FAQ Protocol Support 2
Sixnet Commercial Wireless products use these protocols to provide communications with existing devices and networks.
Click on Protocol to view details.
Async
Tandem
Bisync
TCP/IP
Frame Relay
UDP
Poll/Select
Uniscope
SNA/SDLC
X.25
- WAN Protocol Support
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Optional software available with the Gateways
SNA/SDLC Protocol Support
Systems
Network Architecture/Synchronous Data Link Control (SNA/SDLC) is IBM's
version of bit-oriented protocol. SDLC is the link level protocol, and
SNA provides the intelligence for the connection. SNA and SDLC use a
series of commands to control the flow of data through the network. All
of our products support most of the capabilities used by SNA devices.
SNA consists of specialized hardware and control software.
- Communications Controller (for example: 3745) provides the physical interface for the SNA device and runs the NCP/VS program.
- NCP/VS (Netwrok Control Program/Virtual Storage) controls the network.
- VTAM (Virtual Telecommunications Access Method) controls the access between the application program and the logical unit (terminal).
Connection Flow
A major advantage of SNA is the ability of terminals to switch between
applications. The terminal is attached (bound) to an application by:
- NCP/VS issues an Activate Physical Unit (ACTPU) Command to the controller.
- NCP/VS issues an Activate Logical Unit (ACTLU) Command to the terminal.
- The terminal operator issues a request to VTAM for connection to an application (SSCP-LU session). VTAM can also be configured to automatically initiate this interchang
- VTAM
issues a Bind Session Command (BIND) to the terminal. The terminal is
now connected to the application. The terminal operator can issue a
command (System Request) to terminate the connection with the
application (UNBIND) and reconnect to VTAM so that a different
application can be selected.
There are many different terminal implementations of SNA/SDLC protocol. Each terminal uses different hardware and performs different functions. SNA/SDLC terminals are grouped into two major categories: Remote Job Entry (RJE) and interactive. The RJE terminals are characterized by large amounts of data sent in long bursts. An interactive terminal is characterized by an interchange of small messages between the host and the terminal. SNA/SDLC controls the flow of data more efficiently than bisync. This allows RJE and interactive terminals to share the same communications line.
Further information on the RJE terminals is available here. Information on the 3270 terminals is available here.
Space and Character Compression
Some RJE terminals support space and character compression. With space compression, the transmitting terminal replaces consecutive strings of spaces with a counter. The receiving device restores the spaces. Space compression is used to reduce the amount of data sent over the communications line. Character compression works in the same manner for consecutive occurrences of the same character.
Data Formats
SNA devices can support either ASCII or EBCDIC data formats.
Error Reporting
SNA terminals use several levels of reporting error conditions. In addition to the same type of application messages as used in bisync, SNA devices can use protocol level messages to communicate directly with the network control programs.
SNA Advantages
SNA was developed to address many of the shortcomings encountered with bisync protocol. The structure of SNA/SDLC offers the following advantages:
- Transparency - SNA/SDLC is inherently transparent to the different data formats. Information in the SDLC frame is not interpreted by the network devices.
- Dissimilar Devices - SNA/SDLC restricts the size of each frame. NCP/VS controls the flow of data from each terminal on the communications line so that different stations can send data. With bisync, each terminal sends all of its data before the next terminal can send.
- Line Efficiency - SNA/SDLC is multi-thread with several distinct communication paths existing in different states simultaneously. This provides better throughput than bisync, which is single-thread.
- Diagnostics - SNA/SDLC provides more information on failures than bisync since temporary (soft) errors are communicated to the host. The information allows corrective action to be undertaken before hard failures occur.
The terminal emulation is PU1 or PU 2. The host emulation is a subset of PU4/5. The host emulation performs all of the host functions, including ACTPU, ACTLU, BIND and SSCP-LU. This emulation is designed to activate an SNA device and facilitate data transfer.
To view the available options that can be specified for this protocol, please review the worksheets. If you need further information on SNA/SDLC protocol or our implementation and support for this protocol, please e-mail us: wireless.support@sixnet.com
AM6520 is a proprietary polled protocol developed by Tandem that uses async or sync transmission method. AM6520 access method supports 6510, 6520, 6530, and 6540 terminals.
With async, each terminal is directly connected to the host. With sync transmission, each Tandem terminal can use a unique network ID address. With this mode, each terminal is specifically polled by the host on a multidrop line.
The AM6520 protocol supports the following transmission modes:
| Mode One | Mode Two | Mode Three | Mode Four |
| Async Full-duplex Character mode Point-to-point |
Async Half-duplex Block mode Point-to-point |
Sync Half-duplex Block mode Point-to-point |
Sync Half-duplex Block mode Multipoint (polled) |
The first mode does not use formatting codes. The screen formatting codes are identical for the remaining modes. The only difference between the second and third modes is the transmission method (sync or async).
With the first three modes, the Tandem terminal must be directly connected to the communications controller. A communications controller (or multiplexer) port is required for every terminal. The fourth mode uses a polled, multipoint protocol to connect several terminals to the same port. Each terminal is specifically polled since AM6520 protocol does not support a General poll capability.
