Physical Layer (OSI Layer 1)

The physical layer describes the transportation of raw bits over physical media. It defines signaling specifications and media types and interfaces. It also describes voltage levels, physical data rates, and maximum transmission distances. In summary, it deals with the electrical, mechanical, functional, and procedural specifications for links between networked systems.

Examples of physical layer specifications are

• EIA/TIA-232 (Electronic Industries Association/Telecommunications Industry Association)

• EIA/TIA-449

• V.35

• IEEE 802 LAN and metropolitan-area network (MAN) standards

• Physical layer (PHY) groups Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH)

• Maximum cable distances of the Ethernet standards

Data Link Layer (OSI Layer 2)

The data link layer is concerned with the reliable transport of data across a physical link. Data at this layer is formatted into frames. Data link specifications include frame sequencing, flow control, synchronization, error notification, physical network topology, and physical addressing. This layer converts frames into bits when sending information and converts bits into frames when receiving information from the physical media. Bridges and switches operate at the data link layer.

Because of the complexity of this OSI layer, the IEEE subdivides the data link layer into three sublayers for LANs. Figure C-2 shows how Layer 2 is subdivided. The upper layer is the logical link sublayer, which manages communications between devices. The bridging layer, defined by IEEE 802.1, is the middle layer. The lowest layer is the Media Access Control (MAC) sublayer, which manages the protocol access to the physical layer and ultimately the actual media. Systems attached to a common data link layer have a unique address on that data link layer. Be aware that you might find some references describing this layer as having two sublayers: the Logical Link Control (LLC) sublayer and the MAC sublayer.

Examples of data link layer technologies are

• Frame Relay

• ATM

• Synchronous Data Link Control (SDLC)

• High-Level Data Link Control (HDLC)

• Point-to-Point Protocol (PPP)

• Ethernet implementations (IEEE 802.3)

• Wireless LAN (IEEE 802.11)

Network Layer (OSI Layer 3)

The network layer is concerned with routing information and methods to determine paths to a destination. Information at this layer is called packets. Specifications include routing protocols, logical network addressing, and packet fragmentation. Routers operate at this layer.

Examples of network layer specifications are

• Protocol

  • IPv4, IPv6

  • ICMP

  • ARP

  • Connectionless Network Protocol (CLNP)

• Routing protocols

  • Routing Information Protocol (RIP)

  • Open Shortest Path First (OSPF)

  • Enhanced Interior Gateway Routing Protocol (EIGRP)

  • Intermediate System-to-Intermediate System (IS-IS)

Transport Layer (OSI Layer 4)

The transport layer provides reliable, transparent transport of data segments from upper layers. It provides end-to-end error checking and recovery, multiplexing, virtual circuit management, and flow control. Messages are assigned a sequence number at the transmission end. At the receiving end, the packets are reassembled, checked for errors, and acknowledged. Flow control manages the data transmission to ensure that the transmitting device does not send more data than the receiving device can process.

Examples of transport layer specifications are

• Transmission Control Protocol (TCP)

• Real-Time Transport Protocol (RTP)

• User Datagram Protocol (UDP)

Session Layer (OSI Layer 5)

The session layer provides a control structure for communication between applications. It establishes, manages, and terminates communication connections called sessions. Communication sessions consist of service requests and responses that occur between applications on different devices.

Examples of specifications that operate at the session layer are

• DECnet’s Session Control Protocol (SCP)

• H.245 and H.225

Presentation Layer (OSI Layer 6)

The presentation layer provides application layer entities with services to ensure that information is preserved during transfer. Knowledge of the syntax selected at the application layer allows selection of compatible transfer syntax if a change is required. This layer provides conversion of character-representation formats, as might be required for reliable transfer. Voice coding schemes are specified at this layer. Furthermore, compression and encryption can occur at this layer.

An example of a specification that operates at the presentation layer is Abstract Syntax Notation 1 (ASN.1).

Application Layer (OSI Layer 7)

The application layer gives the user or operating system access to the network services. It interacts with software applications by identifying communication resources, determining network availability, and distributing information services. It also provides synchronization between the peer applications residing on separate systems.

