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

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

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

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

Serial Data Communication (cont’d.) Example: ASCII character lower case I ASCII code is 69 hexadecimal (1101001 binary) Figure 7-1 11-bit transmission code Computer Networking From LANs to WANs: Hardware, Software, and Security

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Troubleshooting Techniques Protocol information sources http://www.protocols.com http://www.whatis.com Computer Networking From LANs to WANs: Hardware, Software, and Security

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