Screen Memory
The major difference between the Tandem terminals is the memory size. The Tandem terminal allocates its RAM memory in two different modes:
- Multi-Page Mode - The terminal supports several pages of memory. Each page consists of a 24 row x 80 column screen.
- Single-Page Mode - The terminal builds a single screen buffer consisting of a large number of 80 column rows. This is called ITI mode.
| Terminal | Pages | Lines |
| 6510 6520 6530 6540 (PC program) |
2 4 12 14 |
48 96 312 350 |
In Single-Page mode, the user can scroll through the screen buffer without interacting with the host. The screen scrolls one line at a time or one 24 line page at a time. In Multi-Page mode, displayed pages can only be changed by the host. The user works with the displayed section of the screen buffer. Prints or transmissions only affect the displayed 24 line section.
Screen Formatting
Tandem AM6520 data streams consist of control codes and data formatting codes. Only the data formatting codes differ from one terminal to another.
| Screen Formatting | Video presentation |
| Protected entry Unprotected entry Numeric Only Alpha(betic) Only Alphanumeric Full numeric Full numeric with space Alpha with space Alphanumeric with space TAB Stops |
Low Intensity Blink Underline Reverse Video Blank (non display) |
| Modified Data (refer to Read Commands) |
Read Commands
The 6530 uses a bit in the attribute to determine which data is sent to the host when an Attention Identifier (AID) key is entered. When the operator (or host application) makes an entry into an unprotected field, the Modified Data Tag (MDT) is set. When the ENTER key is pressed, the terminal scans the screen buffer to determine which fields have the MDT set. The modified fields are then assembled into a block for transmission to the host.
The host can use a Read Command to have all or part of the terminal's screen buffer sent to the host.
Transmissions
The operator enters a Function Key which is sent to the host. The host application issues a Read Command associated with the Function Key to the terminal which causes a transmission to be sent to the network.
Status Line
The 6530 terminal uses a 25th Line to inform the operator of the terminal's status and network activity. The 25th line indicates printer status and operational modes. The host can also write messages to the 25th line
Print Options
The 6530 supports three printing options:
| Screen Print | The operator can print the displayed screen on the attached printer. |
| Host Print | The host can print the displayed screen on the attached printer. |
| Addressable Printer | The host can directly address the attached printer. This operation does not affect the displayed screen. |
This tutorial is intended to supply a brief overview of TCP/IP protocol. Explanations of IP addresses, classes, netmasks, subnetting, and routing are provided, and several example networks are considered.
The IP Address and Classes
Hosts and networks
IP addressing is based on the concept of hosts and networks. A host is essentially anything on the network that is capable of receiving and transmitting IP packets on the network, such as a workstation or a router. It is not to be confused with a server: servers and client workstations are all IP hosts.
The hosts are connected together by one or more networks. The IP address of any host consists of its network address plus its own host address on the network. IP addressing, unlike, say, IPX addressing, uses one address containing both network and host address.
How much of the address is used for the network portion and how much for the host portion varies from network to network.
IP addressing
An IP address is 32 bits wide, and as discussed, it is composed of two parts: the network number, and the host number [1, 2, 3]. By convention, it is expressed as four decimal numbers separated by periods, such as "200.1.2.3" representing the decimal value of each of the four bytes. Valid addresses thus range from 0.0.0.0 to 255.255.255.255, a total of about 4.3 billion addresses. The first few bits of the address indicate the Class that the address belongs to:
| Class | Prefix | Network Number | Host Number |
| A | 0 | Bits 1-7 | Bits 8-31 |
| B | 10 | Bits 2-15 | Bits 16-31 |
| C | 110 | Bits 3-23 | Bits 24-31 |
| D | 1110 | N/A | |
| E | 1111 | N/A |
The bits are labeled in network order, so that the first bit is bit 0 and the last is bit 31, reading from left to right. Class D addresses are multicast, and Class E are reserved. The range of network numbers and host numbers may then be derived:
| Class | Range of Net Numbers | Range of Host Numbers |
| A | 0 to 126 | 0.0.1 to 255.255.254 |
| B | 128.0 to 191.255 | 0.1 to 255.254 |
| C | 192.0.0 to 233.255.255 | 1 to 254 |
Any address starting with 127 is a loopback address and should never be used for addressing outside the host. A host number of all binary 1's indicates a directed broadcast over the specific network. For example, 200.1.2.255 would indicate a broadcast over the 200.1.2 network. If the host number is 0, it indicates "this host". If the network number is 0, it indicates "this network" [2].
All the reserved bits and reserved addresses severely reduce the available IP addresses from the 4.3 billion theoretical maximum. Most users connected to the Internet will be assigned addresses within Class C, as space is becoming very limited. This is the primary reason for the development of IPv6, which will have 128 bits of address space.
Basic IP Routing
Classed IP Addressing and the Use of ARP
Consider a small internal TCP/IP network consisting of one Ethernet segment and three nodes. The IP network number of this Ethernet segment is 200.1.2. The host numbers for A, B, and C are 1, 2, and 3 respectively. These are Class C addresses, and therefore allow for up to 254 nodes on this network segment.
