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The IEEE 802.9 Integrated Services Local Area Network Standard

Gary C. Kessler
December 1993

An edited version of this paper appeared with the title "A Happy Union: IEEE 802.9" in the May, 1994 issue of LAN Magazine.

Introduction

During the 1980s, the use of LANs grew at a phenomenal rate. The late-1980s, in particular, saw user application requirements and bandwidth needs come together with more powerful processors. As the number of PCs attached to LANs grew, so did the individual station's need for bandwidth. This increased usage of the network resulted in reduced performance so that networks were segmented and the average LAN size actually started to decrease. The LAN interconnection market grew out of the resulting need to interconnect LANs over campus and wide areas.

Concurrent with the growth in the use of PCs and LANs, new voice and data services became available from public network service providers. The middle- to late-1980s saw the introduction of the Integrated Services Digital Network (ISDN). The integration of voice and data on a single network offers long-term economic advantages for both the customer and the service provider; the customer needs only a single ISDN port per office instead of separate voice and data jacks, while the service providers only have to maintain and manage a single network rather than two.

ISDNs provide a viable WAN alternative interconnecting LANs, hosts, and PCs for a number of reasons. First, ISDNs will incorporate packet-switching services, including support for X.25 and frame relay, meaning that the current investment in this equipment will be protected. Second, voice/data terminals will become more commonplace as ISDN service expands and integrated voice/data applications become more common. Voice/data terminals will need to be intelligent devices and PCs provide a logical basis for this type of communications equipment.

Finally, the physical integration of voice, image, video, and data on a single network will result in new applications that will logically integrate these different information streams. Human beings are, by our nature, multi-media. We communicate better with a combination of words, images, video, and numerical data rather than through only a single medium. Once voice and data are stored and carried through the same switches and across the same transmission facilities, vendors will create applications that use all of these types of information. This is also the promise of Broadband ISDN (B-ISDN) services made available across Asynchronous Transfer Mode (ATM) networks.

The IEEE 802.9 Standard

The Institute of Electrical and Electronics Engineers (IEEE) 802 committee has been instrumental in creating local and metropolitan area network standards for the international community since its inception in 1980. The IEEE 802 committee was originally composed of 6 working groups, with new ones being added as new projects were developed. In February 1986, the IEEE 802 Executive Committee formed an ad hoc study group on integrated voice/data (IVD) LAN solutions. Within a year, the IEEE 802.9 Working Group was formed with a charter to provide an interface for the "marriage of LANs and ISDN." The working group began to define a standard IVDLAN interface that was compatible with already existing IEEE 802 LAN and International Telecommunication Union Telecommunications Standardization Sector (ITU TSS, formerly the CCITT) ISDN standards, architectures, and services.

The scope of the work charged to the IEEE 802.9 working group is:

  • To develop an integrated voice/data service interface at the medium access control (MAC) and physical layers that is compatible with other IEEE 802 standards and ISDN standards;

  • To develop an interface that operates independently of the backbone network; and

  • To focus on the use of unshielded twisted pair (UTP) as the primary distribution medium. This point is particularly important because of the near-pervasiveness of UTP and the excess bandwidth and capacity that is usually present when UTP is employed for such applications as voice.

For successful deployment, the 802.9 standard must also:

  • Be attractive to both manufacturers and users from the perspectives of economy, installation, and network operation;

  • Support the quality of voice service available today and expected improvements in the future; and

  • Allow for the implementation of a range of both centralized applications (e.g., connection to the public telephone network via a private branch exchange) and distributed applications (e.g., access to shared databases via a LAN file servers and hosts).

By the end of 1990, the IVDLAN standard was almost completed but industry support had fallen off so much that the project was nearly terminated. As ISDN and multimedia applications became increasingly available, however, new enthusiasm was found for this work. Renamed the Integrated Services LAN (ISLAN), the IEEE 802.9 standard was approved as a standard in the fall of 1993 and the vendor involvement in this activity suggests that products will be available by 1995.

Islan Overview

The ISLAN standard defines an interface between integrated services terminal equipment (ISTE) and a backbone network. The standard provides a high bandwidth interface to the desktop in support of packet data service and isochronous (time-sensitive) services. It is intended for operation over an unshielded twisted pair medium.


