First, the theoretical basis:
serial links are commonly used in WAN. It defines two serial link data transmission: asynchronous and synchronous.
1, the asynchronous transmission is to transmit data bytes, and require additional start bit and stop bit to mark the beginning and end of each byte. A binary 0 value start bit, 1 stop bit binary value. In this transmission mode, start and stop bits occupy a large proportion of the transmission data, each byte requires additional transmission overhead.
2, synchronous transmission in frame units to transmit data, required to synchronize the clock present in the communication terminal apparatus and the communication peer. DCE i.e., data communication device, which provides a clock signal for synchronizing data transfer between the DTE and the DCE. I.e., data terminal equipment DTE, DCE is usually generated using a clock signal.
3, ISO is developed HDLC bit oriented communications rules. The information unit is transmitted HDLC frame. Thus, a synchronous transmission.
. 4, PPP protocol is a point to point link layer protocol used for data transmission on the same point to point full duplex asynchronous link, PPP supports both isochronous transfer and asynchronous transfer support.
Second, by the following experiments: if the S1 and R1 R2 / 0/0 interface IP address is not the same network segment, but also enable communication therebetween by HDLC or PPP it?
Two routers are interconnected topology: 
2.1 CONFIGURATION R1 S1 / 0/0 Interface:
#
interface Serial1/0/0
link-protocol ppp
ip address 192.168.1.1 255.255.255.0
#2.2 Configuration of R2 S1 / 0/0 Interface:
#
interface Serial1/0/0
link-protocol ppp
ip address 192.168.2.1 255.255.255.0
#2.3 on R1, ping R2 Interface IP: 192.168.2.1; in R2, pingR1 interface IP: 192.168.1.1. 
2.4 now R1, R2 of S1 / link layer protocol 0/0 interface to: HDLC protocol. Each interface Run: link-protocol hdlc. Tip: y. Huawei Default is ppp protocol.
2.5 on R1, ping R2 Interface IP: 192.168.2.1 nowhere; in R2, pingR1 interface IP: 192.168.1.1 nowhere. 
Third, the comprehensive experiments, the conclusion is based on point to point protocol PPP link layer, you can communicate when the network address is not in the same network segment . And based on HLDC agreement, unreasonable when Internet address is not on the same network segment .
Reason: After the PPP configuration, data link layer connection is established between the routers. Local router sends a host route to the remote router, the routing information contains local interface IP address, mask is 32 bits.
