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Chapter 6 Broadband
specified) but is only available when data is being sent. An example of an VBR-RT connection would
be video conferencing. Video conferencing requires real-time data transfers and the bandwidth
requirement varies in proportion to the video image's changing dynamics.
The VBR-nRT (non real-time Variable Bit Rate) type is used with bursty connections that do not
require closely controlled delay and delay variation. It is commonly used for "bursty" traffic typical
on LANs. PCR and MBS define the burst levels, SCR defines the minimum level. An example of an
VBR-nRT connection would be non-time sensitive data file transfers.
Unspecified Bit Rate (UBR)
The Unspecified Bit Rate (UBR) ATM traffic class is for bursty data transfers. However, UBR doesn't
guarantee any bandwidth and only delivers traffic when the network has spare bandwidth. An
example application is background file transfer.
IP Address Assignment
A static IP is a fixed IP that your ISP gives you. A dynamic IP is not fixed; the ISP assigns you a
different one each time. The Single User Account feature can be enabled or disabled if you have
either a dynamic or static IP. However the encapsulation method assigned influences your choices
for IP address and default gateway.
Introduction to VLANs
A Virtual Local Area Network (VLAN) allows a physical network to be partitioned into multiple logical
networks. Devices on a logical network belong to one group. A device can belong to more than one
group. With VLAN, a device cannot directly talk to or hear from devices that are not in the same
group(s); the traffic must first go through a router.
In Multi-Tenant Unit (MTU) applications, VLAN is vital in providing isolation and security among the
subscribers. When properly configured, VLAN prevents one subscriber from accessing the network
resources of another on the same LAN, thus a user will not see the printers and hard disks of
another user in the same building.
VLAN also increases network performance by limiting broadcasts to a smaller and more
manageable logical broadcast domain. In traditional switched environments, all broadcast packets
go to each and every individual port. With VLAN, all broadcasts are confined to a specific broadcast
domain.
Introduction to IEEE 802.1Q Tagged VLAN
A tagged VLAN uses an explicit tag (VLAN ID) in the MAC header to identify the VLAN membership
of a frame across bridges - they are not confined to the switch on which they were created. The
VLANs can be created statically by hand or dynamically through GVRP. The VLAN ID associates a
frame with a specific VLAN and provides the information that switches need to process the frame
across the network. A tagged frame is four bytes longer than an untagged frame and contains two
bytes of TPID (Tag Protocol Identifier), residing within the type/length field of the Ethernet frame)
and two bytes of TCI (Tag Control Information), starts after the source address field of the Ethernet
frame).
The CFI (Canonical Format Indicator) is a single-bit flag, always set to zero for Ethernet switches. If
a frame received at an Ethernet port has a CFI set to 1, then that frame should not be forwarded as
it is to an untagged port. The remaining twelve bits define the VLAN ID, giving a possible maximum
VMG1312-Bx0D User's Guide
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