Sheraton Gateway Hotel PCCA

January 9, 1999January 11, 1999

TITLE:

The IP Modem Interface Standard

DRAFT

January 11, 1999January 9, 1999

SOURCE:

Portable Computer and Communications Association

Modem Standards Committee

P.O. Box 924

Brookdale, California 95007 U.S.A

Telephone: (408) 338-0924

Fax: (408) 338-7806

Contact: Jim Panian, Editor919 472-7220, .

Christopher Burke, 425 487-5917,
Emil Sturniolo, 330 723-9099,

Peter Rysavy,206 517-5654

TABLE OF CONTENTS

1SCOPE

2REFERENCES

3TERMINOLOGY

3.1COMMONLY USED TERMS......

3.2SUPPORTING DEFINITIONS......

3.3OPEN ISSUES......

4REQUIREMENTS

5IP MODEM ARCHITECTURE......

5.1INTERNET DIAL-UP NETWORKING......

5.2GSM “FAST CONNECT” WIRELESS INTERNET ACCESS......

5.3CDPD WIRELESS INTERNET ACCESS......

5.4EMBEDDED INTERNET: THE IP MODEM ARCHITECTURE......

5.4.1Description......

5.4.2Other Service Agents......

5.4.3Alternatives and cost effectiveness......

5.4.4Additional advantages/applications......

6IP MODEM IMPLEMENTATION

6.1INTRODUCTION......

6.2Virtual Channels......

6.3IP MODEM APPLICATION MODEL......

6.4IP MODEM INTERFACE IS A SIMPLE ROUTER......

6.5LOCALLY HOSTED SERVICE AGENT......

6.6Classes of operation......

6.7IP Address assignments to the IP Modem......

6.7.1IP modem uses statically assigned IP addresses......

6.7.2IP modem uses self-administered IP addresses.......

6.7.3IP modem uses a limited broadcast address.......

6.8IP Address assignment to the DTE......

6.8.1IP modem transfers the IP address to DTE by restarting PPP......

6.8.2IP modem provides a dummy IP address and functions as a proxy......

6.8.3IP modem provides a Virtual Private Network......

6.9Requirements for a DCE implementing ip modem......

6.10REQUIREMENTS FOR A DTE Interfacing to an IP Modem......

7Requirements for a Minimal Implementation of IP Modem......

8AT COMMANDS......

8.1AT +WIPC - Configure THE IP MODEM INTERFACE......

8.2AT +WIPM - Interrogate THE IP MODEM INTERFACE......

8.3AT +WIPA – Assign an IP Address to a Service Agent......

8.4AT +WIPT – Context Management......

8.5AT+WDSM46 – Wireless Data Stack Multiple scanning priority list......

8.6Compound Parameter IP_Spec

9SCENARIOS

9.1CDPD PC CARD IS CONFIGURED TO INVOKE THE IP MODEM INTERFACE......

9.2A DISCONNECTION CANCELS THE IP MODEM INTERFACE......

