I refer to difference between CSMA/CD vs 802.7

Discussion in 'Computing and Networks' started by lse123, May 18, 2010.

  1. lse123

    Thread Starter Active Member

    Oct 17, 2006
    I refer to difference between CSMA/CD vs 802.7 ? What CSMA/CD actually RESTRICTS,...? Means:Same time PCs can not send packects[IN THE SAME cables eg bus topology] ? Or in case except we use SWITCHES between PCs?
  2. dhudes

    New Member

    May 27, 2010
    CSMA/CD is Carrier Sense Multiple Access with Collision Detection.
    Ethernet, IEEE 802.3, is a CSMA/CD network. In half-duplex mode, all nodes on a segment obey the CD protocol: instead of waiting their turn, they all try to talk whenever they are ready and the wire seems available (carrier sense). Nodes listen to their transmission and if they detect a collision they stop and run an exponential backoff algorithm (omitting details). Note that Ethernet uses unshielded twisted pair (UTP), not co-axial, cable though once upon a time 10 megabit did use Thicknet. The Thinnet cable was popular for a long time then came hubs and 10/100/1000baseT. Switches and hubs are different layer of the stack (2 vs 1) but both utilize a star topology at the physical level.

    IEEE 802.7 is dead. http://standards.ieee.org/reading/ieee/std_public/new_desc/lanman/withdrawn/802.7-1989.html

    Here's the abstract:
    Abstract: IEEE 802.7-1989, IEEE Recommended Practices for Broadband Local Area Networks (LANs) specifies the design, installation, and test parameters for a broadband cable medium. The medium supports the communication of 802.3b, 802.4, video, and narrow-band radio frequency (RF) modem devices. The broadband bus topology consists of amplifiers, coaxial cable, and directional couplers that create a full duplex directional medium. Inbound signals flow from user outlet transmitters to a central headend location. The headend processes the signals and then transmits the signals via an outbound path to all the user outlets. The medium utilizes frequency division multiplexing (FDM), which supports the simultaneous coexistence of services. Each service is assigned a unique inbound and outbound channel frequency.

    Clearly the purpose of 802.7 is a MAN broadband situation rather than a LAN. It's appropriate comparison is 802.14 and DOCSIS rather than Ethernet. Neither 802.7 nor 802.14 use CDMA with or without CD. Each user has its own channel using Frequency-Division Multiplexing.