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Chapter 7

Tactical Packet Network

This chapter introduces packet switching and covers the Army TPN architecture which contains the network's hardware and software. It also covers TPN employment and management.


  7-1. Packet switching is a standard for interconnecting many computers. (See Figure 7-1.) Packet switching transmits data from one location (host) to another, just as circuit switching transmits voice from one location to another. Packet switching breaks the data into small packets with addresses and routes each packet to its destination through the network across the quickest and shortest path. The packet switching network uses the established paths of the circuit switch network rather than engineering the same links. Dedicating some trunk group channels to packet switching allows the packet switching network to take advantage of alternative path routing.

Figure 7-1. TPN Overview



7-2. The TPN includes packet-switching overlays to the MSE circuit switched network at ECB and EAC. The MSE overlay at ECB (the MPN) and the TRI-TAC packet overlay at EAC are basically identical and comprise the Army TPN. Typically, a data system must have four essential items to take advantage of the packet network capabilities. These are--

  • A physical interface.
  • Department of Defense (DOD) standard protocols.
  • Tactical name server (TNS) registration software.
  • A user application program written with packet access in mind.

7-3. The MPN is implemented with packet switches in the NC, SEN, LEN, FES, and NMT. The EAC packet overlay is implemented with packet switches in the TTC-39Ds, SENS, and LENS. An NMC manages the network. It is installed in the NMT at ECB and collocated with the CSCE at EAC. Other major components include the interworking gateways and the TNS/message transfer agent (MTA). Figure 7-2 shows the TPN WAN.

Figure 7-2. TPN WAN



7-4. The AN/TYC-20 is a self-contained unit and is the packet switch used in the TPN. The switch provides user access and routes packets through the MSE network. To accomplish this, the switch contains two separate processors. One processor is the main processor which automatically switches and routes data as packets. The other processor is the integral gateway (IGW) (the IGW-side) which acts as a transparent gateway to all LAN hosts.

7-5. One packet switch locates in the NCS, SEN, NMT, and FES shelters. Two packet switches locate in the LEN shelter and the TTC-39D. In MSE and TRI-TAC networks, a small portion of the trunking is dedicated to support the packet overlay. The SEN to NCS/TTC-39D trunks are at 16 kbps, while backbone trunking (NCS/TTC-39D to NCS/TTC-39D, NCS/FES/TTC-39D) and LEN to NCS/TTC-39D trunking are at 64 kbps. This data overlay of packet switches is implemented with almost no impact on voice user grade-of-service. The SEN, LEN, TTC-39D, and FES packet switches are mainly used for host connections and host access into the TPN backbone. The NCS packet switch, however, acts primarily as the backbone trunking packet switch for the TPN.

7-6. The IGW in the AN/TYC-20 acts as a transparent gateway and reverse address resolution protocol (RARP) server for all LAN hosts. The IGW contains two ports for LAN connections: LAN 0 and LAN 1. Both ports pass through separate Institute of Electrical and Electronics Engineers (IEEE) 802.3 transceivers and the shelter's signal entry panel (SEP). The IGW allows all connected TPN LAN hosts to send off LAN IP datagrams without any knowledge of the present TPN topology.

7-7. The TPN packet switch can connect up to 64 hosts on each of its LAN ports. However, with the AN/TYC-20, the IGW is considered one host per LAN appearance. Therefore, only a maximum of 63 hosts can connect to each LAN of the TPN. The IEEE 802.3 standard is 30 LAN hosts per 185 meters of RG-58 Thinlan cable. If using the LAN to full capacity, a repeater should be placed after the first and second 185-meter segment containing 29 or 30 hosts. If using only one segment of 185-meter cable, the TPN can connect only 30 hosts minus the IGW.

7-8. In the NCS, TTC-39D, and FES, there is a further limitation on LAN 0. The switch workstation is connected to LAN 0 and is considered connected to host port 56 at all times. Therefore, LAN 0 at the NCS, TTC-39D, and FES is not used in either shelter. The switch workstation contains the TNS and the MTA. If a user improperly connects to LAN 0, the workstation may be disconnected causing the TNS and the MTA to function incorrectly. This is highly undesirable; therefore, the user community should not connect to this LAN.

