1. Point-to-Point Links: Framing
Section 2.3

2. Framing Demarcates (Separates) units of transfer Goal Challenge
Enable nodes to exchange blocks of data
Challenge
How can we determine exactly what set of bits constitute a frame?
How do we determine the beginning and end of a frame?

3. Why Framing? Synchronization recovery Link multiplexing
Breaks up continuous streams of unframed bytes
Link multiplexing
Multiple hosts on shared medium
Simplifies multiplexing of logical channels
Efficient error detection
Per-frame error checking and recovery

4. Framing Approaches Characteristics Sentinel Length-based Clock based
Bit- or byte-oriented
Fixed or variable length
Data-dependent or data-independent length

5. Framing: Bit Oriented Protocols/ Sentinel-based
Beginning and End of Frame
Marked with a special bit pattern:
Frame length is data-dependent
Problem: what if the special pattern occurs within frame?
Solution: escaping the special characters
E.g., sender always inserts a 0 after five 1s
… and receiver always removes a 0 appearing after five 1s
Frame contents

6. Bit Oriented Protocols/ Sentinel-based: bit stuffing
After receiving 5 1s
next bit
0 >> stuffed bit >> removed
Example
bits received ; bits retained (data):
bits received ; bits retained (data):
1 >> bits received or
0 END of Frame marker
1 Error , frame is discarded; receiver waits for next to start receiving next frame

7. Example Bit Oriented Protocol
ISO High-Level Data Link Control, HDLC protocol
Cyclic Redundancy Check (CRC) field: used to detect transmission errors (Section 2.4)

8. Framing: Byte Oriented Protocols/ Sentinel-based
e.g. Point-to-Point, PPP protocol is similar in concepts as “Bit Oriented/ Sentinel-based” ( p. 84)

9. Framing: Byte Oriented Protocols/ Counter-based
include payload length in header
COUNT: specifies how many bytes are contained in the frame’s body
e.g. Digital Data Communication Message Protocol, DDCMP
Problem: what if the count field gets corrupted?
Causes receiver to think the frame ends at a different place
Solution: catch when CRC fails
Receiver accumulates bytes numbered by bad COUNT field
Recognizes that frame is bad using the error detection field
And wait for the next sentinel (SYN) for the start of a new frame
The beginning of a frame is denoted by sending a special SYN (synchronization) character.

10. Clock-Based Framing Continuous stream of fixed “time-length” frames
Example:
Synchronous Optical Network (SONET)
Dominant for long-distance transmission over optical networks
STS-1: lowest-speed SONET link (51.84 Mbps)
STS-1 frames µsec long
All SONET frames are µsec long
Problems:
Frame synchronization
Clock synchronization

11. SONET Frame synchronization Clock synchronization …
2-byte synchronization pattern starts each frame (unlikely to occur in data)
Wait until pattern appears in same place repeatedly; once every 810 bytes
Clock synchronization
NRZ encoding, payload scrambled (XOR’d) with well-known 127-bit pattern
Ensures that there are plenty of transitions to allow the receiver to recover the sender’s clock
Frame is arranged as 9 rows of 90 bytes each
First 3 bytes of each row are overhead.
First two bytes of the frame: special bit pattern, used to determine start of frame.
Receiver hope pattern appears once every 810 bytes, since each frame is 9 X 90 = 810 bytes long.
special pattern appears in the right place enough times >> sync , receiver interprets frame correctly
The overhead bytes of a SONET frame are encoded using NRZ, the simple encoding described in the previous section where 1s are high and 0s are low. However, to ensure that there are plenty of transitions to allow the receiver to recover the sender’s clock, the payload bytes are scrambled. This is done by calculating the exclusive-OR (XOR) of the data to be transmitted and by the use of a well-known bit pattern. The bit pattern, which is 127 bits long, has plenty of transitions from 1 to 0, so that XORing it with the transmitted data is likely to yield a signal with enough transitions to enable clock recovery. …
Overhead
Payload
9 rows
90 columns
STS-1 frame

12. SONET Multiplexing STS-n Since each frame is 125µs long
STS-1: lowest speed SONET, runs at Mbps
STS-N viewed as N STS-1 sharing a fiber
STS-48 runs at Mbps…you can go faster
Since each frame is 125µs long
STS-1 frame is 810 bytes
STS-3 frame is 2430 bytes
Payload from N STS-1 frames can be linked to form a larger STS-N payload. This is called an STS-Nc link

13. SONET Multiplexing (continued)
A single SONET frame can contain subframes for lower-rate channels
STS-N signal can be used to multiplex the bytes of N STS-1 frames
STS-3 thought of as consisting of Mbps links sharing a fiber (bytes interleaved; STS-1 merged byte wise round-robin into STS-3) OR
Unmerged (single-source) Payload from N STS-1 frames can be linked to form a larger STS-N payload. The link is called an STS-Nc {concatenated} link (e.g. STS-3c is viewed as a single Mbps pipe)
HDR
STS-1
STS-3