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Page 86 / 320 Scroll up to view Page 81 - 85
Chapter 6 Broadband
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86
6.4
WAN Technical Reference
This section provides some technical background information about the topics covered in this
chapter.
6.4.1
Encapsulation
Be sure to use the encapsulation method required by your ISP. The AMG1302/AMG1202-TSeries
supports the following methods.
6.4.1.1
ENET ENCAP
The MAC Encapsulated Routing Link Protocol (ENET ENCAP) is only implemented with the IP
network protocol. IP packets are routed between the Ethernet interface and the WAN interface and
then formatted so that they can be understood in a bridged environment. For instance, it
encapsulates routed Ethernet frames into bridged ATM cells. ENET ENCAP requires that you specify
a gateway IP address in the
Gateway IP Address
field in the wizard or WAN screen. You can get
this information from your ISP.
6.4.1.2
PPP over Ethernet
The AMG1302/AMG1202-TSeries supports PPPoE (Point-to-Point Protocol over Ethernet). PPPoE is
an IETF Draft standard (RFC 2516) specifying how a personal computer (PC) interacts with a
broadband modem (DSL, cable, wireless, etc.) connection. The PPPoE option is for a dial-up
connection using PPPoE.
For the service provider, PPPoE offers an access and authentication method that works with existing
access control systems (for example RADIUS).
One of the benefits of PPPoE is the ability to let you access one of multiple network services, a
function known as dynamic service selection. This enables the service provider to easily create and
offer new IP services for individuals.
Operationally, PPPoE saves significant effort for both you and the ISP or carrier, as it requires no
specific configuration of the broadband modem at the customer site.
By implementing PPPoE directly on the AMG1302/AMG1202-TSeries (rather than individual
computers), the computers on the LAN do not need PPPoE software installed, since the AMG1302/
MTU
The Maximum Transmission Unit (MTU) defines the size of the largest packet allowed on an
interface or connection. Enter the MTU in this field.
For ENET ENCAP, the MTU value is 1500.
For PPPoE, the MTU value is 1492.
For PPPoA and RFC, the MTU is 100-1500.
Apply
Click this to save your changes.
Cancel
Click this to restore your previously saved settings.
Table 15
Network Setting > Broadband > More Connections: Edit: Advanced Setup (continued)
LABEL
DESCRIPTION
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AMG1202-TSeries does that part of the task. Furthermore, with NAT, all of the LANs’ computers will
have access.
6.4.1.3
PPPoA
PPPoA stands for Point to Point Protocol over ATM Adaptation Layer 5 (AAL5). A PPPoA connection
functions like a dial-up Internet connection. The AMG1302/AMG1202-TSeries encapsulates the PPP
session based on RFC1483 and sends it through an ATM PVC (Permanent Virtual Circuit) to the
Internet Service Provider’s (ISP) DSLAM (Digital Subscriber Line (DSL) Access Multiplexer). Please
refer to RFC 2364 for more information on PPPoA. Refer to RFC 1661 for more information on PPP.
6.4.1.4
RFC 1483
RFC 1483 describes two methods for Multiprotocol Encapsulation over ATM Adaptation Layer 5
(AAL5). The first method allows multiplexing of multiple protocols over a single ATM virtual circuit
(LLC-based multiplexing) and the second method assumes that each protocol is carried over a
separate ATM virtual circuit (VC-based multiplexing). Please refer to RFC 1483 for more detailed
information.
6.4.2
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.
6.4.3
VPI and VCI
Be sure to use the correct Virtual Path Identifier (VPI) and Virtual Channel Identifier (VCI) numbers
assigned to you. The valid range for the VPI is 0 to 255 and for the VCI is 32 to 65535 (0 to 31 is
reserved for local management of ATM traffic). Please see the appendix for more information.
6.4.4
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 ENET ENCAP gateway.
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IP Assignment with PPPoA or PPPoE Encapsulation
If you have a dynamic IP, then the
IP Address
and
Gateway IP Address
fields are not applicable
(N/A). If you have a
Static IP Address
assigned by your ISP, then they should also assign you a
Subnet Mask
and a
Gateway IP Address
.
IP Assignment with RFC 1483 Encapsulation
In this case the IP address assignment must be static.
IP Assignment with ENET ENCAP Encapsulation
In this case you can have either a static or dynamic IP. For a static IP you must fill in all the
IP
Address
and
Gateway IP Address
fields as supplied by your ISP. However for a dynamic IP, the
AMG1302/AMG1202-TSeries acts as a DHCP client on the WAN port and so the
IP Address
and
Gateway IP Address
fields are not applicable (N/A) as the DHCP server assigns them to the
AMG1302/AMG1202-TSeries.
6.4.5
Nailed-Up Connection (PPP)
A nailed-up connection is a dial-up line where the connection is always up regardless of traffic
demand. The AMG1302/AMG1202-TSeries does two things when you specify a nailed-up
connection. The first is that idle timeout is disabled. The second is that the AMG1302/AMG1202-
TSeries will try to bring up the connection when turned on and whenever the connection is down. A
nailed-up connection can be very expensive for obvious reasons.
Do not specify a nailed-up connection unless your telephone company offers flat-rate service or you
need a constant connection and the cost is of no concern.
6.4.6
NAT
NAT (Network Address Translation - NAT, RFC 1631) is the translation of the IP address of a host in
a packet, for example, the source address of an outgoing packet, used within one network to a
different IP address known within another network.
6.5
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.
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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 25
Example of Traffic Shaping
6.5.1
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.
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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.

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