Outline

• Encoding
• Quiz
• Framing

Encoding

• We want to be able to write 0 and 1's using our ability to have send the signals low and high between machines.

NRZ Encoding

In the nonreturn to zero (NRZ) encoding we use high for a certain amount of time to represent a 1 and low for a certain amount of time to represent 0.

• The receiver usually keeps an average of the signal seen so far, so if several consecutive 0's or 1's are sent it skews the average, making it hard to distinguish between 0's and 1's. (baseline wander)
• The sender and receiver need to synchronize their clocks in order to know how long a single 0 or 1 is. To do this the receiver typically relies on seeing 0 to 1 or 1 to 0 transitions from the sender so it can align its clock. However, if too many 0's or 1's in a row are sent this is not possible.

NRZI Encoding

In the nonreturn to zero inverted (NRZI) encoding, to send a 1 the sender transitions from the current signal value to the opposite; to send a 0 the sender stays on the current signal.

• This solves the problem of consecutive 1's, but does not help with consecutive 0's.

Manchester Encoding

In the Manchester encoding, one sends the XOR of the clock and the NRZ-encode data.

• This results in a 0 being encoded as a low to high transition and a 1 being encoded as a high to low transition.
• The drawback is the rate at which the signal changes (the baud rate) to encode a single bit has doubled.
• The bit rate/baud rate is only 50% which is somewhat inefficient.

4B/5B encoding

• In 4B/5B encoding, every 4 bits of data are encoded in a 5-bit code sent to the receiver (who must then decode them).
• These codes are sent using NRZI.
• The 5-bit codes are selected such that each one has no more than one leading 0 and no more than two trailing 0s.
• So when sent back-to-back, no pair of 5 bit codes results in more than three consecutive 0s.
• The bit rate/baud rate efficiency of the code is 80% -- so is better than Manchester encoding.
• The book has a table of the 5-bit codes. For instance, 0000 maps to 11110.
• There are 32, five bit strings, 16 are used for data, 7 are not valid, and six are used for control values such as: 11111 -- line is idle, 00000 - line is dead, and 00100 -- halt.

Quiz

Which of the following is used by a network adaptor to directly write memory without CPU involvement?

1. CSR
2. DMA
3. device driver

Framing

• Let's assume we can now send a sequence of bits over a link.
• Since we will be focussing on packet-switched networks, we are interested now in splitting this bit-stream into blocks of data called frames.
• We will first look at this problem for point-to-point links and then for multiple access networks.

Byte-oriented Protocols

• A protocol which views each frame as a collection of bytes rather than bits is called a byte-oriented protocol.
• Such protocols were originally developed to connect terminals to mainframe.
• For example, Binary Synchronous Communication (BISYNC) was developed at IBM and Digital Data Communication Message Protocol (DDCMP) was developed at DEC for this purpose.
• Point-to-point (PPP) is a more recent and widely used example of this approach.
• We'll now discuss two types of byte-oriented framing: sentinel-based framing and byte-count framing.

Sentinel-based Framing

• BISYNC uses special characters called sentinel characters to indicate where frames start and end.
• The SYN character is used to indicate the start of a frame, there is a SOH character to indicate a start of a frame header, an STX character to indicate start of text, and an ETX character used to indicate the end of text.
• A BISYNC frame ends with a CRC (cyclic redundancy check) character used to detect errors in the frame.
• BISYNC uses character stuffing (preceding a control character with a special data-link escape (DLE) character) if a SYN, ETX, etc need to appear in the text or header.

PPP Frames

• PPP is commonly used to carry internet packets over point-to-point links (could be modem; could be long-haul links).
• It was defined in RFC 1661
• Flag is a sentinel with bit pattern 01111110; Address and Control usually contain default values 0xFF and 0x03; Protocol is used to identify the high level protocol such as IP or IPX(Novell).
• The Payload can be up 1500 bytes and character stuffing might be used in it.

Link Control Protocol

• The Protocol field is negotiated using the Link Control Protocol (LCP).
• LCP actually makes use of PPP frames to send control messages to do this negotiation. These messages have as their protocol a special LCP identifier.
• LCP is also used to establish a link between peers when both sides detect that communication over the link is possible.

Byte-count Framing

• In the byte-counting approach, one sends the number of characters in the frame at the start of the frame.
• DDCMP uses this approach. Its frames look like:

  8 bits	8 bits	8 bits	14 bits	42 bits		16 bits
  SYN	SYN	Class	Count	Header	Body	CRC

• So called framing errors occur with both byte-count framing and sentinel framing if the COUNT field is corrected or the ETX field is corrupted. These can be caught using the CRC field.

Bit-oriented Protocols (HDLC)

• The high-level data link control protocol (HDLC) is an example of a bit-oriented protocol -- the payload of a frame is viewed as a sequence of bits which might not necessarily corresponds to bytes in some character encoding.
• HDLC is an ANSI and ISO standard derived from the SLDC -- Synchronous Data Link Control protocol used in IBM mainframes.
• A HDLC frame looks like:

  8 bits	16 bits		16 bits	8 bits
  01111110	Header	Body	CRC	01111110

Bit-stuffing

• If the beginning sequence 01111110 is needed within the body of the message a technique known as bit-stuffing is used:
  • Whenever 11111 is sent within a frame, the sender immediately sends a 0, before sending the next bit.
  • The receiver when receiving these extra 0's immediately removes them from the decoded bit stream.

SONET and Clock-based Framing

• Sychronous Optical Networks (SONET) use a third approach to framing known as clock-based framing.
• SONET was developed at Bell Communication Corporation, ANSI, and it has been adopted by the ITU-T.
• SONET solves both the framing and encoding problem and it also supports multiplexing several low-speed links onto higher speed links.

SONET STS-1 Frames

• An STS-1 frame consists of 9 rows each of 90 bytes.
• The first 3 bytes of each row are overhead, but the remaining "payload" bytes can be used for data.
• The first two bytes of each frame contain a special bit pattern.
• To sync itself with the sender, the receiver looks for this bit pattern repeating every 810 bytes.
• The overhead bytes are encoded using NRZ.
• The payload bytes are encoded by XORing them against a fixed known 127 bit pattern that has plenty of 1's and 0's in the hope of reducing the number of all 1 or all 0 stretches in the sent data.
• An STS-N frame can be built out of N, STS-1 frames by interleaving the bytes of these frames.
• The payload of a frame need not be contained completely within the frame.
• The overhead can be used to specify a synchronous payload envelope which is an offset to the starting position of the payload within a frame. This payload could potentially cross frame boundaries.