CS541 Advanced Networking 1 Spectrum Sharing in Cognitive Radio Networks Neil Tang 3/23/2009.
Framing Neil Tang 9/19/2008
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Framing Neil Tang 9/19/2008. Outline. Framing Byte-Oriented Framing Bit-Oriented Framing Clock-Based Framing. Framing. The main problem is to identify the beginning and the end of each frame. Byte-Oriented Framing. Sentinel Approach (BISYNC and PPP) - PowerPoint PPT Presentation
Transcript of Framing Neil Tang 9/19/2008
CS440 Computer Networks 1
FramingFraming
Neil TangNeil Tang9/19/20089/19/2008
CS440 Computer Networks 2
OutlineOutline
Framing
Byte-Oriented Framing
Bit-Oriented Framing
Clock-Based Framing
CS440 Computer Networks 3
FramingFraming
The main problem is to identify the beginning and the end of each frame.
Frames
Bits
Node A Node BAdaptor Adaptor
CS440 Computer Networks 4
Byte-Oriented FramingByte-Oriented FramingSentinel Approach (BISYNC and PPP)
A frame is composed of several fields whose lengths are multiples of 8 bits (byte)
Character-Stuffing (escaping): precedes the control character with the escaping character (e.g., DLE) if it appears in the body/payload.
Header Body
8 8 8 8 168
CRC
ProtocolControlAddressFlag Payload
88 816168
FlagChecksum
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Byte-Oriented FramingByte-Oriented FramingByte-Counting Approach (DDCMP)
COUNT Field: specifies how many bytes are contained in the frame’s body.
Farming Error: COUNT field is corrupted.
Header Body
8 8 4214 168
CRCCount
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Bit-Oriented Framing (HDLC)Bit-Oriented Framing (HDLC) HDLC denotes both the beginning and the end of a frame with 01111110.
Bit-Stuffing: In the sender, insert a 0 after every five consecutive 1s. In the receiver, after five consecutive 1s, if 0, stuffed; if 1, look at the next bit. If 0, end-of-a-frame marker; otherwise error.
The receiver will discard the frame if error happens and it will start to receive again until next 01111110.
Header Body
8 16 16 8
CRCBeginningsequence
Endingsequence
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Clock-Based Framing (SONET)Clock-Based Framing (SONET)
Overhead Payload
90 columns
9 rows
The first 2 bytes contain a special bit pattern are used to determine where the frame starts.
Each frame has the fixed length of 9 90 = 810 bytes, no bit/byte stuffing is used.
The receiver looks for the special bit pattern consistently hoping to see it appearing every 810 bytes.
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Clock-Based Framing (SONET)Clock-Based Framing (SONET) Multiplexing
Sub-frames are interleaved.
STS-1 STS-1 STS-1
STS-3cHdr