Examples of application layer specifications are

• Telnet

• File Transfer Protocol (FTP)

• Simple Mail Transfer Protocol (SMTP)

• Simple Network Management Protocol (SNMP)

• Network File System (NFS)

• Border Gateway Protocol (BGP)

Network Interface Layer

The TCP/IP network interface layer (also known as network access layer) maps to the OSI data link and physical layers. TCP/IP uses the lower-layer protocols for transport.

Internet Layer

The Internet layer is where IP resides. IP packets exist at this layer. It directly maps to the network layer of the OSI model. Other TCP/IP protocols at this layer are Internet Control Message Protocol (ICMP), Address Resolution Protocol (ARP), and Reverse ARP (RARP).

Host-to-Host Transport Layer

The host-to-host transport layer of TCP/IP provides two connection services: TCP and UDP. TCP provides reliable transport of IP packets, and UDP provides transport of IP packets without verification of delivery. This layer maps to the OSI transport layer, but the OSI model only defines reliable delivery at this layer.

Application Layer

The TCP/IP application layer maps to the top three layers of the OSI model: application, presentation, and session. This layer interfaces with the end user and provides authentication, compression, and formatting. The application protocol determines the data’s format and how the session is controlled. Examples of TCP/IP application protocols are Telnet, FTP, BGP, and Hypertext Transfer Protocol Secure (HTTPS).

Example of Layered Communication

Suppose that you use a Telnet application. Telnet maps to the top three layers of the OSI model. In Figure C-4, a user on Host 1 enables the Telnet application to access a remote host (Host 2). The Telnet application provides a user interface (application layer) to network services. As defined in RFC 854, ASCII is the default code format. No session layer is defined for Telnet (not an OSI protocol). Per the RFC, Telnet uses TCP for connectivity (transport layer). The TCP segment is placed in an IP packet (network layer) with a destination IP address of Host 2. The IP packet is placed in an Ethernet frame (data link layer), which is converted into bits and sent onto the wire (physical layer).

When the frame arrives at Router 1, it converts the bits into a frame; removes the frame headers (data link); checks the destination IP address (network); places a serial link header on the packet, making it a serial frame; and forwards the frame to the serial link (data link), which sends it as bits.

Router 2 receives the bits and converts them into a frame; removes the serial encapsulation headers; checks the destination IP address (network); adds an Ethernet header to the packet, making it a frame; and places the frame on Ethernet 2 (data link). Host 2 receives bits (physical) from the Ethernet cable and converts the bits into a frame (data link). Then the IP protocol is examined and the packet data is forwarded to TCP, which checks the segment number for errors and then forwards the segment to TCP port 23 (Telnet), which is the application.

Hexadecimal Numbers

The hexadecimal numeric system uses 16 digits instead of the 10 digits used by the decimal system. Table C-1 shows the hexadecimal digits and their decimal equivalent values.

Hexadecimal Representation

It is common to represent a hexadecimal number with 0x before the number so that it is not confused with a decimal number. The hexadecimal number of decimal 16 is written as 0x10, not 10. Another method is to put a subscript h to the right of the number, such as 10_h. It is also common to use the term hex when speaking of hexadecimal. Much of the following text uses hex.

Converting Decimal to Hexadecimal

First things first: Memorize Table C-1. There are two ways to convert larger numbers. The first method is to convert decimal to binary and then convert binary to hex. The second method is to divide the decimal number by 16—the residual is the rightmost hexadecimal digit—and then keep dividing until the number is not divisible anymore. For the first method, use the schemes described in later sections. For the second method, follow the examples described here.

First, divide the decimal number by 16. The remainder of the division is the least-significant (first) hexadecimal digit. Continue to divide the quotients (answer) of the divisions by 16 until the quotient is 0. The remainder value of each later division is converted to a hexadecimal digit and prepended to the previous value. The final remainder is the most-significant digit of the hexadecimal equivalent. For large numbers, you might have to divide many times. The following examples make the process clearer.