Each of these nodes have corresponding Ethernet addresses, which are six bytes long. They are normally written in hexadecimal form separated by dashes (02-FE-87-4A-8C-A9 for example).
In the diagram above and subsequent diagrams, we have emphasized the network number portion of the IP address.
Suppose that A wanted to send a packet to C for the first time, and that it knows C's IP address. To send this packet over Ethernet, A would need to know C's Ethernet address. The Address Resolution Protocol (ARP) is used for the dynamic discovery of these addresses [1].
ARP keeps an internal table of IP address and corresponding Ethernet address. When A attempts to send the IP packet destined to C, the ARP module does a lookup in its table on C's IP address and will discover no entry. ARP will then broadcast a special request packet over the Ethernet segment, which all nodes will receive. If the receiving node has the specified IP address, which in this case is C, it will return its Ethernet address in a reply packet back to A. Once A receives this reply packet, it updates its table and uses the Ethernet address to direct A's packet to C. ARP table entries may be stored statically in some cases, or it keeps entries in its table until they are "stale" in which case they are flushed.
Consider now two separate Ethernet networks that are joined by a PC, C, acting as an IP router (for instance, if you have two Ethernet segments on your server).
Device C is acting as a router between these two networks. A router is a device that chooses different paths for the network packets, based on the addressing of the IP frame it is handling. Different routes connect to different networks. The router will have more than one address as each route is part of a different network.
Since there are two separate Ethernet segments, each network has its own Class C network number. This is necessary because the router must know which network interface to use to reach a specific node, and each interface is assigned a network number. If A wants to send a packet to E, it must first send it to C who can then forward the packet to E. This is accomplished by having A use C's Ethernet address, but E's IP address. C will receive a packet destined to E and will then forward it using E's Ethernet address. These Ethernet addresses are obtained using ARP as described earlier.
If E was assigned the same network number as A, 200.1.2, A would then try to reach E in the same way it reached C in the previous example - by sending an ARP request and hoping for a reply. However, because E is on a different physical wire, it will never see the ARP request and so the packet cannot be delivered. By specifying that E is on a different network, the IP module in A will know that E cannot be reached without having it forwarded by some node on the same network as A.
Direct vs. Indirect Routing
Direct routing was observed in the first example when A communicated with C. It is also used in the last example for A to communicate with C. If the packet does not need to be forwarded, i.e. both the source and destination addresses have the same network number, direct routing is used.
Indirect routing is used when the network numbers of the source and destination do not match. This is the case where the packet must be forwarded by a node that knows how to reach the destination (a router).
In the last example, A wanted to send a packet to E. For A to know how to reach E, it must be given routing information that tells it who to send the packet to in order to reach E. This special node is the "gateway" or router between the two networks. A Unix-style method for adding a routing entry to A is
route add [destination_ip] [gateway] [metric]
Where the metric value is the number of hops to the destination. In this case,
route add 200.1.3.2 200.1.2.3 1
will tell A to use C as the gateway to reach E. Similarly, for E to reach A,
route add 200.1.2.1 200.1.3.10 1
will be used to tell E to use C as the gateway to reach A.
It is necessary that C have two IP addresses - one for each network interface. This way, A knows from C's IP address that it is on its own network, and similarly for E. Within C, the routing module will know from the network number of each interface which one to use for forwarding IP packets.
In most cases it will not be necessary to manually add this routing entry. It would normally be sufficient to set up C as the default gateway for all other nodes on both networks. The default gateway is the IP address of the machine to send all packets to that are not destined to a node on the directly-connected network. The routing table in the default gateway will be set up to forward the packets properly, which will be discussed in detail later.
Static vs. Dynamic Routing
Static routing is performed using a preconfigured routing table which remains in effect indefinitely, unless it is changed manually by the user. This is the most basic form of routing, and it usually requires that all machines have statically configured addresses, and definitely requires that all machines remain on their respective networks. Otherwise, the user must manually alter the routing tables on one or more machines to reflect the change in network topology or addressing. Usually at least one static entry exists for the network interface, and is normally created automatically when the interface is configured.
Dynamic routing uses special routing information protocols to automatically update the routing table with routes known by peer routers. These protocols are grouped according to whether they are Interior Gateway Protocols (IGPs) or Exterior Gateway Protocols. Interior gateway protocols are used to distribute routing information inside of an Autonomous System (AS). An AS is a set of routers inside the domain administered by one authority. Examples of interior gateway protocols are OSPF and RIP. Exterior gateway protocols are used for inter-AS routing, so that each AS may be aware of how to reach others throughout the Internet. Examples of exterior gateway protocols are EGP and BGP. See RFC 1716 [11] for more information on IP router operations.
WAN Routing
Our WAN Cards provide a network interface, and do not actually route packets according to IP address, or maintain IP routing information. Packet routing between interfaces is accomplished by the protocol stack, which can send IP based dynamic routing protocols over WAN card. The information and protocols needed for dynamic routing are handled by the protocol stack. In practice, it is almost always better to use explicit static routing table entries rather than relying on dynamic routing.
Click link below for:
Advanced IP Routing
Wireless FAQ Overview