                       --------  --------  --------
                       | ISTE |  | ISTE |  | ISTE |
                       ----+---  ----+---  ----+---
        IEEE 802.9         |         |         |
         Interface    - - -|- - - - -|- - - - -|- - - 
                           |         |         |
                       ----+---------+---------+---- 
                       |                           | 
                       |        ACCESS UNIT        |
                       |                           |
                       --------------+--------------
                                     |
                                     |
                             ================
                             BACKBONE NETWORK

FIGURE 1. Scope of the IEEE 802.9 ISLAN standard. Integrated
services terminal equipment (ISTE) are connected to the access
unit (AU) in a star configuration and the AU, in turn, is attached
to the services backbone network.


Figure 1 shows the scope of the IEEE 802.9 standard. ISTEs are connected to an access unit (AU) in a physical star topology. These ISTEs may take on many forms; a voice TE, for example, might be a telephone, while a data TE might be a PC. A digital bit stream is sent over each point-to-point link between an ISTE and the AU, carrying packet data or isochronous data (such as voice, image, video, and facsimile). These different traffic flows are carried in separate channels on the line using time division multiplexing (TDM). The 802.9 standard describes the interface between the ISTEs and the AU.

 

  -------- ---------- ---------
  |Voice/| |Voice TE| |Data TE|  ------      --------- --------
  |Video/| -------+-- --+------  |ISTE|      |Data TE| | Data |
  | Data |      --+-----+-       ---+--      ----+---- |Server|
  |Server|      |Terminal|          |            |     ---+----
  ----+---      |Adapter |          |            |        |
      |         ----+-----          |            |        |
      |             |   802.9 ISLAN |            |        |
      |             |     INTERFACE |            |        |
   - -|- - - - - - -|- - - - - - - -|- -      ===+========+===== 
      ----------    |   -------------         | PREMISES-BASED |
            ---+----+---+--                   |    BACKBONE    |
            | ACCESS UNIT |                   |                |
            ----------+-+--                   | (802.3/4/5/6,  |
                      | ----------------------|     FDDI)      |
                      |                       ===+=====+========
                      |                          |     |
                      |                          |     |
 =============        |       LAN-PBX/C.O.       |     |
 |    WAN    |     ===+====    interface         |     |
 |           |     | PBX  |-----------------------     |
 | including |-----| C.O. |                        ====+===
 | ISDN and  |     |VOICE |------------------------| HOST |
 |  B-ISDN   |     |SWITCH|   Computer-PBX/C.O.    ========
 =============     ========       interface                

 FIGURE 2. IEEE 802.9 ISLAN interface configuration.


From the perspective of the ISTE, 802.9 only defines the interface to the AU and all of the services appear to be provided by the AU. This implies that the 802.9 standard can apply to two general scenarios. In the first scenario, the ISTEs are connected to a stand-alone LAN, in which case the AU actually does provide the integrated services. In the second scenario, the ISTEs access an integrated services backbone network, in which case the AU is merely a gateway to the backbone. This backbone network may be an existing IEEE 802 LAN, an ISDN (narrowband or broadband), a Fiber Distributed Data Interface (FDDI) metropolitan area network, or an ISLAN (such as 802.6 or FDDI-II). Figure 2 shows some of the possible interface configurations. The AU-to-backbone connection is beyond the scope of the IEEE 802.9 standard and, in any case, is transparent to the ISTEs.

Channels

One of the basic concepts common to ISDN and the 802.9 standard is that of multiple communications channels sharing the physical medium simultaneously. ISDN and 802.9 need to support multiple channels because each channel has a specific purpose or application. The easiest way to view the multiple channels is to compare them to having a multi-line telephone, where each line on the telephone might serve a different user at any given point in time.

The multiple channels share the same physical medium by using time division multiplexing. A TDM system assigns every channel a fixed amount of time on the medium at specified intervals. The time-division multiplexed bit stream between the AU and ISTE comprise several different full-duplex digital channels, each defined for a different purpose. These channels are:

  • The P-channel, or packet data channel, provides an IEEE 802 MAC service for packet-mode (bursty) data. The IEEE 802.9 MAC sublayer for the P-channel is described below.