7-9. The TPN has two general configurations of the packet switch: the six port and the twelve port. The different configurations allow the various MSE and EAC shelters to accommodate distinct arrays of hosts and trunks. The SEN, LEN, and NMT contain the six-port configuration. The six-port configuration has twelve physical ports on the packet switch back plane; however, only six of these ports are configured in the packet switch software and are physically realized on the input/output (I/O) circuit card assembly (CCA). The NCS, FES, and TTC-39D contain the twelve-port configuration. There are twelve physical ports on the packet switch and all ports are configured for operational use.



7-10. The AN/TYC-19 or the T-20 gateway IP router is a communications gateway processor. As a stand-alone device, it resides only in the NCS and TTC-39D as part of its packet switching equipment. The gateway interconnects three different IP networks. These different networks may be networks with different net identifications (IDs) or other types of LANs and WANs (Internet or DISN). The T-20 router provides up to three port interfaces, hence the interconnection of three different packet switch networks. The gateway also supports direct trunk lines to other T-20 gateways.



  7-11. The LEN, SEN, FES, and TTC-39D configurations can connect a wired subscriber to the packet switching network, but they require a signal conversion. The signal data converter (SDC) performs this function. It converts four-wire data into a conditioned diphase (CDP) stream in one direction, and converts the CDP stream into data in the opposite direction. The SDC enables hosts to operate at distances of up to 4 kilometers (2.4 miles).


  7-12. Hosts can be any type of computer that meets the specifications of the TPN and can operate with the protocols prescribed by the network. These hosts can connect through an X.25 interface or as part of a LAN. Hosts classify as either standard hosts or high priority hosts. The high priority hosts are normally LAN hosts because the packet switch does not monitor LAN hosts and the IGW provides the interface, whereas the X.25 hosts connect directly to the packet switch.



7-13. The TNS and the MTA are combined on a single workstation in the NCS, FES, LEN, and TTC-39D. However, the TNS and MTA are running only in the NCS and the TTC-39D, and possibly the FES (if it is configured as an NCS). The TNS and MTA are not running in the LEN or the FES if the FES is configured as a LEN, unless the LEN is booted as an NCS.

7-14. The TNS is a dynamic database consisting of registered hosts and mailboxes whose main function is to answer queries from hosts and from the MTA. The database is dynamic due to the ability of a host or mailbox to relocate anywhere in the TNS network. Thus, when a host relocates, the local TNS receives the new registration information and transmits it to the other TNSs.

7-15. The TNS provides an automatic affiliation process similar to voice users. It performs host address resolution and user registration and provides a means for users to determine the current network location of other users on the network. The TNS network may consist of one or more IP networks. This is because occasionally one network cannot contain all the necessary packet switches.

7-16. The MTA is the e-mail component of the network. It performs e-mail store and forward, absent host coverage, and multiple addressing. The TNS and MTA combine to support mobile users in a tactical environment.




7-17. The two physical interfaces to the packet network are Thinlan (IEEE 802.3 or Ethernet) and four-wire CDP (for X.25 users). Access via IEEE 802.3/Ethernet is by a standard LAN card with the transmission control protocol (TCP)/IP. The length of the coaxial cable cannot exceed 185 meters without a repeater (not supplied with the system) at a maximum access rate of 9.6 kbps. The alternative to IEEE 802.3/Ethernet is X.25 access via a four-wire CDP connection (WF-16 field wire). The four-wire connection provides access at a range of up to 4 kilometers (2.4 miles) with an access data rate of 16 kbps. Figure 7-3 shows user connectivity to a SEN.




7-18. Direct X.25 connection of the TCP/IP host computer to the TPN four-wire CDP X.25 port requires a special interface. Commercial X.25 cards do not support four-wire CDP output, but most cards support synchronous RS-232 signaling levels. Three interface solutions can convert the RS-232 output of a commercial X.25 card to four-wire CDP. These solutions are used to connect TCP/IP hosts to the TPN.

7-19. The first solution is the tactical packet adapter (TPA). It is a self-contained external device and a simple, low-cost synchronous RS-232 to four-wire CDP converter. The installation of the TPA requires only a cable connection to the computer network adapter to the TPA and the connection of the CDP lines to the binding post.

7-20. The second solution is the MSE data interface device (MDID) (all models). The MDID is a simple, low-cost synchronous RS-232 to four-wire CDP converter.

7-21. The third solution is the tactical terminal adapter (TTA). It is a simple, low-cost synchronous RS-232 to four-wire CDP converter.

