Encyclopedia
`of Networking 85
`Telecommunications
`
`Covers emerging technologies:
`a|l-
`' al networks,
`bro
`nd access,
`wir
`computing, Q03,
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`cific
`
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`technologies: Mic
`Cisco<:’iB
`uniper‘?
`Nortel?Su " mi others
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`
`
`
` Addresses vendo-
`
`Certified Network Engineer and au
`of the e t—se||ing
`nic Editi lZisE:r:iR-NED
`Encyclopedia of Networking, Elec
`
`APL 1013
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`IPR2015—00369
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`
`McGraw-Hill
`
`Encyclopedia of Networking
`& Telecommunications
`
`Tom Sheldon
`
`Osborne/McGraw-I-Iill
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`

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`McGraw-I-Iill Encyclopedia of Networking 82: Telecommunications
`
`Copyright © 2001 by The McGraw-Hill Companies. All rights reserved. Printed in the
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`

`
`
`
`HDLC (lliglhleirel llata Link C-ontrol)
`
`581 —
`
`'--High-Lev_el Data Link Control)
`"LC is a bit—o1'iented, link layer protocol for the transrnission of data over synchronous
`_.orl<s. It is an ISO standard, but is a superset of IBM's SDLC (Synchronous Data Link Control)
`wfiocol. SDLC was the successful follow-up to the BISYNC communication protocol and was
`gmally introduced with IBM SNA (Systems Network Arcllitecture) products. Another name
`
`
`
`.\'..-{*1.H"A.‘r.
`
`
`
`LC is the widely accepted name for the protocol. There are some incompatibilities between
`'-'-:-and HDLC, depending on the vendor.
`_
`is bit oriented, meaning that the data is monitored bit by bit. Transmissions consist
`data without any special control codes. Information in the frame contains control and
`- commands, however. HDLC supports full-duplex transmission in which data is transmitted -
`
`Z
`_
`
`-
`
`-and control data links for X25, ISDN, and frame relay networks.
`-
`
`‘en an HDLC session is established, one station, called the primary station, is designated
`finage the flow of data. The other station (or stations) is designated as the secondary station. _
`__ary station issues cornntands, and the secondary stations issue responses. There
`possible connection methods, as shown in Figure H-1. The top two support either
`
`
`
`HDLC connection methods
`
`
`
`

`
`582
`
`Hlllfl (lligll-level Ilata link Control)
`
`point—to—point connections between two systems, or multipoint connections between a p '
`station and two or more secondary stations:
`
`I The normal mode is unbalanced because the secondary station can only transmit wfi‘
`permitted to do so by the primary station.
`
`I The asynchronous mode is also unbalanced, but the secondary station may initiate at
`transmission on its own.
`'
`
`I The asynchronous balanced mode is designed for point-to—point Connections betw _
`two computers over a duplex line. Each station can send commands and responses
`its own line and" receive commands and responses on the duplexed line. This is the
`used to connect stations to X25 packet-switched networks.
`
`_
`
`The HDLC frame defines the structure for delivering data and command/response
`between communicating systems. The frame is pictured in Figure H-2 and described here:
`
`u
`
`
`
`end of the HDLC frame. If any portion of‘ the data in the frame contains more
`1 hits, a zero-bit insertion technique inserts a 0 bit to ensure that data is not mistaken __
`a flag.
`.
`'-
`
`I The Address field generally contains the address of a secondary station. This field
`normally 8 bits, but extended addressing is possible for-multipoint connections than
`contain many different addresses. A broadcast address can also be inserted in the fiél
`to send messages to all stations in a multipoint connection.
`"
`
`I The Control field identifies the information contained in the frame as data, command
`or responses. Commands are sent by the primary station, and responses are sent by--7
`secondary station. The control information can acknowledge frames, request retransmissi
`of frames, or request "a suspension of transmission, as well as other commands
`and responses.
`'
`
`of flow
`
`Direction
`
`

