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Page 116 / 424 Scroll up to view Page 111 - 115
Chapter 5 Broadband
P-2612HNU-Fx User’s Guide
116
Multiplexing
There are two conventions to identify what protocols the virtual circuit (VC) is
carrying. Be sure to use the multiplexing method required by your ISP.
VC-based Multiplexing
In this case, by prior mutual agreement, each protocol is assigned to a specific
virtual circuit; for example, VC1 carries IP, etc. VC-based multiplexing may be
dominant in environments where dynamic creation of large numbers of ATM VCs is
fast and economical.
LLC-based Multiplexing
In this case one VC carries multiple protocols with protocol identifying information
being contained in each packet header. Despite the extra bandwidth and
processing overhead, this method may be advantageous if it is not practical to
have a separate VC for each carried protocol, for example, if charging heavily
depends on the number of simultaneous VCs.
Traffic Shaping
Traffic Shaping is an agreement between the carrier and the subscriber to regulate
the average rate and fluctuations of data transmission over an ATM network. This
agreement helps eliminate congestion, which is important for transmission of real
time data such as audio and video connections.
Peak Cell Rate (PCR) is the maximum rate at which the sender can send cells. This
parameter may be lower (but not higher) than the maximum line speed. 1 ATM
cell is 53 bytes (424 bits), so a maximum speed of 832Kbps gives a maximum
PCR of 1962 cells/sec. This rate is not guaranteed because it is dependent on the
line speed.
Sustained Cell Rate (SCR) is the mean cell rate of each bursty traffic source. It
specifies the maximum average rate at which cells can be sent over the virtual
connection. SCR may not be greater than the PCR.
Maximum Burst Size (MBS) is the maximum number of cells that can be sent at
the PCR. After MBS is reached, cell rates fall below SCR until cell rate averages to
the SCR again. At this time, more cells (up to the MBS) can be sent at the PCR
again.
If the PCR, SCR or MBS is set to the default of "0", the system will assign a
maximum value that correlates to your upstream line rate.
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The following figure illustrates the relationship between PCR, SCR and MBS.
Figure 21
Example of Traffic Shaping
ATM Traffic Classes
These are the basic ATM traffic classes defined by the ATM Forum Traffic
Management 4.0 Specification.
Constant Bit Rate (CBR)
Constant Bit Rate (CBR) provides fixed bandwidth that is always available even if
no data is being sent. CBR traffic is generally time-sensitive (doesn't tolerate
delay). CBR is used for connections that continuously require a specific amount of
bandwidth. A PCR is specified and if traffic exceeds this rate, cells may be
dropped. Examples of connections that need CBR would be high-resolution video
and voice.
Variable Bit Rate (VBR)
The Variable Bit Rate (VBR) ATM traffic class is used with bursty connections.
Connections that use the Variable Bit Rate (VBR) traffic class can be grouped into
real time (VBR-RT) or non-real time (VBR-nRT) connections.
The VBR-RT (real-time Variable Bit Rate) type is used with bursty connections that
require closely controlled delay and delay variation. It also provides a fixed
amount of bandwidth (a PCR is 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.
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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).
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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 number of 4,096
VLANs. Note that user priority and VLAN ID are independent of each other. A
frame with VID (VLAN Identifier) of null (0) is called a priority frame, meaning that
only the priority level is significant and the default VID of the ingress port is given
as the VID of the frame. Of the 4096 possible VIDs, a VID of 0 is used to identify
priority frames and value 4095 (FFF) is reserved, so the maximum possible VLAN
configurations are 4,094.
Multicast
IP packets are transmitted in either one of two ways - Unicast (1 sender - 1
recipient) or Broadcast (1 sender - everybody on the network). Multicast delivers
IP packets to a group of hosts on the network - not everybody and not just 1.
Internet Group Multicast Protocol (IGMP) is a network-layer protocol used to
establish membership in a Multicast group - it is not used to carry user data. IGMP
version 2 (RFC 2236) is an improvement over version 1 (RFC 1112) but IGMP
version 1 is still in wide use. If you would like to read more detailed information
about interoperability between IGMP version 2 and version 1, please see sections
4 and 5 of RFC 2236. The class D IP address is used to identify host groups and
can be in the range 224.0.0.0 to 239.255.255.255. The address 224.0.0.0 is not
assigned to any group and is used by IP multicast computers. The address
224.0.0.1 is used for query messages and is assigned to the permanent group of
all IP hosts (including gateways). All hosts must join the 224.0.0.1 group in order
to participate in IGMP. The address 224.0.0.2 is assigned to the multicast routers
group.
At start up, the ZyXEL Device queries all directly connected networks to gather
group membership. After that, the ZyXEL Device periodically updates this
information.
DNS Server Address Assignment
Use Domain Name System (DNS) to map a domain name to its corresponding IP
address and vice versa, for instance, the IP address of www.zyxel.com is
204.217.0.2. The DNS server is extremely important because without it, you must
know the IP address of a computer before you can access it.
The ZyXEL Device can get the DNS server addresses in the following ways.
TPID
2 Bytes
User Priority
3 Bits
CFI
1 Bit
VLAN ID
12 Bits
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1
The ISP tells you the DNS server addresses, usually in the form of an information
sheet, when you sign up. If your ISP gives you DNS server addresses, manually
enter them in the DNS server fields.
2
If your ISP dynamically assigns the DNS server IP addresses (along with the
ZyXEL Device’s WAN IP address), set the DNS server fields to get the DNS server
address from the ISP.

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