ISDS 409 Exam 2 (chapter 4 & 5)
Amplifier
- Boost both noise and signal for transmission - Typically used on analog circuits
Error Rate
- Example error rate: 1 in 500,000 meaning there is 1 bit in error for every 500,000 bits transmitted
Polling
- Process of transmitting to a client only if asked or permitted - Types of Polling 1. Roll call 2. Hub or Token
Hamming Code
- an example of forward error correction
IPv4 decimal equivalent example
- 128.192.56.1
IPv4 Header
- 192 Bits (24 Bytes) - uses 32 bit addresses
IPv6 Header
- 320 bits (40 Bytes) - uses 128 bits addresses - Flow name: packets are dedicated by name
Asynchronous Transmission Efficiency
- 7-bit ASCII (info bits), 1 parity bit, 1 stop bit, 1 start bit - Transmission Efficiency = 7/10 or 70% - e.g., V.92 modem with 56 Kbps 39.2 Kbps effective rate
Controlled Access Transmission Graphical Display
- A linear function, graph depicts a steady growth of traffic with response time - better with a network of 20+ computers
Network Layer is responsible for?
- Addressing and routing of messages - Selects the best path from computer to computer until the message reaches destination
Network Layer Functions
- Addressing: Assignment of (internet) addresses - Routing: Process of deciding what path a packet must take to reach destination
SDLC Transmission Efficiency
- Assume 100 info characters (800 bits), 2 flags (16 bits), Address (8 bits), Control (8 bits), CRC (32 bits) - Transmission Efficiency = 800/864 or 92.6% e.g., V.92 modem with 56 Kbps 51.9 Kbps effective rate
Checksum
- Calculated by adding decimal values of each character in the message then dividing the total by 255 then saving the remainder, typically of 1 byte and using it as the checksum - If the sender and receiver checksum values are equal, the message is presumed to contain no errors.
Cyclic Redundancy Check (CRC)
- Computed by calculating the remainder to a division problem. Similar to checksum technique but contain a different fixed number - Most powerful and most common
Logical Link Control (LLC) (top sublayer of Data Link)
- Connects to and communicates with network layer above 1. Transmitting end LLC: surround the PDU with the Ethernet frame. 2. Receiving End LLC: removes the data link layer PDU and passes the message it contains to Network Layer
Roll Call Polling
- Controller check each client periodically to see if it wants to transmit - Clients can also be prioritized so that they are polled more frequently.
Media Access Control (MAC) (bottom sublayer of Data Link)
- Controls the physical hardware 1. Transmitting End: takes data link layer PDU from LLC and convert into street of bits and control when to transmit those bits 2. Receiving End: receives bits from physical layer and translate into coherent PDU. Then passes the data link layer PDU to LLC
Data Link Protocols: Synchronous Transmission
- Data sent in a large block called a Frame - Includes a series of synchronization (SYN) characters. Used to help receiver recognize incoming data
Flow Control with ARQ: Continuous
- Ensuring that sender is not transmitting too quickly for the receiver - Both sides agree on the size of the "sliding window" - or number of messages that can be handled by receiver without causing significant delays
Flow Control with ARQ: Stop n Wait
- Ensuring that sender is not transmitting too quickly for the receiver - Receiver sends an ACK or NAK when it is ready
Synchronous Transmission Sub-Protocols: SDLC
- Frame begins and ends with a special bit pattern, known as the flag. -The address field identifies the destination. - The length of the address field is usually 8 bits but can be set at 16 bits; all computers on the same network must use the same length.
Functions of Transport Layer
- Linking to the Application Layer - Segmenting - Session management
Transmission Control Protocol (TCP)
- Links the app layer to the network layer - Performs segmenting and reassembly - 192 bits (24 bytes)
Synchronous Transmission Sub-Protocols: Ethernet II
- Marking the end of a frame with bipolar signaling, send 1s and 0s
Functions of a Data Link Layer
- Media Access Control - Error Control - Message Delineation
Implications for Management: Data Link Layer
- Minimize costly customization - Minimize costly translation among many protocols - Less training, simpler network management
TRIB efficiency
- Number of info bits accepted / total time required to get the bits - (Number of info bits) (Prob. of successful transmission) / (Time it takes to transmit these bits) + (Propagation delay)
Network Errors
- Occur during transmissions, controlled by network hardware and software - 2 categories of network error: 1. Corrupted data - data changed from what it was 2. Lost data - cannot locate the data at all.