  • The D-channel, or signalling channel, is a 16- or 64-kilobit per second (kbps) channel that corresponds to the ISDN D-channel. In an ISDN, the D-channel is used primarily for the exchange of signaling information between the user and the network for the provision of user services (called bearer services). The ITU-TSS Q.930 protocol family is used for user-network signaling for call control and the access to bearer services. The secondary function of the ISDN D-channel is to carry user packet-mode data. The 802.9 D-channel may be restricted for user-network signaling in some applications, but other applications may support packet data transfer over this channel.

    The ISDN basic rate interface (BRI) specifies use of a 16- kbps D-channel, while the primary rate interface (PRI) uses a 64-kbps D-channel. The 802.9 standard will support both rates, primarily to facilitate interoperability with today's ISDN BRI terminals.

  • The B-channel, or bearer services channel, is a 64-kbps channel that is functionally identical to the ISDN B-channel. ISDN circuit-mode bearer services such as voice and video and, optionally, packet-mode data services, are provided on the B-channel. A 64-kbps rate is used on this channel because that corresponds to the rate of a single digital voice channel. Two B-channels are required by the 802.9 standard, corresponding to the two B-channels on the ISDN BRI.

  • The C-channel, or circuit switched channel, is intended for circuit switched services that require a bit rate greater than that available from a single B-channel. The C-channel operates in increments of 64 kbps; Cm is used to indicate the size of the C-channel, where m is the number of 64-kbps multiples. C-channels are conceptually similar to ISDN H-channels, which are higher rate channels equivalent to some defined number of B-channels. The 802.9 C-channels correspond to ISDN B- and H-channels as follows:

      C1 = B = 64 kbps
      C6 = H0 = 384 kbps
      C24 = H11 = 1.536 megabits per second (Mbps)
      C30 = H12 = 1.920 Mbps

802.9 Protocol Overview

The IEEE 802.9 protocol architecture is shown in Figure 3. Like other IEEE and ANSI LAN standards, these protocols correspond to the physical and data link layers of the Open Systems Interconnection (OSI) reference model.


          I E E E   8 0 2 . 9          OSI

 ---------------------------------- = = = = = =
 |      |802.2| Appropriate  |    |
 |Layer | LLC |   layer 2    |LAPD|   Data
 |Mgmt. |-----| protocol for |    |   Link
 |Entity| MAC | isochronous  |    |   Layer
 |      | (P) | service (B,C)|(D) |
 |------+-----+--------------+----| = = = = = = 
 |      |Hybrid Multiplexing (MUX)|
 |Layer |-------------------------|
 |Mgmt. | Physical Signaling (PS) |   Physical
 |Entity|-------------------------|    Layer
 |      |     Physical Media      |
 |      |     Dependent (PMD)     |
 -------+-------------------------| = = = = = = 
        |     PHYSICAL MEDIUM     |
        ---------------------------

FIGURE 3. IEEE 802.9 protocol architecture. (Appropriate
channel is shown in parenthesis.)


The 802.9 interface must provide support for a number of different services depending upon the user application and the channel being used. For this reason, several different protocols are supported that correspond to the OSI data link layer:
  • The P-channel is a packet data channel that will use a MAC scheme and frame format specific to the 802.9 standard. Like other IEEE 802 LANs (and ANSI's FDDI), the IEEE 802.2 Logical Link Control (LLC) protocol acts as the upper sublayer of the data link layer on the P-channel.

  • The 802.9 D-channel is essentially the same as the ISDN D-channel. Therefore, the 802.9 access unit will use the same data link protocol as ISDN, namely the Link Access Procedures for the D-channel (LAPD), described in ITU-TSS Recommendations Q.920 and Q.921. Control of B- and C-channel services will be accomplished using basic ISDN call control procedures, described in the ITU-TSS Q.930-series recommendations. The D-channel can also be used to support other ISDN services, such as frame relay and packet services, although this has not yet been defined in the IEEE 802.9 standard.

  • The B- and C-channels are used to carry bit streams related to the requested bearer services. As in ISDN, no data link layer is specified for bearer channels since any protocol may be used that has been agreed to on an end-to-end basis. The B-channel was originally intended for any 64-kbps isochronous service, such as digital voice, but its scope has expanded to include other circuit-mode services such as switched 56 and 64 kbps digital data, and Group 4 (digital) facsimile. Packet data transfers typically use either the ITU-TSS Recommendation X.25 Link Access Procedures Balanced (LAPB) or LAPD protocol. The C-channels, like ISDN H-channels, are wideband isochronous channels for high-speed packet- and circuit-mode services, such as high-speed data transfers, video services, and image transfers.

