note3 - Computer Networks Framing Saad Mneimneh Computer...

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Computer Networks Framing Saad Mneimneh Computer Science Hunter College of CUNY New York “Who framed Roger rab bit ”? A detect ive, a woman, and a rab bit in a network of trouble 1 Introduction We will skip the physical layer and the detail of how communication signals are sent and received. In doing so, we are simply going to assume (a correct assumption) that we are capable of sending bits over a single link . Using this abstraction, the DLC layer becomes responsible for reliable transmission of packets over a single link . Reliable transmission of packets means that every packet is delivered once, only once, without errors, and in order. To achieve this goal, the DLC layer performs three operations: Framing: determines start and end of packets (all we see is a string of bits) and their order by inserting packet headers Error detection: determines when errors exist using packet trailers Error correction: retransmits packets containing errors 2 Framing The main problem is to decide where successive packets start and end. Therefore, the DLC encapsulates the packet into a frame by adding its own header and trailer. packet header trailer frame Figure 1: From packet to frame Moreover, if there is a period of idle fills between successive frames (a synchronous bit pipe keeps on sending even if there is no data), it becomes also necessary to separate 1
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idle fills from frames. Even when idle fills are replaced by dead periods (intermittent bit pipe), the problem is not simplified, because often there are no dead periods. For instance, where is the data in the following string of bits: ... 01011010110010110101101110101110 ... 2.1 Character based framing Character based codes, such as ASCII (7 bits and 1 parity bit), provide binary repre- sentation for keyboard characters and terminal control characters. The idea in char- acter based framing is that such codes can also provide representation for various communication characters: SYN: a string of SYN characters provide idle fill between frames when a sending DLC has no data to send (but a synchronous modem requires bits) STX: start of text ETX: end of text SYN STX header packet ETX CRC SYN SYN SYN frame trailer added by DLC for error detection (later) Figure 2: Character based frame One disadvantage of character based framing is that the frame must contain an integer number of characters; otherwise, the characters cannot be read correctly. This also raises a question: how do we send binary data? For instance, a packet is a binary string of arbitrary length. Even more important is that a packet may possibly contain,
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note3 - Computer Networks Framing Saad Mneimneh Computer...

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