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Wide area network

Wide area Network (WAN,Wide Area Network) usually spans a large physical area, covering a range from tens of kilometers to thousands of kilometers. It can connect multiple cities or countries, or span several continents and provide long-distance communication to form an international remote network. The local area network is usually connected to the wide area network as the end user of the wide area network. The wide area network is generally established, managed and maintained by telecommunications departments or companies, and provides paid services for communication, traffic statistics and billing problems to the whole society, such as home users accessing the Internet through ADSL or optical fiber access to Internet, that is, the wide area network.

As shown in the figure, LAN 1 and LAN 2 are connected through a WAN line, and the interface on the router in the figure that connects to the WAN is the Serial interface, the serial interface. There are several standards for Serial interfaces, and the figure shows two interfaces: synchronous WAN interface card and non-channelized E1/T1 WAN interface.

WAN links can have different protocols. As shown in the figure, the serial link between the AR1 router and the AR2 router uses the HDLC protocol, the serial link between AR2 and AR3 uses the PPP protocol, AR3 and AR4 are connected by a frame Relay switch, and the Frame Relay protocol is used.

Different links use different data link layer protocols, and each data link layer protocol defines the corresponding data link layer encapsulation (header). Packets are encapsulated into different frames when they pass through different links. The picture shows that when PC1 sends a packet to PC2, it first goes through Ethernet, encapsulates the packet into Ethernet frame, encapsulates the packet into HDLC frame on the link between AR1 and AR2, encapsulates the packet into PPP frame on the link between AR2 and AR3, encapsulates the packet into frame relay frame on the link between AR3 and AR4, and encapsulates the packet into Ethernet frame when sent from AR4 to PC2.

Next, we introduce several common protocols used in WAN links, and grab packets to let you see the frame formats of different data link layer protocols.

HDLC protocol

High-level Data Link Control, Advanced data Link Control, or HDLC for short, is a bit-oriented link layer protocol.

HDLC formulated by ISO is a bit-oriented communication rule. The unit of information transmitted by HDLC is in frames. As a typical bit-oriented synchronous data control protocol, HDLC has the following characteristics:

1. The protocol does not depend on any kind of character encoding set.

two。 Data messages can be transmitted transparently, and the "0 bit insertion method" for transparent transmission is easy to be implemented in hardware.

3. Full-duplex communication can send data continuously without waiting for confirmation, and has high transmission efficiency of data link.

4. All frames are checked by CRC, and the information frames are numbered, which can prevent missed or re-received, and the transmission reliability is high.

5. The transmission control function is separated from the processing function, which has greater flexibility and more perfect control function.

6. Frame format: HDLC frame format includes address domain, control domain, information domain and frame check sequence.

HDLC is formulated by the International Organization for Standardization (ISO). It is a data link layer protocol that has been widely used in the field of communication. However, with the progress of technology, the reliability of the communication channel has been greatly improved than in the past. It is no longer necessary to use very complex protocols (including numbering, error detection and retransmission technology) in the data link layer to achieve reliable data transmission. As a narrowband communication protocol, the application of HDLC in the company gradually disappears, and the scope of application is gradually reduced, but it is only used to transmit data through some private networks. Transparent transmission, that is, transparent transmission, means that the transmission network is only responsible for transmitting the services that need to be transmitted to the destination node, regardless of the transmission business, while ensuring the quality of transmission without processing the transmitted business.

Let's configure the link between AR1 and AR2 routers to use the DHLC protocol. Grab the packet to analyze HDLC frame format.

Configuration on AR1

[AR1] interface Vlanif 1

[AR1-Vlanif1] ip address 192.168.0.1 24

[AR1-Vlanif1] quit

[AR1] interface Serial 2-0-0

[AR1-Serial2/0/0] ip address 192.168.1.1 24

[AR1-Serial2/0/0] display this-- Interface configuration

[V200R003C00]

#

Interface Serial2/0/0

Link-protocol ppp-the default data link layer protocol is PPP

Ip address 192.168.1.1 255.255.255.0

#

Return

[AR1-Serial2/0/0] link-protocol?-View all supported data link layer protocols

Fr Select FR as line protocol

Hdlc Enable HDLC protocol

Lapb LAPB (X.25 level 2 protocol)

Ppp Point-to-Point protocol

Sdlc SDLC (Synchronous Data Line Control) protocol

X25 X.25 protocol

[AR1-Serial2/0/0] link-protocol hdlc-- specifies that the data link layer protocol is HDLC

[AR1-Serial2/0/0] quit

[AR1] ip route-static 192.168.2.0 24 192.168.1.2-- A route added to the 192.168.2.0 Universe 24 network segment

Configuration on AR2.

[AR2] interface Vlanif 1

[AR2-Vlanif1] ip address 192.168.2.1 24

[AR2-Vlanif1] quit

[AR2] interface Serial 2-0-1

[AR2-Serial2/0/1] ip address 192.168.1.2 24

[AR2-Serial2/0/1] link-protocol hdlc

[AR2-Serial2/0/1] quit

[AR2] ip route-static 192.168.0.0 24 192.168.1.1

As shown in the figure, right-click the AR2 router, click "data capture packet" à "Serial 2-0-1", in the Select Link Type dialog box that appears, select HDLC, open the packet grab tool in PC1 ping PC2.

In the packet capture tool, click on the ICMP protocol, you can see that the data link layer is the Cisco HDLC protocol, which means that it is the HDLC protocol defined by Cisco. The header of the Cisco HDLC protocol has three fields, the address field, the control field and the protocol field. Compared to Ethernet frames, there is no destination MAC address source MAC address.

Teacher Han Ligang's apprenticeship course project

Http://edu.51cto.com/topic/819.html?qd=hlg

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