The functions corresponding to the OSI physical layer are accomplished by three sublayers in the 802.9 protocol model. Briefly, these sublayers are:

  • The hybrid multiplexing (HMUX) sublayer multiplexes bits from the B-, C-, D-, and P-channels into a single bit stream between the ISTE and AU. This sublayer provides the interface between the physical layer and the user/control information.

  • The physical signaling (PS) sublayer provides an interface between the multiplexed bit stream and the actual physical bit stream on the line. The PS sublayer appends maintenance information to the frame, calculates parity and adds the appropriate parity bit, scrambles the bit stream, and appends framing information.

  • The physical media dependent (PMD) sublayer defines the electrical and mechanical characteristics of the specific medium being used; in this case, unshielded twisted pair. This sublayer defines the specific signaling scheme, cable and connector characteristics, and electrical properties of the transmitter and receiver.

Finally, the Layer Management Entities (LMEs) are part of the overall network management facilities of the interface. Each sublayer has a specific interface to its LME. The combination of all LMEs and the inter-LME communication define the network's Management (MT) entity. It is fully intended that the network management features of the ISLAN standard will conform to OSI standards for system and layer management. Furthermore, management of the ISLAN interface will also conform to those standards defined for managing the ISDN user-network interface.

Physical Layer Features

The 802.9 standard specifies that the ISTE and AU should be connected over unshielded telephone twisted pairs (UTTP), as defined in the EIA/TIA-568 premises wiring standard. Two different PMDs have been defined, which balance different speed and distance requirements.

The low-speed PMD operates at 4.096 Mbps over a distance of up to 450 meters (m), using a Partial Response Class IV (PR4) encoding scheme. A high-speed PMD operates at 20.48 Mbps over a distance of up to 135 m, and uses a 4-point carrierless AM/PM (4-CAP) encoding scheme. Both PR-4 and 4-CAP are used to achieve very high speeds over UTP in other standards, such as FDDI and ATM.

The physical connector for 802.9 ISTEs and AUs is an 8-pin modular connector (specified in ISO standard 8877), commonly referred to as an RJ-45. This is the same connector specified for the ISDN basic rate interface and the IEEE 802.3 TYPE 10BASET standard. Pin assignments for the connector are:

       PIN |   FUNCTION
       ----+---------------
        1  | ISTE Transmit
        2  | ISTE Transmit
        3  | ISTE Receive
        4  | Not used
        5  | Not used
        6  | ISTE Receive
        7  | Reserved
        8  | Reserved

As shown, pins 1/2 will be used for transmission in the ISTE-to-AU direction and pins 3/6 will be used for transmission in the AU-to-ISTE direction. An interface power supply to the ISTE is not a requirement of the standard, but pins 7/8 may be used for this purpose. Furthermore, phantom power may be supplied by the AU over pins 1/2 and 3/6, although the standard does not include any specifications for this. The standard does state, however, that any further 802.9 work with respect to powering should, as much as possible, be consistent with the ISDN BRI physical layer standard (ITU-TSS Recommendation I.430).

The PS sublayer's major functions include frame synchronization and scrambling. Frame synchronization ensures that the transmissions between ISTE and AU remain aligned and that the receiver is correctly interpreting the incoming transmission. Scrambling of the bit stream prior to transmission helps reduce the affects of electromagnetic interference (EMI) and aids in clock recovery.

The HMUX sublayer must take the bits from the incoming B-, C-, D-, and P-channels and place them into a single outgoing bit stream. The HMUX can operate in one of several modes:

  • Mode 0 - IEEE 802 Service Only: This mode is used by devices that have implemented only the 802.9 MAC and the entire payload is dedicated to the P-channel. There is no support for other bearer services in this mode.

  • Mode 1 - BRI ISDN Service Only: This mode is used by ISTEs that implement only the ISDN BRI and provides no support for IEEE 802 data services or the IEEE 802.9 MAC scheme.

