2.2.3. Network Requirements 
The HG 3500 / IPDA components
place high demands on the network via which they are coupled to their
IP endpoints. Just like signaling links between the CC and the access
points, Voice-over-IP data streams are sensitive to packet runtime, runtime
variance (jitter) and packet loss.
The availability and reliability of a HG 3500 / IPDA system depends
heavily on the quality of the IP network used.
Therefore, the network must be examined prior to installation with regard to its suitability
for
VoIP for the use of HG 3500 / IPDA.
All OpenScape 4000 components must be connected to their own
ports on Layer 2 switches.
Using hubs together with OpenScape 4000 can cause problems. For this
reason, they are not permitted for use in corresponding VoIP scenarios.
If hubs are available, they should be replaced with a switch.
In order to achieve a high Quality of Service (voice quality/real
time behavior of signaling), the support of IEEE 802.1 p/q VLAN Tagging
and IETF RFC 2474 DiffServ is recommended (see Section 2.4,
“Network Data”).
IMPORTANT:
If an access point is connected over Wide
Area Networks in which the available bandwidth is extremely
restricted, the control information (signaling) and voice data (payload)
for access points must be given priority over other services.
Frequently, only explicitly familiar TCP/IP or UDP/IP port numbers
are supported in the customer LAN for security reasons. All others are
then blocked in such cases. Precisely which port numbers must be supported
for the HG 3500 component in the network is described in Chapter
20, “IP Ports”
IP Interface MTU
The OpenScape 4000 IP interfaces use an IP interface MTU of 1500 bytes
by default. In order to ensure optimal IP network throughput, it is recommended
to likewise operate the IP network both for the IPDA LAN (Signaling/Media
- Voice over IP) and the Management LAN with a MTU of 1500 bytes.
If this is not possible, for example because of WAN routers (PPPoE,
IPSEC,...) with a smaller MTU, it is recommended to use TCP - MSS Clamping
in these routers. This router configuration results in the OpenScape
4000 software components adapting the TCP packet size for the next TCP
three-way handshake.
IMPORTANT:
This procedure applies for TCP, but not for UDP.
The UDP packets (e.g. RTP media) are generally smaller than 500 bytes
however.
Exception: SIP-Q via UDP
In case of problems, it is possible to configure the IP interface
MTU for CGW/OpenScape 4000 SoftGate modules by means of the module's
WBM. The IP interface MTU should not be set however to a value under
1000 bytes.
Furthermore, the OpenScape 4000 software stacks generally also react
to ICMP Fragmentation Needed Packets, which then results in the packet
sizes being adapted. This procedure can lead to longer runtimes however.