1. Chapter 7 Low-Level Protocols
Computer Networking From LANs to WANs: Hardware, Software, and Security
Chapter 7
Low-Level Protocols 1

2. Objectives Describe the format of a serial data transmission
List the differences between SLIP and PPP
Explain the operation of the Logical Link Control sublayer
Discuss the role of NetBEUI, NetBIOS, and NetBIOS over TCP/IP in a Windows network
Computer Networking From LANs to WANs: Hardware, Software, and Security

3. Serial Data Communication
Communication over single communication line
One bit at a time (least significant bit first)
11-bit transmission waveform standard
Used for transmitting 7-bit ASCII characters
First bit: start bit (always low)
Identifies beginning of new transmission
Next seven bits: ASCII code
Last data bit: parity bit
Last two transmission frame bits: stop bits (always high)
May be one, one and one-half, two stop bits
Computer Networking From LANs to WANs: Hardware, Software, and Security

4. Serial Data Communication (cont’d.)
Parity bits
Determine error in the received character
Odd parity
Occurs if odd number of 1s
Even parity
Occurs if even number of 1s
Transmission error:
Received parity not in line with expected parity
Computer Networking From LANs to WANs: Hardware, Software, and Security

5. Serial Data Communication (cont’d.)
Example: ASCII character lower case I
ASCII code is 69 hexadecimal ( binary)
Figure bit transmission code
Computer Networking From LANs to WANs: Hardware, Software, and Security

6. Serial Data Communication (cont’d.)
Each waveform bit takes same amount of time
Related to baud rate of the serial transmission
Baud rate
Generally: number of bits per second in a transmission
Actually: number of transitions per second
Advantages
Simple connection requirements
Single transmit wire, single receive wire (plus ground)
Asynchronous communication not possible
Start bit used to synchronize transmitter and receiver
Computer Networking From LANs to WANs: Hardware, Software, and Security

7. Serial Data Communication (cont’d.)
UARTs (universal asynchronous receiver transmitter)
Digital devices handling serial data transmission
Parallel input data converted to serial output data
Serial input data converted into parallel output data
Figure 7-2 The UART
Computer Networking From LANs to WANs: Hardware, Software, and Security

8. SLIP First protocol to transmit TCP/IP over dial-up lines
Provides basic method to encapsulate TCP/IP data
Not provided
Error detection
Internet Protocol (IP) address assignments
Link testing
Synchronous communication
Transmission protocols other than TCP/IP
Compressed SLIP (CSLIP) available
Replaced Point-to-Point Protocol (PPP)
Computer Networking From LANs to WANs: Hardware, Software, and Security

9. PPP Provides serial connection encapsulation PPP frame provides:
TCP/IP, IPX, NetBEUI, AppleTalk, other protocols
PPP frame provides:
Error detection
Checksum value tests data validity
Ability to assign IP addresses
As needed during session duration
Link testing
Mechanism to periodically test PPP link operation status
Computer Networking From LANs to WANs: Hardware, Software, and Security

10. PPP (cont’d.) PPP frame provides: (cont’d.)
Synchronous, asynchronous communication modes
Security (user name, password authentication)
Password Authentication Protocol (PAP)
Challenge Handshake Authentication Protocol (CHAP)
Compression
Operates on layers 1 (Physical) and 2 (Data-Link)
Figure 7-3 PPP frame format
Computer Networking From LANs to WANs: Hardware, Software, and Security

11. PPP (cont’d.) Table 7-1 Comparing SLIP and PPP
Computer Networking From LANs to WANs: Hardware, Software, and Security

12. PPPoE Allows multiple LAN users to share a single Internet connection
Encapsulates PPP within an Ethernet frame
User establishes PPPoE session through the single ISP Internet connection
Each PPPoE user session can be monitored (billing)
Computer Networking From LANs to WANs: Hardware, Software, and Security

13. PPPoE (cont’d.) Code field: PPPoE frame type (discovery, session)
Session ID, Ethernet frame’s source and destination MAC addresses
Uniquely identify PPPoE session
Length field: payload size or user data, sent
Figure 7-4 PPPoE frame format
Computer Networking From LANs to WANs: Hardware, Software, and Security

14. Logical Link Control Data-Link layer contains two parts
Figure 7-5 Data-Link layer details
Computer Networking From LANs to WANs: Hardware, Software, and Security