  • Mode 2 - 802 & BRI ISDN Service Only: This mode supports only the ISDN BRI and IEEE 802.9 data services. Dynamic bandwidth management functions are not supported, meaning that the ISLAN C-channel is not used.

  • Mode 3 - Dynamic Bandwidth Management Service: This mode is for full support of ISLAN services, including the dynamic use of C-channels and bandwidth negotiation over the D-channel.

Modes 4 through 7 are currently reserved for future definition.

TDM Frame Structure

The bit stream exchanged between an ISTE and the AU is called a TDM Frame. A single TDM frame carries data from the B-, C-, D-, and P-channels, as well as additional synchronization, control, and maintenance information. A TDM frame is generated 8,000 times per second, or once every 125 microseconds; this corresponds to the sampling rate necessary to digitize human voice. Each octet (8 bits) in the frame, then, represents a 64-kbps channel.

The 802.9 standard supports an ISDN BRI, which comprises two B-channels and a single D-channel (designated 2B+D). Since each channel in an 802.9 TDM frame operates at 64 kbps while the ISDN BRI D-channel operates only at 16 kbps, the 802.9 D-channel will support both rates.


   0   1   2   3   4   5   6   7   8     ooo       N-1
 --------------------------------------- - - ----------
 |SYN|TDM|HMC|res| D |B1 |B2 |AC |      PAYLOAD       |
 |   |MTN|   |   |   |   |   |   |                    |
 --------------------------------------- - - ----------

 a) Default TDM frame format.

   0   1   2   3   4   5   6
 -----------------------------
 |SYN|TDM|HMC|res| D |B1 |B2 |
 |   |MTN|   |   |   |   |   |
 -------------------------------------------------------------- 
 |AC | 8           63 octets of Packet Payload Space       70 |
 --------------------------------------------------------------
 |AC | 72          63 octets of Packet Payload Space      134 |
 --------------------------------------------------------------
 |AC | 136         63 octets of Packet Payload Space      198 |
 --------------------------------------------------------------
 |AC | 200         63 octets of Packet Payload Space      262 |
 --------------------------------------------------------------
 |AC | 264         56 octets of Packet Payload Space    319 |
 ------------------------------------------------------------

 b) 20.48 Mbps TDM frame.

FIGURE 4. IEEE 802.9 TDM frame formats.



Figure 4a shows the default TDM frame structure, comprising the following fields:
  • Synchronization (SYN): Used to establish TDM frame synchronization between the ISTE and AU. The SYN field contains a 7-bit Frame Alignment Word that, when detected, indicates the first octet of the frame. (The eighth bit is reserved and currently unused.)

  • TDM Maintenance (TDM_MTN): Used to transmit local physical layer status and control information to the device at the other end of the link. This octet is controlled by the layer management entities at the two ends. Functions include loopback testing and parity checking.

  • Hybrid Multiplexer Control (HMC): An 802.9 ISLAN interface can support a variety of services that may require dynamic bandwidth allocation. ISDN-like call control mechanisms will be used on the D-channel for this purpose. The configuration of the bandwidth within the TDM frame, however, must use some procedure so that a given ISTE and AU are always in agreement about their use of the TDM channels. This field indicates the speed of the D-channel (16 or 64 kbps), the mode of the HMUX (0-3, as described above), and whether the exchange of this information is complete or not.

  • Reserved (RES): Reserved channel; use to be determined.

  • D: The 16- or 64-kbps D-channel. The D-channel may be restricted to conveying signaling information only. All information in this channel will be packetized according to the ISDN call control procedures defined in Recommendation Q.930.

  • B1 and B2: One octet from each of the two ISDN B-channels. The B-channels may be used for any ISDN bearer service and may be non-switched, packet switched, or circuit switched.

  • Access Control (AC): This field contains information related to the 802.9 MAC scheme for the P-channel, which is briefly described below.

  • Payload: The Payload field has two parts. The first octet is called the Service Identifier (SID) and indicates the format of the data to follow. Current SID options support use of an 802.9-specific frame format (described below) or LAPD. The remaining octets are called the Payload Information field and carry P- and/or C-channel data. C-channels will carry isochronous (time sensitive) information. Therefore, time slots within this field will usually be pre-allocated for the C-channels and extra time slots will be used to carry non-isochronous P-channel data.

