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In this paper, we describe a new protocol for framing asynchronous protocol data units (PDUs) over a general-purpose point-to-point communications channel. The generality of this protocol-we call it simple data link (SDL) protocol-allows it to be used to transport most PDU types, such as IP/IPX datagrams, 802.3 frames, and ATM cells. Its low implementation complexity makes it particularly suitable for high-speed transmission links such as SONET/SDH point-to-point links, SONET/SDH paths, 1 and wavelength links in an optical network. Point-to-point transmission links deliver the incoming information stream in a sequential and orderly fashion, which greatly simplifies frame boundary delineation and recovery procedures at the SDL layer.While the focus of this paper is on the primary function of SDL-to provide PDU framing and delineation-we have designed SDL to facilitate many traf-♦ A Simple Data Link Protocol for High-Speed Packet Networks Bharat T. Doshi, Subrahmanyam Dravida, Enrique J. Hernandez-Valencia, Wassim A. Matragi, M. Akber Qureshi, Jon Anderson, and James S. Manchester Many popular point-to-point data link protocols use the high-level data link control (HDLC) framing mechanism, which delineates protocol data units (PDUs) by means of a special bit pattern or flag. When such a flag occurs in the payload portion of a frame, an escape byte used to pad the transmitted byte stream enables the receiver to differentiate between a true framing flag and an occurrence of the flag pattern in the user information. The need to process each byte in the incoming byte stream to identify the flag pattern makes this frame delineation method increasingly more complex and expensive to implement as the interface speed increases. In addition, the byte stuffing operation performed when the flag pattern appears in the user information stream expands the carried traffic, interfering with quality of service (QoS) management procedures and making the link susceptible to malicious attacks. These factors limit the scalability and QoS management capabilities of the flagbased data link protocols, particularly at high link rates. The increasing popularity of the packet over SONET (POS) protocol stack makes it desirable to find alternative framing protocols. In this paper, we present a simple data link (SDL) protocol. Based on using a Length Indicator field and a header cyclic redundancy check (CRC), rather than a flag, to delineate frames, SDL is inherently scalable to high speeds and provides constant transmission overhead. We discuss the error control, delineation, and resynchronization mechanisms used by SDL, and we describe a link scrambler designed to protect against malicious attacks that attempt to generate very low bit transition density on the line. We also present additional link control functions designed to make SDL suitable for next-generation multiservice networks.86 Bell Labs Technical Journal x January-March 1999 fic engineering functions needed in the next-generation multiservice packet networks. In particular, w...

Asynchronous transfer mode (ATM) networks carry fixed-size cells within the nettivity to packet telephony. These applications and others are described in some detail later.In applications like the ones mentioned above, there are two primary reasons to transmit small packets across ATM networks:• When small native packets are generated away from the ATM network-that is, in a digital cellular mobile terminal-and the packet boundaries need to be recovered at the destination outside the ATM network.• When the bit rate of the native application is low and the requirement on the end-to-end delay prohibits accumulation of bits to fill an ATM cell before sending the cell out to its destination. In the latter case, small packets are generated even if the packetization is done at the ATM network edge. For these applications, partial filling of ATM cells when using existing AALs may cause unacceptable loss in bandwidth efficiency. This inefficiency is a concern especially when the total traffic demand requires relatively low-speed leased lines, which have high cost relative to bit rate. In many cases, the cost penalty may nullify many advantages offered by an ATM backbone. This necessitates a new AAL for carrying small packets.Our initial motivation for designing a new AAL for small packets was the wireless infrastructure application mentioned above. The solution was motivated by the fact that many native mode connections (for example, cellular voice calls) may share two end points of the ATM network. In such a case, the solution uses one ATM connection Panel 1. Abbreviations, Acronyms, and Terms AAL-asynchronous transfer mode adaptation layer ADSL-asymmetric digital subscriber line ATM-asynchronous transfer mode BER-bit error rate CBR-constant bit rate CDMA-code division multiple access CES-circuit emulation service CID-connection identification CPS-common part sublayer CRC-cyclical redundancy check DCS-digital cross-connect system DLCI-data link connection identifier DTMF-dual-tone multifrequency signaling HEC-header error check IETF-Internet Engineering Task Force IP-Internet protocol ITU-T-International Telecommunication UnionTelecommunication Standardization Sector LI-length indicator LLC-logical link connection MAN-metropolitan area network MSC-mobile switching center OA&M-operations, administration, and maintenance OSF-offset field PBX-private branch exchange PCS-personal communications services PDC-personal digital cellular PH-packet handler PHY-physical (typically used with "layer") POTS-"plain old telephone service" PPP-point-to-point protocol PRI-primary rate interface PSTN-public-switched telephone network PTI-payload type indicator RES-reserved RPM-radio port multiplexer RP-radio port SMAAL-small packet multiplexed asynchronous transfer mode adaptation layer SN-sequence number SS7-Signaling System 7 SSCF-service-specific convergence function SSCS-service-specific convergence sublayer STF-start field STM-synchronous transfer mode TA-terminal adapter TDM-time division multiplexing UU-user to user VCC-virtual chan...

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