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Page 76 / 344 Scroll up to view Page 71 - 75
Chapter 5 Broadband
P-2601HN(L)-F1 Series User’s Guide
76
5.3
Technical Reference
This section provides some technical background information about the topics
covered in this chapter.
5.3.1
Encapsulation
Be sure to use the encapsulation method required by your ISP. The ZyXEL Device
supports the following methods.
5.3.1.1
PPP over Ethernet
The ZyXEL Device 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 ZyXEL Device (rather than individual
computers), the computers on the LAN do not need PPPoE software installed,
Sustain Cell
Rate
The Sustain Cell Rate (SCR) sets the average cell rate (long-term) that
can be transmitted. Type the SCR, which must be less than the PCR.
Note that system default is 0 cells/sec.
This field is available only when you select
Non Realtime VBR
or
Realtime VBR
.
Maximum Burst
Size
Maximum Burst Size (MBS) refers to the maximum number of cells that
can be sent at the peak rate. Type the MBS, which is less than 65535.
This field is available only when you select
Non Realtime VBR
or
Realtime VBR
.
Apply
Click
Apply
to save your changes back to the ZyXEL Device.
Back
Click
Back
to exit this screen without saving.
Table 7
Broadband: Add/Edit: Bridge Mode (continued)
LABEL
DESCRIPTION
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Chapter 5 Broadband
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77
since the ZyXEL Device does that part of the task. Furthermore, with NAT, all of
the LANs’ computers will have access.
5.3.1.2
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 ZyXEL Device
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.
5.3.2
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.
5.3.3
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.
5.3.4
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.
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Chapter 5 Broadband
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78
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, then you only need to fill in the
IP
Address
field and not the
Gateway IP Address
field.
5.3.5
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.
5.4
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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Chapter 5 Broadband
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The following figure illustrates the relationship between PCR, SCR and MBS.
Figure 16
Example of Traffic Shaping
5.4.1
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.
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.

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