`
`HIJML (Handheld Device Markup language)
`
`583
`
`A communication session starts by establishing connections between primary and
`secondary stations. The primary station transmits a special frame to either a single station
`or to multiple stations toinitiate a setup procedure. The secondary stations respond with
`information that is used for error and flow control during the session. When everything is
`set up, data transmission begins; and when data transmission ends, the primary station sends
`a frame to initiate a disconnection of the session.
`_ As mentioned, HDLC forms the basis for data link layer control in X25 packet-switching
`networks. A subset of HDLC is LAP~B (Link Access Procedure-Balanced), which is designed
`for‘ point—to-point connections, so the Address field is not necessary to identify the secondary
`stations. It provides the frame structure, error, and flow-control mechanisms for an asynchronous
`.balanced-mode session. Another subset of HDLC is the LAP-D (Link Access Procedure D—channel)
`protocol associated with ISDN (Integrated -Services Digital Network). The D channel is the signaling
`channel that controls the data flowing through the two B (bearer) channels. Think of the B channels
`as two separate phone lines and the D channel as the line that sets up the call.
`Two Internet RP-Cs are related to HDLC. These are RFC 2687, ”PPP in a Real-Time Oriented
`l_-lDLCl—like Framing,” (September 1999) and RFC 1662, “PPP in HDLC-like Framing,” (July 1994).
`
`'3'
`
`Related Entries Data Communication Concepts; Data Link Protocols; Framing in Data Transmissions;
`(Integrated Services Digital Network); LAP (Link Access Procedure); LLC (Logical Link Control);
`Point-to-Point Communications; PPP (Point-to—Point Protocol); SDLC (Synchronous Data Link Control);
`Serial Communication and Interfaces; Synchronous Communications; and X25
`
`
`
`‘inktionary!——Torn Sheldon’s Encyclopedia of
`Ntitworking updates
`Tutorial at RAD Data Communications
`
`'Fail'hurSt's I-IDLC protocol tutorial
`
`'- Jl3Yl1Chr0nous HDLC FAQ
`
`http: / /www.linktionary.com/h/hd1c.html
`
`http: / /www.rad.com / networks /' 1994/hdld
`hdlc.htIn
`
`http:/ /www.erg.abdn.ac.uk/users/gorry/course/
`dl—pages{hdlc.htrnl
`
`http: / [ftp.uni-erlangen.de/pub/other/
`Documents/ISO/async-HDLC
`
`-'_§,l= —__Handhe|d Device Markup Language)
`Was developed by Unwired Planet (now Phone.com) as a markup language for
`l’9__C_l<e.t-sized devices such as cellular phones and two-way pagers that conform to Internet
`t5_‘11d_fl_1‘ds and protocols, and that are constrained to a few lines of display, a limited keypad,
`little memory. It is a markup language similar to HTML (Hypertext Markup Language)
`‘mcept but designed for developing applications and services for cellular devices» It is
`- -_-.Pal'ible with all Web servers, uses little memory, makes efficient use of Wireless T1€tW01'1<
`aflfimissions, and supports integrated phone features like voice. More iI1f0rITlafi0I1 is at the
`0Dn:
`-
`ryl Web site.
`
`
`
`-- mm.---
`
`

`
`1006
`
`PPP (Point-to—Point Prntolcol)
`
`complete, the user's computer is an extension of the Internet and the user's serial port and
`modem have the same functionality as a network interface card connected to the service
`provider's network.
`'
`PPP provides full-duplex bidirectional links that deliver packets in order. This is an .
`important point. The PPP link is point to point as the name implies. It is a circuit dedica
`the user over which packets flow from the user’s computer to the service provider's route;-_-:
`provides a flow of frames into which datagrams are encapsulated and sent across the link;
`can think of boxcars crossing the PPP link. PPP also supports multiplexing of different
`layers protocols, such as TCP/ll’, NetBEUI, and IPX, on the same link.
`Most PPP'i1nplementation.s use framing derived from HDLC (High—level Data Link
`Protocol) as described in RFC 1662 (PPP in HDLC-like Framing, July 1994). This framing
`method is used over asynchronous and synchronous lines using serial communication
`protocols such as El.A—232—E, EIA—422, and EIA-423, as well as CCITI‘ V24 and V.35. But
`other framing mechanisms are supported, as described in the following RFCs. These
`methods are typically used to support router—to-router links in enterprise and Internet
`backbone networks:
`
`1
`
`'
`
`
`
`I RFC 2363 (PPP over FUNI, July 1998) describes how PPP can use ATM FUNI (Fram
`User Network Interface) on point-to-point ATM connections.
`'
`
`I RFC 2516 (A Method for Transmitting PPP Over Ethernet, February 1999) describes
`how to build PPP sessions and encapsulate PPP packets over Ethernet.
`
`I RFC 2615 (PPP over SONET/SDH, June 1999) describes "IP—over-PPP-over-SONET/ "H
`SDH,” a technique that is used to send IP packets directly over optical networks
`"
`without using ATM. See “PoS (Packet over SONET)” for more information.
`
`'
`
`As mentioned, PPP’s standard framing method is derived from HDLC, which itself is
`derived from IBM's SDLC (Synchronous Data Link Protocol). The frame format is pictured in__
`Figure P—9 and described here:
`
`
`
`I Delimiters Mark the beginning and ending of the frame
`
`I Address Holds the destination address
`
`Control Holds the sequence number to ensure proper handling
`
`._
`
`Protocol
`
`Identifies the protocol contained in the frame (IP, IPX, AppleTalk, and so on)
`
`Data Contains the data, which can Vary in length
`
`Frame Check Sequence Calculates a check sum used for error checking
`
`I Figure P-9.
`
`The Point-toPoint Protocol frame format