ed.). Indianapolis: Howard W. Sams & Company. ISBN0-672-27270-9.
• Pooch, Udo W.; Greene, William H.; Moss, Gary G. (1983). Telecommunications and Networking. Boston: Little, Brown and Company. ISBN0-316-71498-4.
• Schatt, Stan (1991). Linking LANs: A Micro Manager's Guide. McGraw-Hill. ISBN0-8306-3755-9.
• Systems Network Architecture General Information(PDF). First Edition. IBM. January 1975. GA27-3102-0.
• Systems Network Architecture Concepts and Products(PDF). Second Edition. IBM. February 1984. GC30-3072-1.
• Systems Network Architecture Technical Overview. Fifth Edition. IBM. January 1994. GC30-3073-04.
• Systems Network Architecture Guide to SNA Publications. Third Edition. IBM. July 1994. GC30-3438-02.

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External links[edit]

• APPN Implementers Workshop Architecture Document repository
• SNA protocols quite technical
• Related whitepapers sdsusa.com
• Advanced Function for Communications System Summary(PDF). Second Edition. IBM. July 1975. GA27-3099-1. Retrieved May 22, 2014.
• Systems Network Architecture Formats. Twenty-first Edition. IBM. March 2004. GA27-3136-20.
• Systems Network Architecture - Sessions Between Logical Units(PDF). Third Edition. IBM. April 1981. GC20-1868-2.
• Systems Network Architecture - Introduction to Sessions between Logical Units(PDF). Third Edition. IBM. December 1979. GC20-1869-2.
• Systems Network Architecture Format and Protocol Reference Manual: Architectural Logic(PDF). Third Edition. IBM. November 1980. SY20-3112-2.
• Systems Network Architecture: Transaction Programmer's Reference Manual for LU Type 6.2. Sixth Edition. IBM. June 1993. GC30-3084-05.
• Systems Network Architecture Type 2.1 Node Reference. Fifth Edition. IBM. December 1996. SC30-3422-04.
• Systems Network Architecture LU 6.2 Reference: Peer Protocols. Third Edition. IBM. October 1996. SC31-6808-02.

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