The smallest supported TDM frame contains 64 octets; at 8000 frames per second, then, this yields a line rate of 4.096 Mbps. At rates above 4.096, the AC field may need to be periodically repeated to minimize buffering, as in the 20.48 Mbps TDM frame shown in Figure 4b.

MAC Frame Structure

P-channel data will be carried in an 802.9 MAC frame which, in turn, is transported in the Payload field of a TDM frame. Figure 5 shows the fields of the MAC frame, which are described below:

  • Length (LEN): A 2-octet field indicating the length of the MAC frame, excluding the Length and FCS fields. The maximum MAC frame size is 5,119 octets.

  • Frame Control (FC): A 1-octet field containing the priority of the frame. The priority is a 3-bit value from 0 (lowest) to 7 (highest). The remaining bits are reserved and set to 0.

  • Destination Address (DA) and Source Address (SA) fields: Specifies the address of the intended destination station(s) for this frame and the address of the station sending this frame, respectively. The address fields are 48 bits in length and conform to other IEEE 802 48-bit addresses. The first address bit transmitted in the DA field is called the individual/group (I/G) bit and indicates if this address specifies an individual station or a group of stations. In the SA field, the I/G-bit is set to 0 and ignored. The second bit transmitted is called the universal/local (U/L) bit and indicates whether the specified address is part of a locally-administered addressing plan (0) or administered by a central authority, such as the IEEE (1). The remaining 46 bits contain the actual station address. The 46-bit field yields roughly 64 trillion possible station addresses.

  • Information: This field contains up to 1500 octets of user data.

  • Frame Check Sequence (FCS): A 4-octet field containing the remainder from the CRC-32 calculation, used to detect bit errors in the SID and MAC frame.

 ------------------------------- - - -------------- 
 | LEN | FC  | DA  | SA  |    Information   | FCS |
 ------------------------------- - - -------------- 
    2     1     6     6                        4

FIGURE 5. P-channel MAC frame format.

MAC frames will, in all likelihood, be larger than a single Payload field and, therefore, 802.9 MAC frames will have to be fragmented so that they can be carried in multiple Payloads. A bit in the AC field indicates whether the following Payload contains the first fragment of a frame or not.

P-Channel Access Control

The 802.9 standard defines a point-to-point P-channel so that an ISTE can access LAN services (Figure 6). The bandwidth of each P-channel will vary according to the services offered by the individual ISTE. Furthermore, the bandwidth available for the operation of a given P-channel will depend upon how much of the payload fields reserved for P- and C-channels are dedicated to the isochronous C-channels.


 -------   -------   -------        ------- 
 |ISTE |   |ISTE |   |ISTE | o o o  |ISTE |
 |  1  |   |  2  |   |  3  |        |  n  |
 ---+---   ---+---   ---+---        ---+---
    |         |         |              |
    |         |         |              |
    P1        P2        P3   o o o     Pn
    |         |         |              |
    |         |         |              |
 ---+---------+---------+--------------+---
 |                                        |
 |        A C C E S S   U N I T           |
 |                                        |
 ------------------------------------------

FIGURE 6. Each ISTE has a point-to-point P-channel
connection to the AU. The bandwidth of the P-channels
will vary according to the needs of the individual ISTEs
and the total available bandwidth.


Access to the P-channel by the ISTE and AU is controlled by a scheme called the Request/Grant protocol. The Request/Grant protocol has the following general characteristics:
  • It is associated only with the transmission of 802.9 MAC frames on the P-channel.

  • The transmission of MAC frames from an ISTE to the AU is governed and controlled by the AU.

  • The transmission of MAC frames from the AU to an ISTE may or may not be governed and controlled by the intended ISTE receiver; if configured, the AU may send MAC frames to an ISTE whenever it is ready without waiting for permission.

In general, the Request/Grant protocol works as follows (Figure 7). The AC field of the TDM frame contains one GRANT-bit and 3 request bits, called REQ3, REQ2, and REQ1.