1SCOPE...... 3

2REFERENCES...... 3

3TERMINOLOGY...... 3

3.1COMMONLY USED TERMS...... 3

3.2SUPPORTING DEFINITIONS...... 4

3.3OPEN ISSUES...... 5

4REQUIREMENTS...... 5

5IP MODEM ARCHITECTURE...... 6

5.1INTERNET DIAL-UP NETWORKING...... 6

5.2GSM “FAST CONNECT” WIRELESS INTERNET ACCESS...... 7

5.3CDPD WIRELESS INTERNET ACCESS...... 8

5.4EMBEDDED INTERNET: THE IP MODEM ARCHITECTURE...... 9

5.4.1Description...... 9

5.4.2Other Service Agents...... 11

5.4.3Alternatives and cost effectiveness...... 11

5.4.4Additional advantages/applications...... 12

6IP MODEM IMPLEMENTATION...... 13

6.1INTRODUCTION...... 13

6.2Virtual Channels...... 13

6.3IP MODEM APPLICATION MODEL...... 14

6.4IP MODEM INTERFACE IS A SIMPLE ROUTER...... 15

6.5LOCALLY HOSTED SERVICE AGENT...... 15

6.6IP Address assignments to the IP Modem...... 16

6.6.1IP modem uses statically assigned IP addresses...... 16

6.6.2IP modem uses self-administered IP addresses....... 16

6.6.3IP modem uses a limited broadcast address....... 16

6.7IP Address assignment to the DTE...... 16

6.7.1IP modem transfers the IP address to DTE by restarting PPP...... 16

6.7.2IP modem provides a dummy IP address and functions as a proxy...... 17

6.7.3IP modem provides a Virtual Private Network...... 18

6.8Requirements for a DCE implementing ip modem...... 19

6.9REQUIREMENTS FOR A DTE Interfacing to an IP Modem...... 19

7AT COMMANDS...... 20

7.1AT +WIPC - Configure THE IP MODEM INTERFACE...... 20

7.2AT +WIPM - Interrogate THE IP MODEM INTERFACE...... 21

7.3AT +WIPA – Assign an IP Address to a Service Agent...... 23

7.4Compound Parameter IP_Spec...... 25

8SCENARIOS...... 27

8.1CDPD PC CARD IS CONFIGURED TO INVOKE THE IP MODEM INTERFACE...... 27

8.2A DISCONNECTION CANCELS THE IP MODEM INTERFACE...... 28

1SCOPE

This is the draft PCCA IP modem standard.

2REFERENCES

Table 1. References

3TERMINOLOGY

3.1COMMONLY USED TERMS

IP Modem - DCE which, in at least one operating mode, generates and interprets Internet Protocol (IP) datagrams conveyed over the DDL, typically by means of one or more Virtual Remote End Systems. See also Service Agent DCE (SA-DCE).

Service Agent -A Virtual Remote End System within an IP Modem, with which the DTE interacts by means of IP datagrams transferred across the DDL having a specified IP address, protocol type, and port number/frame type (whichever is applicable).[1]

SA-DCE -Service Agent DCE. An IP Modem incorporating at least one sService aAgent.

DDL -DTE-DCE Data Link. The communication circuit and procedures between DCE and DTE, which are assumed to provide at least half-duplex, substantially reliable bi-directional transfer of an octet stream. All other attributes of the DDL are irrelevant to general architectural principles of the IP Modem.

Network -A means of interconnecting two or more End Systems. The means of interconnection and interconnection protocols (whether data, voice, audio, video, integrated services, virtual, etc.) are irrelevant to general architectural principles of the IP Modem.

Service -Any task performed by an End System on behalf of a user, another End System, or itself. Typical services include data conversion, I/O, data distribution, routing, database access, notification, etc.

AT Command Service Agent -
A sService aAgent that provides a standards-based IP interface to an AT command set interpreter (e.g. by accepting AT commands encapsulated in UDP, and by issuing responses encapsulated in UDP).

PPTP-PPTP is a tunneling protocol defined by the PPTP Forum that allows PPP packets to be encapsulated within Internet Protocol (IP) packets and forwarded over any IP network, including the Internet itself. PPTP provides support for virtual LAN connection establishment/release and encapsulation of higher level protocol frames within the Generic Routing Encapsulation (GREv2) over IP. GREv2 encapsulation is connectionless, and is carried directly on top of IP. PPTP provides for congestion control using a sliding window mechanism.

3.2SUPPORTING DEFINITIONS

DTE -Data Terminal Equipment. Any terminal, computer, or information appliance capable of providing commands and data to operate DCE. DTE does not necessarily incorporate a user interface, but may. Typical DTE performs computing, service, and user interface functions.

DCE - Data Circuit-Terminating Equipment. Any device (e.g. wireless modem, smart phone) that connects DTE to one or more real or virtual Networks. The phrase “circuit-terminating” is historical and does not imply any operating characteristic of any Network. DCE does not necessarily incorporate a user interface, but may. Typical DCE interconnect with and transfer information between real or Virtual Remote End Systems.

End System -A generalized system component which performs the combined functions of DTE and DCE. DTE and DCE interconnected via a DDL form an End System, but the designation End System does not imply incorporation of either DCE or DTE.
NOTE: Although PCCA STD-101 distinguishes between an End System and an Intermediate System, the distinction is irrelevant with respect to general architectural principles of the IP Modem; the term End System is used in this document for either type of system.

Local End System -Term used to discuss interactions among End Systems. The Local End System is the end system being discussed; the Remote End System is the “other” End System with which it interacts.

Remote End System -Term used to discuss interactions among End Systems. An End System connected to another through a Network. See Local End System.

VRES-Virtual Remote End System. An End System indistinguishable from a Remote End System, but implemented internally to the Local End System (e.g. by software or other means within the DCE). Typically used to implement a sService aAgent.

Virtual Channel -A communications path between an application on the DTE and a service agent.

3.3OPEN ISSUES

1. We need to architect the BeaconDirectory service agent. The service location protocol is a possibility; or a lightweight directory service.

2.We need to decide upon the requirements for a minimal implementation of IP modem.

1. Can 1.X addresses be reserved for use by IP modem service agents?

4.The use of term "+WS46 Service Agent" needs to be finalized. There may also be more than one +WS46 Bridge Service Agent active; one for each new interface added to the internal router.

