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New Member

Serial interface

When we ping from one router to another router which are connected only from serial interface. The packect has to go down to physical layer to be transmitted into wire . Now since serial doesnt have a MAC address what will the router put in the frames that has to be formed when sending packets

plz suggests..

3 REPLIES
New Member

Re: Serial interface

Well, oyu didn't specify what type of serial link this is. There are choices. Inmost cases, serial links have a tendency to be point to point, so the need for a MAC address is minimal/none. In those cases where there is some form of a switched path (ATM and frame relay are just two examples), there tends to be a path identifier at the data link layer to indicate the path to a particular station. For frame relay, we have a data link connection identifier (DLCI) for identifying a path for another data link layer station. A MAC address only really becomes crucial when you have a shared media access at the data link layer. This is the case with Ethernet, token ring, FDDI, etc.

pw

New Member

Re: Serial interface

Hi thanx for prompt reply ..

WElll the serial link is p2p on Leased line . so it means no DATA Link frame would be formed before transmission of the packets in G.703 form to other end of the router . ??? Plz confirm

New Member

Re: Serial interface

Whoa, not so fast! If this is a point to point link (versus a switched WAN link such as frame relay), then you are dealing iwth a garden hose, in effect. The water comes in one and and can only go out the other end. Because of this, the need for a MAC address is only for management purposes, not addressing. As such, the protocols that can be used for a point-to-point serial link are PPP and Cisco's HDLC (among others). Look at each:

PPP header:

Flag—A single byte that indicates the beginning or end of a frame. The flag field consists of the binary sequence 01111110.

Address—A single byte that contains the binary sequence 11111111, the standard broadcast address. PPP does not assign individual station addresses.

Control—A single byte that contains the binary sequence 00000011, which calls for transmission of user data in an unsequenced frame. A connectionless link service similar to that of Logical Link Control (LLC) Type 1 is provided. (For more information about LLC types and frame types, refer to Chapter 16.)

Protocol—Two bytes that identify the protocol encapsulated in the information field of the frame. The most up-to-date values of the protocol field are specified in the most recent Assigned Numbers Request For Comments (RFC).

Data—Zero or more bytes that contain the datagram for the protocol specified in the protocol field. The end of the information field is found by locating the closing flag sequence and allowing 2 bytes for the FCS field. The default maximum length

of the information field is 1,500 bytes. By prior agreement, consenting PPP implementations can use other values for the maximum information field length.

Frame check sequence (FCS)—Normally 16 bits (2 bytes). By prior agreement, consenting PPP implementations can use a 32-bit (4-byte) FCS for improved error detection.

***NOTE - The address field does not assign single station addresses!

Now for HDLC:

http://www.wildpackets.com/images/compendium/Cisco_HDLC_Frame.gif

In Cisco HDLC (cHDLC), the Address field is always one byte and takes only one of two values: 0x0F (unicast), or 0x8F (broadcast).

These protocols (PPP and HDLC) sit at layer 2 of the OSI model. G.703 is effectively a layer 1 (physical layer) standard. It basically defines how those 1's and 0's will go out on the wire in a manner that is recognizable to the distand physical layer device, such as a CSU/DSU. Each layer functions on top of the other layers and they have dependencies. IP (Internet Protocol) will not work unless some layer 2 protocol is properly functioning, such as Ethernet, token ring, LAPB, PPP, or HDLC. These layer 2 protocols are not much good unless the layer 1 protocols are working properly. G.703 is a layer 1 protocol. Expect to see a web page transmitted over a leased line by taking application layer output (HTTP) and pasing it down to TCP (Transmission Control Protocol), that passes it to IP, which passes it to HDLC, which might use G.703 as the transmission standard at the physical layer. Does this make sense?

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