15. Logical Link Control (cont’d.)
Three types of service:
Type 1: Connectionless communication (unreliable)
Type 2: Connection-oriented communication (reliable)
Type 3: Acknowledged connectionless communication
LLC protocol:
Based on HDLC (High-Level Data-Link Control)
Provides mechanism for sending commands and responses over a communication link
Computer Networking From LANs to WANs: Hardware, Software, and Security

16. Logical Link Control (cont’d.)
DSAP and SSAP fields
Indicate how payload interpreted
SNAP (subnetwork access protocol) payload use
When high-level protocol handled by LLC sublayer
Figure 7-6 Logical Link Control PDU
Computer Networking From LANs to WANs: Hardware, Software, and Security

17. Logical Link Control (cont’d.)
MAC sublayer
Interfaces LLC sublayer with different network technologies
LLC sublayer does not know details
Allows the same LLC sublayer to work with different network technologies
Additional responsibilities
Controlling access to network media
Recovering from errors
Addressing (working with MAC addresses)
Computer Networking From LANs to WANs: Hardware, Software, and Security

18. NetBIOS
Provides all functionality to share resources between networked computers
Utilizes three types of services:
Name: Finding and naming machines
Session: Connection-oriented reliable transfer of messages
Datagram: Connectionless non-reliable datagram transfer
NetBIOS message main component
SMB (server message block)
Provide all functionality possible under NetBIOS
Computer Networking From LANs to WANs: Hardware, Software, and Security

19. Table 7-2 Sample NetBIOS commands
NetBIOS (cont’d.)
Table 7-2 Sample NetBIOS commands
Computer Networking From LANs to WANs: Hardware, Software, and Security

20. NetBIOS (cont’d.) Machine names
NetBIOS names that identify each machine
Up to 15 characters (letters, numbers, limited set of symbols)
Figure 7-7 Windows 9x Network Identification display
Figure 7-8 Invalid NetBIOS computer
name error message
Computer Networking From LANs to WANs: Hardware, Software, and Security

21. NetBEUI Transport mechanism to deliver NetBIOS messages over a LAN
Does not conform to OSI model
Uses Transport, Network, LLC part of Data-Link
Not a routable protocol
Uses MAC addresses to specify source, destination computers
Only used on small networks
Windows Internet Name Service (WINS)
Maps NetBIOS names to IP addresses
Computer Networking From LANs to WANs: Hardware, Software, and Security

22. NetBEUI (cont’d.) NetBIOS over TCP/IP
NetBIOS messages transported using TCP
Used to share resources over a WAN
View Network properties window
NETSTAT displays active NetBIOS ports
Some may cause computer vulnerability
Table 7-3 NetBIOS over TCP/IP port assignments
Computer Networking From LANs to WANs: Hardware, Software, and Security

23. Figure 7-9 Active network connections on a Windows system
Computer Networking From LANs to WANs: Hardware, Software, and Security

24. Figure 7-10(a) Network Properties showing
protocol bindings on a Windows 9x computer
Computer Networking From LANs to WANs: Hardware, Software, and Security

25. NetBEUI (cont’d.) Communication channel
Formed between protocol and adapter
When protocol bound to network adapter
Protocol cannot use an adapter until it is bound to it
Multiple protocols
May be bound to single adapter or multiple adapters
NetBEUI originally used by IBM LAN Manager network
Now adapted by Microsoft for use in Windows
Computer Networking From LANs to WANs: Hardware, Software, and Security

26. Figure 7-10(b) Windows 9x NetBEUI Properties window
Computer Networking From LANs to WANs: Hardware, Software, and Security

27. Figure 7-10(c) Windows 9x NetBEUI Properties window
Computer Networking From LANs to WANs: Hardware, Software, and Security

28. Figure 7-11 NetBEUI Properties windows on Windows 2003 server
Computer Networking From LANs to WANs: Hardware, Software, and Security

29. Troubleshooting Techniques
Protocol information sources
Computer Networking From LANs to WANs: Hardware, Software, and Security

30. Summary Low-level protocols Serial data communication
Establish serial communication
Exchange data over different hardware technologies
Provide peer-to-peer communication
Serial data communication
One bit at a time over a single communication line
Parity bits help determine errors in the received waveform
Many protocols available
SLIP, PPP, PPPoE, NetBIOS, NetBEUI
Computer Networking From LANs to WANs: Hardware, Software, and Security