1. Can DHCP be used to assign IP addresses to the DTE as IP addresses assigned by the network change when the IP modem attaches to different IP networks?

4 REQUIREMENTS

1. Describe a next-generation model of fixed and mobile computing services consistent with projected evolution of computing, mobility, telecommunication, and Internet architectures.

1. Enable multiple applications on a DTE to interact simultaneously with a SA-DCE. Some examples are:

 Phonebook management

 SMS management

 User interface menus provided for PC card wireless modems.

 Battery level/signal strength

1. Provide a well-defined Internet integrated services migration path for legacy AT command-based telecommunication applications (e.g. data, fax, voice telephony, DCE-based phonebook, mobile short messaging and control panel functions), with minimal application impact.

1. Support time-constrained applications such as voice, video, and interactive gaming.

1. Support multiple, simultaneous applications that perform circuit-switched data/fax and packet-based protocols that use AT commands.

56IP MODEM ARCHITECTURE

The IP Modem architecture results from simple extension of the Internet dial-up networking paradigm, and is fully consistent with PCCA STD-101 (Dialing and Control for Character Mode DCE on Wireless Data Services), PCCA ANX-101 L (Command Extensions for CDPD Modems), and contemporary wireless Internet remote access techniques such as those used by GSM and CDPD.

6.1INTERNET DIAL-UP NETWORKING

Figure 1 illustrates the dial-up networking paradigm familiar to most Internet users.

Figure 1. Internet Dial-Up Networking Paradigm

The DTE and DCE of Figure 1 are interconnected by an RS-232 or similar communication port using V.24 circuits. A DTE application or service opens an Internet connection, causing the DTE to issue AT commands to the DCE. These AT commands instruct the DCE to dial the telephone number of an Internet Service Provider (ISP). Dialing and subsequent data link protocol negotiation creates a durable two-way circuit between the DCE and a modem bank connected to the ISP. The DCE informs the DTE that the circuit is available by transmitting a CONNECT code; the modem bank similarly informs the ISP of the connection. The DCE and modem bank then switch themselves to a mode in which they can transceive data.

The ISP is capable of routing internet traffic (IP datagrams) between the DTE and other Internet-enabled sites, such as WWW sites or an email host. Both the ISP and the DTE expect to transmit and receive IP datagrams and certain other signaling (such as call termination) on the circuit.

The DTE composes IP datagrams and forwards them to the DCE, which forwards them to the modem bank, which forwards them to the ISP, which forwards them to the desired Internet site, host, or server. Once the circuit has been established, a host, site, or server reverses the path to send datagrams to the DTE. The circuit persists until either the DTE or the ISP “hangs up”, disconnecting the DTE from the Internet. Both the DCE and the modem bank respond to a “hang up” by preparing themselves to process future AT commands.

Most of this behavior is invisible to the user of the DTE. However, since telephone calls aren’t guaranteed to work (e.g. busy signal, system failure, improper cabling) the DCE often provides feedback to the DTE or directly to the user during dialing.

6.2GSM “FAST CONNECT” WIRELESS INTERNET ACCESS

Several techniques are used to access the Internet using a GSM phone. One new technique, “fast connect”, uses slightly different connections than those of Figure 1. Every effort is made to conceal these differences from the user (and from DTE application software); the illusion of dial-up networking is preserved. Figure 2 illustrates the GSM “fast connect” wireless internet access paradigm.

Figure 2. GSM “Fast Connect” Wireless Internet Access Paradigm

As in Figure 1, the DTE and DCE of Figure 2 are interconnected by an RS-232 or similar communication port using V.24 circuits. A DTE application or service opens an Internet connection, causing the DTE to issue AT commands to the DCE. These AT commands instruct the DCE to dial the telephone number of an Interworking Function (IWF) maintained by the GSM network operator. The DCE is specially designed to interact directly with the IWF, rather than with the PSTN. Dialing and subsequent data link protocol negotiation creates a durable two-way circuit between the DCE and the IWF. The DCE informs the DTE that the circuit is available by transmitting a CONNECT code; the IWF is implicitly aware of the connection. The DCE and IWF then switch themselves to a mode in which they can transceive data.

The IWF is capable of routing internet traffic (IP datagrams) between the DTE and other Internet-enabled sites, such as WWW sites or an email host. Both the IWF and the DTE expect to transmit and receive IP datagrams and certain other signaling (such as call termination) on the circuit.

The DTE composes IP datagrams and forwards them to the DCE, which forwards them to the IWF, which forwards them to the desired Internet site, host, or server. Once the circuit has been established, a host, site, or server reverses the path to send datagrams to the DTE. The circuit persists until either the DTE or the IWF “hangs up”, disconnecting the DTE from the Internet. The DCE responds to a “hang up” by preparing itself to process future AT commands.

