The Cisco CallManager Express is a product under the Unified Communications Products suite of Cisco. In the past it was known as CCME (Cisco Call Manager Express) but now the new name is Cisco Unified Communications Manager Express.

It is an IP Telephony system (IP PBX) for small to medium size businesses of up to 250 IP phones capacity. Basically, a CallManager Express system is a normal Cisco Integrated Services Router (models 1800, 2800, 3800) which has the CallManager software installed on the router’s flash memory. The router hosting the callmanager system can work also as normal Internet Border router or as WAN Router connecting to other enterprise sites. The CallManager software provides call control and IP telephony functionality to internal IP phones. For connectivity to the PSTN network, voice interface cards can be installed on the CallManager router (such as voice BRI, PRI etc).

In this post we will describe three common deployment models for a CallManager Express system as it is implemented in real world enterprise environments. The three deployment models are Single Site, Multi Site with Distributed Call Processing, and Multi Site with Centralized Call Processing.

Single Site Deployment Model

This is the most common scenario and is usually found in smaller business environments. See the picture below:

Basically a single CallManager Router system is installed which usually provides also the Internet connectivity for the office. If you are a little flexible with your budget, I would recommend installing a firewall in front of the CallManager router to protect it from Internet attacks. All IP Telephony services are provided on the LAN network for internal IP Voice communication. Any call beyond the LAN uses the PSTN network. There are no telephony services provided over an IP WAN.

Characteristics and Best Practices

• Maximum of 250 IP phones can be supported.
• Arrange your internal switch to have two VLANs (one for Voice and one for Data Traffic).
• Use G.711 codec for all IP phone calls on the LAN (80kbps bandwidth per call) for best voice quality.
• You can also install a Voice Mail card on the router to offer voice mail functionality to users.
• Use appropriate Voice Interface Cards on router for PSTN connectivity.
• You can use dual router for redundancy if needed.
• Try to avoid connecting the CallManager router directly to the Internet. Use a firewall as border internet device.
• Dial Plan is simplified. If DID (Direct Inward Dialing) is required, then arrange your dial plan and internal IP phone numbering accordingly.

Multi Site with Distributed Call Processing Model

The multi site model consists of two or more independent sites, each with its own CallManager Express system installed (distributed call processing) as shown in the figure below.

All the sites are interconnected over an IP WAN which can be offered via Leased Lines, Frame Relay, ATM, MPLS Layer2/3 VPN, IPSEC VPN over the Internet etc. All sites have also local PSTN connectivity which can serve as backup to the WAN telephony connectivity or for local inbound and outbound PSTN calls. 

Characteristics and Best Practices

• PSTN Call cost savings when using the IP WAN for calls between sites.
• Bypass long distance call charges (toll bypass) by routing calls through remote site callmanager systems which are closer to the PSTN number dialed. For example, you have one site in New York and one in California. Calls from NY to California can be routed over the IP WAN towards Cal office and then get out to PSTN from the Cal office.
• No loss in functionality for IP WAN failure because there are independent Call processing units in each site.
• Recommended to install a GateKeeper (Cisco IOS gatekeeper) to provide call admission control and dial-plan resolution.
• Use G.729 or G.723 codec for IP calls over the WAN to save bandwidth.
• Use a SIP proxy if you are using SIP instead of H323.

Multi Site with Centralized Call Processing Model

This implementation scenario is suitable for an Enterprise that has a big central office with several smaller branches. One centralized CallManager system can be installed to the Central Site offering call processing and IP Telephony service to both the central site as well as to the remote small branches. The remote branches are equipped only with IP phones (no callmanager system). This is shown in the figure below:

The remote branches are connected to the central site over an IP WAN or even using IPSEC VPN over the Internet. The IP phones located to the remote sites should have IP connectivity to the Central CallManager system, where they are registered. PSTN access is offered only on the Central Site. That is, the call of a remote branch user calling a PSTN number is routed over the WAN to the Central Site and then routed out to the PSTN.

Characteristics and Best Practices

• Cost savings in hardware (only one central callmanager express)
• Easier to manage (centralized management for all IP phones).
• Disadvantage in redundancy since remote sites depend heavily on the availability of WAN lines.
• Use G.729 or G.723 for inter-site calls.
• Savings in PSTN line costs.
• Remote sites must not have many IP phones (10-20 maximum).

All the above deployment models apply also for the other Cisco IP Telephony solution, the Cisco Unified Communications Manager system which is for bigger implementations compared to the Express solution.

One of the most frequent problems encountered from users of DSL internet connectivity (especially PPPoE DSL service) is when they experience incomplete connections or unreliable data transfers when they communicate with internet servers. A prominent example is when a user accesses a web site and some times it works and some times does not. All these problems occur mainly because of MTU (Maximum Transmission Unit) and MSS (Maximum Segment Size) values that need to be adjusted on the customer’s broadband DSL router connecting the office to the ISP.
 
In this example we will consider a Cisco 800 series broadband router (e.g 850, 870 etc) connected with a DSL line using PPPoE protocol (you can find out if you are using such a connectivity method from your ISP. It is a very common DSL access method in USA and the rest of the world also).  

Before going any further, let’s put down some numbers regarding packet sizes:

• Normal Ethernet packets have an MTU value of 1500
• PPP protocol uses 8 bytes header size
• IP Header is 20 bytes
• TCP Header is 20 bytes

 
So, an Ethernet connected device (PC computer, Web Server etc) by default will send traffic with maximum 1500 bytes (1500 includes all headers plus data).

A broadband Cisco 800 router working with PPPoE DSL connectivity has two interfaces connected: One virtual WAN “Dialer” interface facing the ISP (attached on the physical FastEthernet 4 port) and one internal “Vlan 1” interface facing the LAN network (see the post here for an example how to configure PPPoE http://www.cisco-tips.com/configuring-pppoe-for-cisco-router-520-and-for-850-870/ ).

We need to adjust two values (one for MTU and one for MSS) on each router interface (dialer and vlan1). On the Dialer interface (WAN interface) we need to adjust the MTU value to 1500-8 = 1492 in order to accommodate the 8-bytes PPP header size. On the Vlan1 interface we need to adjust the MSS value to 1500-8-20-20 = 1452 bytes (this is 1500 minus the PPP, TCP, and IP headers). So, always remember that to obtain the MSS value you need to subtract the TCP and IP headers as well. Let’s see a configuration snapshot below:

Cisco800(config)# interface dialer 1
Cisco800(config-if)# ip mtu 1492
Cisco800(config-if)# exit

Cisco800(config)# interface vlan 1
Cisco800(config-if)# ip tcp adjust-mss 1452

If you still experience problems with web traffic access, then you can still reduce the MSS value lower than 1452 (some people suggest lowering this value to 1412).