            ISTE                                ACCESS UNIT
    ----------------                           ----------------
               |                                  |
 PDU ready to  |                                  |
 be sent...    |                                  |
               |--- REQ-bit = 1 ----------------->|
               |                                  |
               |                                  |  Allocate buffer...
               |                                  |
               |                                  |  Ready to receive... 
               |<--------------- GRANT-bit = 1 ---|
               |                                  |
 Send PDU...   |                                  |
               |--- Payload = MAC frame --------->|
               |                                  |
               |--- Payload = MAC frame --------->|
               |              o                   |
               |              o                   |
               |              o                   |
               |--- Payload = MAC frame --------->|
               |                                  |
 Finished...   |                                  |
               |                                  |
               |                                  |
 

FIGURE 7. The 802.9 MAC Request/Grant protocol.


When an ISTE is ready to send a frame, it sets the appropriate REQ-bit that corresponds to the MAC frame's priority (in the FC field). Although there is no direct relationship between the request priority in the AC field of the TDM frame and the priority value in the FC field of the MAC frame, the standard recommends that frame priorities 6 and 7 map to REQ3 (high), 3 through 5 map to REQ2 (medium), and 0 though 2 map to REQ1 (low).

When the AU sees an incoming REQ-bit, it must ensure that adequate buffer space is available to accommodate P-channel MAC frames. When it is ready to receive a MAC frame, it sets the GRANT-bit to 1 in a TDM frame going back to the ISTE. Note that if buffers in the AU are available for each P-channel, all ISTEs could theoretically send 802.9 MAC frames simultaneously. Due to the bursty nature of data traffic from the ISTEs, however, AUs will probably be designed with fewer receive buffers than the number of P-channels; in this case, some ISTEs may incur some delay before receiving permission to transmit.

When the ISTE sees the GRANT-bit set, it may send one complete MAC frame. Recall that a single MAC frame will probably be sent in multiple Payload fields of 802.9 TDM frames.

This scenario would be reversed if the AU's transmissions are controlled by the ISTE.

Compatibility With Other Standards

One of the major additions to the current IEEE 802.9 specification are descriptions of how an ISLAN fits in with existing and emerging protocols. Some of these interworking issues are:

  • ITU TSS Recommendation Q.931 describes basic ISDN call control for the establishment, maintenance, and termination of connections. IEEE 802.9 describes a subset of Q.931, called Q.93x, that can be used for ISLAN call control purposes for connections on the B- and C-channels. Q.93x is very similar to draft Recommendation Q.93B, describing B-ISDN extensions to Q.931.

  • Public ATM services started to become available in the U.S. in late 1993. 802.9 also describes how the TDM frame can be mapped onto cells in an ATM network.

  • Many addressing schemes are currently used in today's local and wide area networks, including IEEE 802 addresses, ITU TSS Recommendation E.164 international ISDN numbers, X.121 data network identifiers, F.69 telex addresses, and ISO data country codes. All of these schemes are independent of each other using a different address format, coding scheme, and length. IEEE 802.9 describes interworking between these different plans.

The IEEE 802.9 standard also provides a detailed specification of managed objects for the definition of network and layer management, as well as security control for multimedia connections.

Status Of IEEE 802.9

Draft 20 (dated May 17, 1993) of the IEEE 802.9 standard was circulated for letter ballot in August 1993. It successfully passed the ballot stage and was established as an official IEEE standard in the fall of 1993.

Although no ISLAN products nor product announcements have yet appeared on the market, several companies actively participated in the IEEE 802.9 standards process and have expressed interest in developing such products, including AT&T Paradyne (Largo, FL), Ericsson (Anaheim, CA), Hitachi America (Brisbane, CA), IBM (Boca Raton, FL), LUXCOM (Fremont, CA), National Semiconductor (Santa Clara, CA), and NEC (Princeton, NJ). Stevens Institute of Technology (Hoboken, NJ) has announced that they will provide a beta test site for testing 802.9 equipment and applications. Their Advanced Telecommunications Institute (ATI), in fact, has also expressed an interest in developing a consortium to develop 802.9-based products.

It is important to note that the 802.9 standard and ISLAN products cannot stand alone since they are conceptually associated with ISDN and B-ISDN. The development of these products, then, will only succeed if there is concurrent deployment of ATM in the local and/or wide area backbone.

ACKNOWLEDGEMENTS: I would like to thank Professor Dhadesugoor R. Vaman, Director of the Advanced Telecommunications Institute at Stevens Institute of Technology and Chair of the IEEE 802.9 Committee, for his assistance and encouragement in the preparation of this article.

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