6.3CDPD WIRELESS INTERNET ACCESS

CDPD and other packet switched networks also offer wireless internet access, using a topology that differs from both dial-up access and GSM remote access. Both the PSTN and GSM are circuit-switched, whereas CDPD is packet-switched.

The idea of dial-up networking might seem superfluous on a packet-switched network, but every effort is made in CDPD to preserve the illusion of dial-up networking, in order to make the technology more familiar to the user (and to DTE applications). Figure 3 illustrates the paradigm.

Figure 3. CDPD Wireless Internet Access Paradigm

Compare Figure 3 to Figure 2. The GSM IWF is missing, replaced by a Mobile Data Intermediate System (MDIS) internal to the CDPD network. The MDIS acts like an Internet bridge / router, giving the appearance that the CDPD network is fully integrated into the Internet.

Next, compare Figure 3 to Figure 1. The ISP is missing, replaced by a Virtual Remote End System (VRES) incorporated into the DCE and emulating an Internet Service Provider.

The circuit-switched connection provided by the PSTN (Figure 1) or the GSM Network (Figure 2) is replaced in CDPD (Figure 3) by a software state machine within the DCE. This state machine simply keeps track of what AT commands the DTE issues; when an appropriate ATD command is issued, the state machine places the DCE into a mode in which it can transceive data.

While in data transceiving mode (corresponding to the On-Line Data State of a conventional telephone modem) software within the DCE transfers data received from the DTE to the internal VRES instead of over the CDPD network. The VRES interprets the data stream exactly as an ISP would, i.e. as a series of IP datagrams encapsulated in SLIP or PPP. It removes the encapsulation, and used CDPD protocols to forward each IP datagram to the MDIS. The MDIS then forwards the datagram to the addressed Internet site, host, or server. Traffic from a site, host, or server follows the reverse path.

The DTE disconnects from CDPD in the same way as it does for dial-up or GSM, but the internal processing within the DCE is quite different for CDPD. Instead of signaling a “hang up” to the MDIS, the CDPD modem simply adjusts its internal state machine and prepares to accept additional AT commands.

Metricom’s Ricochet wireless network employs similar techniques, and similar techniques have been proposed for future packet-switched GSM systems (iDEN, GPRS, UMTS).

6.4EMBEDDED INTERNET: THE IP MODEM ARCHITECTURE

Figures 1 through 3 illustrate the continuous evolution of the AT command set Internet access paradigm for Data Circuit-Terminating Equipment (DCE). The IP Modem Architecture extrapolates this evolutionary trend, integrating portions of the Internet itself into the DCE (requirement 1) while supporting legacy applications and providing an Internet-friendly interface to non-internet communication services (requirement 2, 3).

Figure 3 introduced the idea of a VRES inside of the DCE, offering services previously provided by an ISP on the other side of the wired or wireless network. Moving the ISP into the DCE gives the DCE control over datagram routing, opening the door to implementing an “intranet” and a “firewall” within the DCE, as shown in Figure 4.

Figure 4. IP Modem Architecture (DCE Only)

6.4.1 Description

Figure 4 illustrates the IP Modem Architecture. The DCE described in Figure 4 can be used in various system configurations such as those shown in Figure 1, Figure 2, or Figure 3 if appropriate Service Agents are included.

The DCE behaves as an IP modem only for specific settings of +WS45 and +WS46. The remainder of this description assumes that the appropriate +WS45 and +WS46 settings are selected.

The DTE and DCE are interconnected via a DTE-DCE Data Link, DDL, as defined elsewhere in this document. The DDL is used to convey AT Commands, IP Datagrams, and possibly other (legacy) types of information between DTE and DCE.

The DCE services the DDL using an AT State Machine. The AT State Machine keeps track of the current settings of +WS45 and +WS46, other parameter settings, and whether the IP Modem is in Command State, On-Line Command State, or On-Line Data State. When the modem is in Command State or On-Line Command State, the AT State Machine monitors for state changes (e.g. disconnect, ring) but passes all other data to the AT Command Interpreter, which processes AT commands, updates internal parameter values, executes action commands, and returns responses. When the modem is in On-Line Data State, the AT State Machine monitors for state changes (e.g. via +++ or a drop in the DTR modem control line) but passes all other data to the Internal IP Router. In other words, in an IP Modem the ATD command connects the DTE to the Internal IP Router, not to the communication network. The Internal IP Router, and portions of the AT State Machine, are equivalent to the ISP Virtual Remote End System (ISP VRES) described for CDPD modems.