Parity Checking
- Oldest and simplest error detection. - add a parity bit to the message, then the receiving end will check by calculating the parity bit (1 bit) to see if the message ends with an odd number of 1s or even number of 1s - can catch odd number of errors but miss even number in data block
Hub/Token Polling
- One computer starts the poll. 1. sends message (if any) 2. passes the token on to the next computer
Functions of TCP/IP (Transmission Control Protocol/Internet Protocol)
- Performs error checking - Transmits large files with end-to-end delivery assurance - Used by the Internet - Used by almost all backbone and wide area networks - Most commonly used protocol on LANs
User Datagram Protocol
- Protocol used when sender needs to send a single small packets to the receiver (e.g., DNS request), faster - Uses a small header ony 8 bytes containing 4 fields. 1. Source port 2. Destination port 3. Length 4. CRC-16 - Does not check for lost messages
Repeater
- Regenerate a clean signal for transmission. - Typically used on digital circuits
Case 3: Unknown Address
- Sends a DNS request (encapsulated by a UDP datagram) to the local DNS server
Automatic Repeat reQuest (ARQ) Retransmission
- Simplest, most effective, least expensive - 2 different types of ARQ 1. Stop N Wait 2. Continuous ARQ
IPv4 addressing
- The first n bits of an IP address represent the specific network - The last (32 - n) bits of the IP address identify the host on that specific network - These (32 - n) bits are often called the "host portion" - 2 ^ (32-n)
Contention transmission
- Transmit whenever the circuit is free. Prone to collisions when more than one computer tries to transmit at the same time. -used in Ethernet LANs
Case 2: Known Address, Different Subnet
- Use subnet mask to determine that the destination is NOT on the same subnet
Synchronous Transmission Sub-Protocols: Point-to-Point
- Used on dial-up lines from home PCs
Synchronous Transmission Sub-Protocols: HDLC
- Uses controlled access MAC - Basis for many other Data Link protocols - Longer addresses than SDLC -Larger sliding window size
Synchronous Transmission Sub-Protocols: Ethernet 802.3ac
- VLan tag
Contention Transmission Graphical Display
- an exponential function, graph depicts an exponential growth of traffic with response time - better with a network of 20 computers or less
Access Request
- client computers that want to transmit send a request to transmit to the device that is controlling the circuit - similar to a classroom situation in which the instructor calls on the students who raise their hands.
Controlled Access Transmission
- controls the circuit and determines which clients can transmit at what time. - there are 2 controlled access techniques 1. access request 2. polling
Centralized routing
- decisions made by 1 central comp. - used on small, host-based networks
decentralized routing
- decisions made by each router independently of one another - used on the internet
IP address
- does not change from end to end. Source and destination addresses stay the same so message communication can be send back and forth to the correct source/destination.
Data Link Protocols: Asynchronous Transmission
- each character is transmitted independently of all other characters. - start and stop bit are the opposite of each other. Typically, the start bit is a 0 and stop is a 1.
burst error
- errors are not uniformly distributed in time e.g. 1 in 500k is 100 bits error for every 50,000,000 bits transmitted
attenuation
- lose of power a signal suffers as it travels from transmitting computer to receiving computer.
Line noise and distortion
- major cause for errors in data transmission - manifestations: extra bits, flipped bits, and missing bits
Data Link Layer is responsible for?
- moving messages from one device to another - control the way messages are sent on the physical media.
Implications for Management: Transport & Network Layer
- moving toward a single standard based on TCP/IP - decreased cost of buying and maintaining network
Qos Routing
- provided RTP - Defines and assigns priorities to "classes of service" - Timely delivery of packets is important - Real-time app: - VoIP (high priority) - E-mail (low priority)
Connection-Oriented Messaging
- provided by TCP - TCP connection also called a session - packet deliveries are acknowledged
Connectionless Messaging
- provided by UDP - each packet is sent independently, more streamline and faster than TCP
Continuous ARQ (full duplex)
- sender does not wait for ACK after sending a message, it immediately sends the next on. - While transmitting messages, sender examines stream of returning ACK, if it receives an NAK, the sender retransmits the needed messages.
Stop N Wait ARQ (half duplex)
- sender stops and waits for a response from receiver after each data packet
errors prevention
- shielding cables - moving cables away from noise sources - install amplifiers or repeaters to prevent attenuation
How can TCP serve several App Layer protocols at the same time?
- standardized port numbers or addresses
Case 1: Known Address, Same subnet
- subnet mask to see that the destination is on the same subnet as itself
Throughput
- total number of info bits received per sec, after taking into account the overhead bits and the need to transmit frames contains errors. - More accurate definition of efficiency - Uses TRIB
Human errors
- typo of a number, controlled through the application programs
Dynamic routing algorithms: link state dynamic routing
- uses a variety of information types to decide how to route a packet (more sophisticated) - e.g., # of hops, congestion, speed
Dynamic routing algorithms: distance vector dynamic routing
- uses the least number of hops to decide how to route a packet
MAC address
- will change from node to node because it is device specific
IPv6 decimal equivalent example
2001:0890:0600:00d1:0000:0000:abcd:f010 convert to 2001:890:600:d1: :abcd:f010 - compressed notation eliminates leading 0s within each block and blocks that are all 0s
