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Chapter 5 Broadband
VMG8324-B10A / VMG8324-B30A Series User’s Guide
66
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.
The following figure illustrates the relationship between PCR, SCR and MBS.
Figure 32
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
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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.
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.
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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
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 Device queries all directly connected networks to gather group membership. After
that, the 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 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 Device’s WAN IP
address), set the DNS server fields to get the DNS server address from the ISP.
IPv6 Addressing
The 128-bit IPv6 address is written as eight 16-bit hexadecimal blocks separated by colons (:). This
is an example IPv6 address
2001:0db8:1a2b:0015:0000:0000:1a2f:0000
.
IPv6 addresses can be abbreviated in two ways:
•
Leading zeros in a block can be omitted. So
2001:0db8:1a2b:0015:0000:0000:1a2f:0000
can
be written as
2001:db8:1a2b:15:0:0:1a2f:0
.
•
Any number of consecutive blocks of zeros can be replaced by a double colon. A double colon can
only appear once in an IPv6 address. So
2001:0db8:0000:0000:1a2f:0000:0000:0015
can be
written as
2001:0db8::1a2f:0000:0000:0015
,
2001:0db8:0000:0000:1a2f::0015
,
2001:db8::1a2f:0:0:15
or
2001:db8:0:0:1a2f::15
.
IPv6 Prefix and Prefix Length
Similar to an IPv4 subnet mask, IPv6 uses an address prefix to represent the network address. An
IPv6 prefix length specifies how many most significant bits (start from the left) in the address
compose the network address. The prefix length is written as “/x” where x is a number. For
example,
2001:db8:1a2b:15::1a2f:0/32
means that the first 32 bits (
2001:db8
) is the subnet prefix.
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