User Manual

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2009 Industrial Wireless Guidebook
Cellular Networks
3
3.2 Private IP Solution
There are two limiting factors you are almost assured to encounter when setting up a wired LAN with private IP
addresses in an office: (1) short transmit latency and (2) limited IP addresses to connect to the Internet for the
cellular WAN interface. As a result, delay times for the WAN interface are very different from the local area network’s
delay times. WAN port IP addresses are also very different from those in office LANs.
Private IP vs. Public IP
From the LAN point of view:
According to RFC 1918 standards, office networks are allocated private IP addresses according to class
A, B, and C. The Internet Assigned Numbers Authority (IANA) has reserved the following three blocks of IP
address space for private networks:
10.0.0.0 - 10.255.255.255 (10/8 prefix)
172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
192.168.0.0 - 192.168.255.255 (192.168/16 prefix)
From the cellular WAN port point of view:
Normally, IP addresses for cellular WAN ports will have:
Type of Address Description Role(s) of Configured Devices
Floating private IP
address
Mobile operator keeps a pool of
private IP addresses and assigns one
to the GPRS subscriber
Always Client role to access server
Class A 10.xx.xx.xx
Floating public IP
address
Mobile operator keeps a pool of public
IP addresses and assigns one to the
GPRS subscriber
Can be Client or Server role, IP
address is always changeable, needs a
notification mechanism to update public
IP address
Fixed public IP
address
Mobile operator keeps a dedicated IP
address for each SIM card based on
the SIM card’s (IMSI) ID code and user
service level
Can be Client or Server role,
IP address is fixed, needs a special bill
rate from operator
As you can see from the table above, the kind of WAN IP address obtained from your cellular operator will
affect network planning and determine the role of the devices configured with the IP address.
Private IP addresses are suitable for Client role.
Public IP addresses are suitable for Client role and Server role.
Delay Time
Latency in a packet-switched network is measured either one-way (the time from the source sending a packet
to the destination receiving it) or round-trip (the one-way latency from source to destination plus the one-way
latency from the destination back to the source). Round-trip latency is more often quoted, because it can be
measured from a single point. Note that round trip latency excludes the amount of time that a destination
system spends processing the packet. Many software platforms provide a service called ping that can be used
to measure round-trip latency. Ping performs no packet processing; it merely sends a response back when it
receives a packet (i.e., performs a no-op) so it is a relatively accurate way of measuring latency.
Where precision is important, one-way latency for a link can be more strictly defined as the time from the start
of packet transmission to the start of packet reception. The time from the start of packet reception to the
end of packet reception is measured separately and called “Serialization Delay.” This definition of latency is
independent of the link’s throughput and the size of the packet, and is the absolute minimum delay possible
with the link latency of the LAN you can measure by specified device with input to output delay time in
serialization.