Section 3 Internet Based Connections (Xdsl, Cable, Etc.) - NEC Sl2100 Networking Manual

SL2100
• IP Precedence - Layer 3 QoS:
Allows you to specify the class of service for a packet. You use the three precedence bits in the IPv4
header type of service (ToS) field for this purpose. Using the ToS bits, you can define up to six
classes of service. Other devices configured throughout the network can then use these bits to
determine how to treat the packet in regard to the type of service to grant it. These other QoS
features can assign appropriate traffic-handling policies including congestion management and
bandwidth allocation. By setting IP Precedence levels on incoming traffic and using them in
combination with QoS queuing features, you can create differentiated service. (Refer to Program
84-10 : ToS Setup on page 5-24
• Differentiated service (Diffserv) - Layer 3 QoS:
Provides services differentiated on performance using weighted priority queuing. DiffServ requires
that edge routers classify traffic flows into a member from a set of categories based on the TCP/IP
header fields in what is called a micro flow. Because the Diffserv is present in every packet header,
each node can provide differentiated services per-hop. Refer to Program
page 5-24 for detailed programming information.
S 3 INTERNET BASED CONNECTIONS (XDSL, CABLE,
ECTION
ETC.)
Internet-based connections are becoming increasingly popular. This is mainly due to the speed and
cost of xDSL and cable modem connections. For data applications, these types of connection are
generally acceptable. For Voice over IP applications several issues should be taken into consideration.
Asymmetric Data Rates
On many Internet based connections, there are different data rates for upstream and downstream. For
example 1Mbps down and 256Kbps up. This works well for Internet access, as generally you
download files from the Internet to your PC and transmit less information in the other direction. For
VoIP, speech uses the same amount of bandwidth in both directions, which means that the amount of
simultaneous calls can not exceed the amount of "upstream" bandwidth available.
Contention
Most Internet based connections specify a contention ratio. This is typically 50:1 for home users or
20:1 for business users. This specifies the number of users subscribed to a single connection to the
Internet Service Provider (ISP). This indicates how many users share the bandwidth with other users
on the Internet, which means that the speeds that you are quoted are not necessarily accurate – you
receive less than these figures.
It is unlikely that all subscribers are using a connection at the same time, so these figures are not quite as
bad as they first seem.
Network Address Translation (NAT)
Usually, the equipment that your ISP provides (cable modem, ADSL router, etc.) uses Network
Address Translation. This allows several devices to share one public IP address. The issues relating
to the use of NAT are outlined in Firewalls and NAT below.
VPN
Due to the use of NAT, and non-routable IP addressing, it may be necessary to implement a VPN
solution. This is outlined in VPN Tunneling below. (Refer to
Tunnelling on page 6-6.)
QoS
As discussed earlier, it is essential to have some form of Quality of Service implemented. With Internet
based connections, we are not in control of the many routers, switches and other network hardware
6-4
for detailed programming information.)
84-10 : ToS Setup on
Virtual Private Network (VPN)
Network Design Considerations
ISSUE 1.0