POTS-to-POTS Call Routing Considerations bandwidth

POTS-to-POTS Call Routing Considerations

Occasionally, you must route calls between POTS dial peers. CAC might cause this, such as when a call from a PBX is denied admission to the WAN and sent back to the PBX. Or a call from an analog phone might be routed over the PSTN. The ability to switch calls between POTS dial peers is enabled by default on Cisco gateways.

Many of the CAC mechanisms that are described in this chapter use hairpinning, or tromboning, in which a call both enters and exits on the same interface. Suppose that a router doing toll bypass receives an analog call from its PBX. This call is matched to an incoming POTS dial peer. Typically the analog call is terminated, regenerated as a VoIP call, and then routed out a VoIP dial peer. However, the CAC configuration of the gateway might cause the call to be rejected for sending over the IP network. When a call is rejected, you can redirect (or hairpin) it back to the PBX. Both legs of the call thus traverse the same router interface. The PBX then looks for an alternate route, such as out to the PSTN.

A hairpinned POTS call is terminated on the gateway and then routed out another POTS dial peer as a new call leg. When this happens, DSPs are assigned to the incoming leg of the POTS call and also to the outgoing (hairpinned) leg. These DSP resources stay assigned for the duration of the call; thus, each hairpinned call doubles the DSP usage.

Routing between ISDN POTS dial peers can use TDM switching, but this must be done across a bus that supports it. With TDM switching, DSP resources are assigned when the call is received, but media DSPs are dropped after the call is switched to another POTS port. No DSP is needed for the media because an internal TDM connection is made between the incoming and outgoing ports on the TDM bus. This is an advantage with nonvoice calls, such as modem, fax, and video.

Older routers, such as the 1700, 2600, 3600, and 3700 series, support TDM switching only between ports on certain network modules. Both legs of the call must stay on the same module. Cisco Integrated Services Routers (ISR) support TDM switching across the backplane; thus, POTS-to-POTS calls can be routed between different network modules. Other Cisco gateways, such as the Access Server (AS) and Integrated Access Device (IAD) lines, also support TDM switching across their backplane, and thus, can route POTS calls between interfaces. These are typically used in a service provider environment.

Modules that support intramodule TDM switching include these:

• NM-HDV
• NM-HDV2
• NM-HDV2-1T1/E1
• NM-HDV2-2T1/E1
• NM-HD-1V/2V/2VE
• EVM-HD-8FXS/DID
• AIM-VOICE-30
• VWIC

ISRs allow intermodule TDM call routing between the following modules:

• VWIC
• NM-HDV2
• NM-HD-1V/2V/2VE
• EVM-HD-8FXS/DID

You must synchronize clocking when you are switching between these ports and modules. Chapter 7, “Connecting to PBXs,” describes how to do this.

Case Study: Implementing Gateway-Controlled RSVP

The link between the New York and the Leeds office tends to be fairly congested, so the decision is made to implement RSVP between the two gateways. In addition, LLQ and CVWFQ are applied to the IP WAN interface. RSVP signaling is marked with DSCP 31 and is given a share of the interface bandwidth.

Example 11-12 shows the configuration of the New York gateway to implement RSVP and add the desired queueing. Only the New York router configuration is shown, but you must configure the Leeds router similarly. Use the same required and acceptable QoS dial peer configuration on both gateways. The LLQ and CBWFQ configuration should be the same on both routers, also.

!Identifies RTP traffic
NY-GW1(config)#ip access-list extended RTP
NY-GW1(config-ext-nacl)#permit udp any any range 16383 32767
NY-GW1(config-ext-nacl)#permit udp any range 16383 32767 any
!
!Identifies H.323 signaling traffic
NY-GW1(config-ext-nacl)#ip access-list extended H323
NY-GW1(config-ext-nacl)#permit tcp any eq 1720 any
NY-GW1(config-ext-nacl)#permit tcp any any eq 1720
!
!
!Creates a class for voice media matching ACL RTP
NY-GW1(config-ext-nacl)#class-map match-all VOIP-Media
NY-GW1(config-cmap)#match access-group name RTP
!
!Creates a class for voice signaling matching ACL H323
NY-GW1(config-cmap)#class-map match-all VOIP-Signal
NY-GW1(config-cmap)#match access-group name H323
!
!Creates a class for RSVP signaling matching DSCP configured on
  interface S0/0
NY-GW1(config)#class-map RSVP-Signal
NY-GW1(config-cmap)#match dscp 31
!
!Creates a policy for LLQ and CBWFQ
NY-GW1(config-cmap)#policy-map RSVP
NY-GW1(config-pmap)#class VOIP-Media
NY-GW1(config-pmap-c)#priority percent 30
NY-GW1(config-pmap-c)#class VOIP-Signal
NY-GW1(config-pmap-c)#bandwidth percent 5
NY-GW1(config-pmap-c)#class RSVP-Signal
NY-GW1(config-pmap-c)#bandwidth percent 5
NY-GW1(config-pmap-c)#class class-default
NY-GW1(config-pmap-c)#fair-queue
NY-GW1(config-pmap-c)#random-detect
!
!Configures the interface for RSVP and applies the policy
NY-GW1(config-pmap-c)#int s0/0
NY-GW1(config-if)#ip rsvp bandwidth 400 40
NY-GW1(config-if)#ip rsvp resource-provider none
NY-GW1(config-if)#ip rsvp data-packet classification none
NY-GW1(config-if)#ip rsvp signalling dscp 31
NY-GW1(config-if)#service-policy output RSVP
!
!Configures the dial peer to Leeds for RSVP
NY-GW1(config-if)#dial-peer voice 50300 voip
NY-GW1(config-dial-peer)#destination-pattern 50[34]..
NY-GW1(config-dial-peer)#session target ipv4:10.40.25.1
NY-GW1(config-dial-peer)#req-qos guaranteed-delay
NY-GW1(config-dial-peer)#acc-qos guaranteed-delay
NY-GW1(config-dial-peer)#forward-digits 4