thanks for this kind of technical websites. I have cisco router 2800 and its was working when I attend to change the teleport provider on NS6 stallite. the router could not connect with COMTECH modem via serial and when Im using the Command SH SERIAL INT 0/0/0 the:
DCD=down DSR=down DTR=down RTS=down CTS=down for some reason I dont know why. please see the configuration of the router

Current configuration : 1383 bytes
!
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
service password-encryption
!
hostname wahbi
!
boot-start-marker
boot-end-marker
!
enable secret 5 $1$l87/$arNz8agO2JJx4/x8WqzOB1
enable password 7 02110553090F
!
aaa new-model
!
!
aaa authentication login default local
!
aaa session-id common
!
resource policy
!
mmi polling-interval 60
no mmi auto-configure
no mmi pvc
mmi snmp-timeout 180
ip subnet-zero
ip cef
!
!
no ip dhcp use vrf connected
!
!
username wahbi password 7 03135A030406
!
!
interface FastEthernet0/0
ip address X.X.X.X 255.255.255.240
speed auto
!
interface Serial0/0
ip unnumbered FastEthernet0/0
no ip route-cache cef
no ip route-cache
no ip mroute-cache
load-interval 30
no keepalive
no fair-queue
ignore dcd
no cdp enable
!
ip classless
ip route 0.0.0.0 0.0.0.0 Serial0/0
!
ip http server
ip http access-class 23
ip http authentication local
no ip http secure-server
ip http timeout-policy idle 60 life 86400 requests 10000
!
access-list 99 permit X.X.X.X
access-list 99 remark hosts allowed to send snmp queries to the router
snmp-server community Router2800 RO 99
snmp-server packetsize 2048
snmp-server location Router2800
!
control-plane

where X are number

please I need your help with regards

What is the exact model number of the router, and what version of IOS are you running?

Dear sir;
the number of cisco router:2821
and the IOS: Cisco IOS Software, 2800 Software (C2800NM-IPBASE-M), Version 12.4(3i), RELEASE SOFTWARE (fc2)