Getting Connected (Chapter 2 Part 1)

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Presentation transcript:

Getting Connected (Chapter 2 Part 1) Networking CS 3470, Section 1 Sarah Diesburg

Five Problems Encoding/decoding Framing Error Detection Error Correction Media Access

Five Problems of Chapter 2 The physical links carry signals (electromagnetic radiation) How do we turn signals into bits that are recognized at the receiver? This is known as the encoding problem R S 

Five Problems of Chapter 2 Delineating the sequence of bits into complete messages is called framing. When does a frame start? If there's a specific pattern to mark the frame's start/end, what happens if that same bit pattern is embedded in the data/payload? Byte-oriented Bit-oriented Clock-based

Five Problems of Chapter 2 Data verification Has the data been corrupted? If data has been corrupted, can we take the appropriate action? This is the error detection problem. CRC 2-D parity Checksums

Five Problems of Chapter 2 Error Recovery Frames that are damaged will need to be retransmitted. This is the reliability problem ARQ Stop and wait Sliding window Concurrent channels

Five Problems of Chapter 2 How do you arbitrate, or self-regulate access to a shared link? This is the media-access problem Ethernet Token Ring Wireless

Perspectives on Connecting

Perspectives on Connecting All the links seem the same, yet can be characterized in different ways We can characterize them by the services and bandwidth they provide Service Bandwidth (typical) Dail-up 28-56 Kbps ISDN (Integrated Services Digital Network) 64-128 Kbps DSL (Digital Subscriber Line) 128 Kbps – 100 Mbps CATV (CAble TV) 1 – 40 Mbps FTTH (Fibre To The Home) 50 Mbps – 1 Gbps

Perspectives on Connecting All the links seem the same, yet can be characterized in different ways Another way to characterize links is by their physical makeup Copper – DSL, coaxial, cat5e/cat6 cables Optical fiber – FTTH Air – Wireless

Links Regardless of the form, links propagate signals in the form of electromagnetic waves These links provide the foundation to propagate binary information/bits (0’s and 1’s) – otherwise known as encoding

Visualizing Links Network adaptors connect nodes to links Why abbreviated NIC? Signals travel between signaling components NIC = network interface card, another term for network adaptor

Data Encoding (signals) In this class, we can assume that it’s possible to transmit a pair of high (1) or low (0) signals …which brings us to the problem of encoding binary data on these two signals

Tick, tock! Let's synchronize the clock Encoding and decoding processes are driven by a clock Every clock cycle the sender transmits a bit and the receiver recovers a bit 1 1 Clock

Encoding Encoding schemes NRZ (Non-return to zero) NRZI (Non-return to zero inverted) Manchester 4B/5B

NRZ Non-Return to Zero The simplest thing to do is to map “1” onto the high signal and “0” onto the low signal There are a few challenges to this approach 1 1 NRZ Clock

NRZ Non-Return to Zero Baseline Wander caused by signal averaging Receiver keeps average of signal it has received so far and uses average to distinguish highs and lows E.g., average of 0 and 1 is 0.5 – anything higher than 0.5 is a “1” Problem occurs when to many consecutive 1’s or 0’s cause average to change Not to be confused with the mind wander phenomenon, which happens during long PowerPoint presentations.

NRZ Non-Return to Zero Clock recovery required when signal remains constant too long Receiver uses high-low transitions to mark the clock boundaries What happens when we send a lot of consecutive 1’s or 0’s?

NRZI Non-Return to Zero Inverted Transition on the half-cycles 1's indicated by a transition (low-to-high, high-to-low) 0's are where there is no transition Takes care of problem of consecutive 1’s Still a problem for consecutive zeros 1 1 NRZI Clock

Manchester Encoding Transition on the half-cycles low-to-high indicates a zero high-to-low indicates a one Receiver is able to synchronize clock every cycle 1 1 Manchester Clock

Baud rate The baud rate is the rate at which the signal changes The bit rate is the rate at which you can transmit information For the same baud rate, NRZ and NRZI have twice the bit rate as Manchester.

Different Encoding Strategies So Far…

4B/5B encoding 4-bit payload in a 5-bit gift box Goal is to improve upon Manchester (50% efficiency), but to avoid baseline wander and clock drift Insert extra bits into bit stream to break up long sequences of 0’s and 1’s 5-bit codes selected such that there are never more than three consecutive zero's. Resulting codes transmitted through NRZI encoding

4B/5B 5 bits (32 patterns) to represent 4 bits (16 patterns) 5-bit patterns with no more than 1 leading zero 5-bit patterns with no more than 2 trailing zero's 16 leftovers 7 not valid Others control signals 11111 (idle) 00000 (dead) 00100 (bad)

Framing We know how to transmit bits on a link between two nodes Now we need to figure out how to send distinct messages in frames (Think packets at the link layer) Why would we want to break up messages into frames instead of just a bit stream?

Framing Framing Protocols Framing Approaches Bi-sync HDLC PPP SONET Sentinel Approach Byte-counting approach

Sentinel Approach Use sentinel characters to designate where frames start and end Bi-sync frame format (IBM and mainframes) HEADER BODY SYN SYN SOH STX ETX CRC

Sentinel Approach SYN STX ETX SOH CRC Synchronization Start of Text End of Text SOH Start of Header (Why no EOH?) CRC Cyclic Redundancy Check

Character Stuffing How do you handle the situation where the body contains STX, ETX, SOH, etc? Escape out ETX with at Data Link Escape Character (DLE) Now, how do you deal with a body that has a DLE in it? Also known as character stuffing Examples in programming

Byte-counting Protocols Just like with C strings (and files), we can detect the end of the string (or file) in two ways Special character An extra length field Same is true in framing In the byte-counting approach, we detect the end of the frame with an extra “Count” field

Byte-counting Protocols DECNET DDCMP SYN: 8 CLASS: 8 Count:14 Header: 42 What happens if the count field gets corrupted? HEADER CLASS COUNT BODY SYN SYN CRC

Bit-oriented Protocols Unlike byte-oriented protocols, these protocols don’t care about bytes Could be transmitting ASCII (7-bits) Pixel values in an image …

Bit-oriented Protocols High-Level Data Link Control (HDLC) protocol Denotes beginning and end of a frame with the delimiter: 0 1 1 1 1 1 1 0 Also transmitted anytime link is idle to keep clocks synchronized Still has bit stuffing problem if special delimiter occurs in body 5 1's; zero ALWAYS follows in the body. Note that bit stuffing forces frames with the same payload length to have different realized sizes.

...and then there's Sonet Synchronous Optical Network standard Dominant standard for long-distance transmission of data over optical networks Every frame is exactly the same size! Has some special bit pattern to tell receiver where frame starts and ends, with no bit stuffing

Sonet How does the receiver know where each frame starts and ends? Receiver looks for it consistently (once every fixed number of bytes) Encoded using NRZ To combat NRZ clock recovery problem, XORs data to be transmitted to a well-known bit patten Can XOR encoded data with well-known bit pattern to decode