Frequently Asked Questions (FAQ)

1:

The number 53 may be the first time a binary number is not used in networking, so how did ATM cells come to be 53 bytes in length?

A1:

During the standardization process, a conflict arose within the CCITT as to the payload size within an ATM cell. The U.S. wanted 64-byte payloads; the Europeans and Japanese wanted 32 payloads. The 48-byte payload plus the 5-byte header was the compromise.

The 5-byte header was chosen because 10 percent of payload (5 bytes) was perceived as the upper bound on the acceptable overhead.

2:

What is ATM LANE?

A2:

ATM LAN Emulation (LANE) enables an ATM network to be used as a LAN backbone for hubs, bridges, and switches. ATM LANE enables LAN hosts connected to "legacy" LANs to communicate though a LAN-to-ATM hub/bridge/switch with an ATM-attached device (a file server, for example) without requiring the traffic to pass through a router. The purpose of LANE is to enable common protocols, such as IP, IPX, AppleTalk, and DECnet, to ride over an ATM backbone.

LANE does not replace routers or routing, but provides a LAN switching solution in the wiring closets of large LANS.

3:

What is "traffic shaping"?

A3:

Traffic shaping is forcing traffic to conform to a certain behavior, often in one of two scenarios:

  • Worst case An application will generate 10 Mbps of data for a maximum burst of 5 seconds; for example, streaming multimedia, bulk file transfer.

  • Worst case plus average case An application will generate 10 Mbps of data for a maximum burst of 5 seconds and its average over any 10 second interval will be no more than 5 Mbps; for example, several users accessing a corporate intranet simultaneously.

Traffic shaping can be performed by any one (or more) of the following three functions:

  • Acceptance Each ATM switch is configured with the option of accepting a virtual circuit request based on the traffic parameters declared by the customer/end-user.

  • Policing Each ATM switch ensures that the customer/end-user equipment keeps its promises and does not consume more bandwidth than agreed upon at initial connection setup.

  • Traffic Shaping Occurs in the CPE using information about the policing functions in order to change the traffic characteristics of the traffic stream to get the lowest CLR possible.

For example, an IP router attached to an ATM network might delay some cells in order to reduce the peak rate and rate variance without affecting throughput. An MPEG codec that was operating in a situation where delay wasn't a problem might operate in a CBR mode.

4:

What is the difference between QoS and CoS?

A4:

QoS refers to the network mechanisms that determine which packets have priority. CoS refers to feature sets, or groups of services, that are assigned to users based on company policy. If a feature set includes priority transmission, CoS is implemented in QoS functions within the network routers and switches.



Network Sales and Services Handbook
Network Sales and Services Handbook (Cisco Press Networking Technology)
ISBN: 1587050900
EAN: 2147483647
Year: 2005
Pages: 269

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