7-22. Host computers still require X.25 and TCP/IP to use the TPN. Host registration software is also required to take full advantage of the TPN.

Figure 7-3. User Connectivity to a SEN


  7-23. IP is the protocol used in layer three (or four) of the International Standards Organization (ISO) seven layer stack model. This protocol builds a message into an IP datagram. The datagram contains a header, a source address, a destination address, the data, and an error-checking mechanism.

7-24. An IP address (source or destination) is composed of four bytes (or octets). It is constructed in the following format: Xl. X2. X3. X4, where "X" can take the values of 0-255. This address form is the decimal dot notation or simply the IP address. The TPN supports both Class A and Class B IP addresses. Networks that have more than 65,536 but less than 16,777,216 hosts use Class A addresses. Networks that have more than 256 hosts but less than 65,536 hosts use Class B addresses. The TPN is licensed legally to use only the Class B addresses.

7-25. The IP address in the TPN is similar to the telephone number of the circuit switch network. The switches have to know a user's IP address and/or number to route information from one user to another. In the TPN, the packet switch node can automatically assign the connecting host an IP address. Hosts connecting to a packet switch, however, must have the required software for the automatic assignment of an IP address. If the host does not have this software, the switch operator must manually assign an IP address. Whereas, if the required software is present, the IP address is obtained without any user knowledge of the TPN topology.


7-26. An X.25 host should have Auto X.25 functions, and a LAN host should have RARP functions for automatic assignment of IP addresses, respectively. TPN X.25 hosts obtain their IP address from their connecting packet switch by sending a CALL REQUEST packet. The packet switch responds to the CALL REQUEST packet with an INCOMING CALL packet.

7-27. Once the host obtains its address, it can begin talking with the rest of the network and can register itself with the TNS. This is a nonstandard implementation for TPN users. User communities must develop and implement the software to perform this automated method of attaining an IP number.

7-28. For LAN hosts, the TPN can connect up to 63 host port numbers per user LAN. Again, only 29 hosts may connect per 185-meter segment of Thinlan cable. TPN LAN hosts automatically obtain their IP address from the IGW by using the RARP. A low-level protocol binds addresses dynamically instead of using a static table that lists each host's physical address and corresponding IP address. The IGW is the RARP server for all TPN LAN hosts. No other RARP server may attach to the TPN LANs. There are multiple times at which the host may send a RARP request. One of the most common procedures is to send a RARP request to the IGW for an IP address as the host is booting up.

7-29. A new IP address is required each time a host obtains a new physical connection to the TPN, or if the host is reconnected to a LAN after having powered down, or if the host is otherwise disconnected from the LAN. The RARP request contains the requesting host's 48-bit hardware IP address. The IGW responds to the RARP request with a RARP response to the requesting host. The RARP response includes the IP address and hardware address for the originator and the IGW. The IGW RARP server assigns IP addresses from highest port to lowest port; therefore, if hosts do not have RARP or the requirement is to assign IP addresses manually, the assignment is from lowest to highest. (See Figure 7-4.)



7-30. Registration allows a host and associated mailboxes to register with the TNS allowing the host to communicate with all hosts in the network. After the host receives its name, registration involves two separate processes. The first is to obtain the host's IP address from the packet switch network. The second is to register the host and associated mailboxes with the TNS as described in SR-43A and SR-45. (See Figure 7-4.)

Figure 7-4. Host Address Assignment and Registration with the TNS


  7-31. The NMC (AN/TYQ-54) manages the network, and it is a comprehensive real-time network monitoring and control system for the TPN. It is a computer workstation that monitors and controls the packet switching equipment. The NMC's hardware components include a central processing unit (CPU), a color monitor, a keyboard, and a trackball. Its software, known as the Integrated Management System (IMS), enables the NMC operator to observe activity in a tactical network and to diagnose any problems that may arise. (See Figure 7-5 and Figure 7-6.)


  • Constant watch on network components.
  • Recording and displaying device status changes and network events.
  • Operator querying of specific network components real-time status.


  • Network operators can remotely issue commands to control network components (for example, diagnostics, throughput, and software downloads).

Statistics Collection:

  • Gathers statistical data for further processing.


  • Processes statistics and monitors information to produce management reports.

Figure 7-5. Packet NMC Functions

Figure 7-6. Packet NMC in the NMT

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