Brought to you by EarthWeb
IT Library Logo

Click Here!
Click Here!


Search the site:
 
EXPERT SEARCH -----
Programming Languages
Databases
Security
Web Services
Network Services
Middleware
Components
Operating Systems
User Interfaces
Groupware & Collaboration
Content Management
Productivity Applications
Hardware
Fun & Games

EarthWeb Direct EarthWeb Direct Fatbrain Auctions Support Source Answers

EarthWeb sites
Crossnodes
Datamation
Developer.com
DICE
EarthWeb.com
EarthWeb Direct
ERP Hub
Gamelan
GoCertify.com
HTMLGoodies
Intranet Journal
IT Knowledge
IT Library
JavaGoodies
JARS
JavaScripts.com
open source IT
RoadCoders
Y2K Info

Previous Table of Contents Next


IEEE 802.3 recommends a generic media access control frame format for use in data transmit and receive tasks. All frames are preceded by a preamble.

The media access control frame structure is flexible enough to accommodate baseband and broadband implementations. Both baseband and broadband implementations require addition of a preamble to the frame during transmission. In the broadband implementation, the frame is encapsulated with a preamble and postamble. The following various frame fields defined in IEEE 802.3 are shown in Exhibit 1-1-14.


Exhibit 1-1-14.  Frame Format for IEEE 802.3 Standard

  Baseband preamble. A 56-bit pattern used by the physical layer signaling circuitry to establish bit synchronization and identify the first bit of the frame.
  Broadband preamble. The length of the preamble is 45 bits. The first 20 bits are used by the recipient to establish synchronization. These synch bits are followed by a 2-bit unscramble mode delimiter field. The final 23 bits of the preamble constitute the seed.
  Broadband postamble. The postamble is a 23-bit pattern that follows the last bit of the frame check sequence and functions as the broadband end-of-frame delimiter.
  Start frame delimiter (SFD). This 8-bit 01111110 sequence indicates the start of a frame.
  Destination address (DA). This identifies the stations that are to receive the frame. The bit pattern indicates either a unique physical address, a multicast-group address, or a global address. The choice of a 2- or 6-octet address is an implementation decision and must be the same at any given time for all stations on a particular LAN.
  The source address (SA). This specifies the station that sent the frame. The size of this field must be the same as that of the DA field.
  Length. A 16-bit field that specifies the number of logical link control bytes that follow in the data field.
  The logical link control data. This field is prepared at the logical link control level, and its size must be in octet units.
  Pad field. Added if necessary; a sequence of bits is added o the logical link control data field to ensure that the frame is long enough for proper collision detection operation. The minimum and maximum sizes for the data field are a function of the maximum frame size and address size definitions for a given implementation.
  The frame check sequence (FCS). Contains a 32-bit cyclic redundancy check value. This value is calculated based on the contents of all fields up to but not including the FCS.

The IEEE 802.3 standard specifies a CMSA/CD media access control operation at the media access control sublayer. On receiving a request for transmission, the transmit data encapsulation component constructs a protocol data unit frame using logical link control data. The transmit media access management samples the communication channel. When the medium becomes clear, transmission begins and the logical link control is informed. In the case of a collision, the transmit media access management transmits a jam signal to ensure that all transmitting stations have detected the collision. The transmission is terminated and retry begins after a back-off interval.

When a station enters the data receive mode, the physical layer signaling interface detects an incoming frame and discards the leading synch bits, the preamble, and the start frame delimiter. The physical layer signaling then forwards the data to the receive data decapsulation component. This process continues until the carrier-sense signal goes off. Then, the receive data decapsulation component evaluates the destination address and checks the data for transmission errors. If the address is valid and data is error free, the frame is forwarded to the logical link control; frames with invalid addresses or data transmission errors are discarded or passed to network management for further processing.

The transmit and receive media management activities are supported by the physical layer’s three components: the physical layer signaling (PLS), attachment unit interface (AUI), and physical medium attachment (PMA).

The physical layer signaling shields the media access control layer from the intricacies of establishing, maintaining, and tearing down the serial communications pipe. This interface supports transmit-receive bit streams and channel-allocation and contention-resolution tasks. The physical signaling layer uses five primitives. Those can be grouped according to their functions as shown in Exhibit 1-1-15.


Exhibit 1-1-15.  Physical Signaling Layer Primitives

The attachment unit interface includes specifications for cable connectors and transmission circuitry used to interconnect the physical layer signaling and media access unit in compliance with the following characteristics:

  The AUI must be capable of driving 50 m of cable.
  The AUI must permit data terminal equipment to test the AUI, AUI cable, medium attachment unit (MAU), and medium itself.
  The AUI must support the MAU for baseband coaxial cable, broadband coaxial cable, and baseband optical fiber.

The MAU is the portion of the physical layer between the medium-dependent interface and the AUI. It links the medium to the connector cable and contains the electronic circuitry that sends, receives, and manages the encoded signals impressed on and recovered from the trunk coaxial cable.

The medium-dependent interface is the mechanical and electrical interface between the trunk cable medium and the medium attachment unit.

The physical layer options for IEEE 802.3 are listed in Exhibit 1-1-16. The recommendation calls for the media access control layer to use the parameter values shown in Exhibit 1-1-17 and the typical specifications given in Exhibit 1-1-18.


Exhibit 1-1-16.  IEEE 802.3 Physical Layer Options


Exhibit 1-1-17.  IEEE 802.3 Recommended Media Access Control


Exhibit 1-1-18.  Typical Specifications for IEEE 802.3

For baseband implementation the IEEE 802.3 standard uses a 10M-bps channel with a minimum frame of 512 bits. The network can use 500-m segments for coaxial cable and 1-km segments for optical fiber. The distance between the cable tap and the station should not exceed 50 m.


Previous Table of Contents Next

footer nav
Use of this site is subject certain Terms & Conditions.
Copyright (c) 1996-1999 EarthWeb, Inc.. All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of EarthWeb is prohibited. Please read our privacy policy for details.