Conversion Example C-1 Convert 26 to Its Hex Equivalent

Conversion Example C-2 Convert 96 to Its Hex Equivalent

Conversion Example C-3 Convert 375 to Its Hex Equivalent

Conversion Example C-4 Convert 218 to Its Hex Equivalent

Converting Hexadecimal to Decimal

To convert a hex number to decimal, take the rightmost digit and convert it to decimal (for example, 0xC = 12). Then add this number to the second rightmost digit multiplied by 16 and the third rightmost digit multiplied by 256. Do not expect to convert numbers larger than 255 on the CCNP exam, because the upper limit of IP addresses in dotted-decimal format is 255 (although Token Ring numbers reach 4096). Some examples follow.

Conversion Example C-5 Convert 177_h to Decimal

Conversion Example C-6 Convert 60_h to Decimal

Conversion Example C-7 Convert 100_h to Decimal

Conversion Example C-8 Convert 1DA_h to Decimal

Alternative Method for Converting from Hexadecimal to Decimal

Another way is to convert from hex to binary and then from binary to decimal. The following sections discuss converting from binary to decimal.

Binary Numbers

The binary number system uses two digits: 1 and 0. Computer systems use binary numbers. IP addresses and MAC addresses are represented by binary numbers. The number of binary 1s or 0s is the number of bits, short for binary digits. For example, 01101010 is a binary number with 8 bits. An IP address has 32 bits, and a MAC address has 48 bits. As shown in Table C-2, IPv4 addresses are usually represented in dotted-decimal format; therefore, it is helpful to know how to convert between binary and decimal numbers.

MAC addresses are usually represented in hexadecimal numbers; therefore, it is helpful to know how to convert between binary and hexadecimal.

The CCNP candidate should memorize Table C-3, which shows numbers from 0 to 16 in decimal, binary, and hexadecimal formats.

Converting Binary to Hexadecimal

To convert binary numbers to hex, put the bits in groups of 4, starting with the right-justified bits. Groups of 4 bits are often called nibbles. Each nibble can be represented by a single hexadecimal digit. A group of two nibbles is an octet, 8 bits. Examples follow.

Conversion Example C-9 Convert 0010011101 to Hex

Conversion Example C-10 Convert 0010101001011001000010110001 to Hex

Converting Hexadecimal to Binary

This procedure is also easy. Just change the hex digits into their 4-bit equivalents. Examples follow.

Conversion Example C-11 Convert 0DEAD0 to Hex

Conversion Example C-12 Convert AA0101 to Hex

Converting Binary to Decimal

To convert a binary number to decimal, multiply each instance of 0 or 1 by the power of 2 associated with the position of the bit in the binary number. The first bit, starting from the right, is associated with 2⁰ = 1. The value of the exponent increases by 1 as each bit is processed, working leftward. As shown in Table C-4, each bit in the binary number 10101010 has a decimal equivalent from 0 to 128 based on the value of the bit multiplied by a power of 2 associated with the bit position. This is similar to decimal numbers, in which the numbers are based on powers of 10: 1s, 10s, 100s, and so on. In decimal, the number 111 is (1*100) + (1*10) + (1*1). In binary, the number 11111111 is the sum of (1*2⁷) + (1*2⁶) + (1*2⁵) + (1*2⁴) + (1*2³) + (1*2²) + (1*2¹) + (1*2⁰) = 128 + 64 + 32 + 16 + 8 + 4 + 2 + 1 = 255. For 10101010, the result is 128 + 0 + 32 + 0 + 8 + 0 + 2 + 0 = 170. Examples follow.

Conversion Example C-13 Convert 10110111 to Decimal

Conversion Example C-14 Convert 00011011 to Decimal

Conversion Example C-15 Convert 11111111 to Decimal

Converting Decimal to Binary Numbers

This procedure is similar to converting from hex to decimal (by dividing), but now you divide the decimal number by 2. You use each residual to build the binary number by prepending each residual bit to the previous bit, starting on the right. Repeat the procedure until you cannot divide anymore. The only problem is that for large numbers, you might have to divide many times. You can reduce the number of divisions by first converting the decimal value to a hexadecimal value and then converting the intermediate result to the binary representation. After the following example, you will read about an alternate method suitable for use with decimal values between 0 and 255 that can be represented in a single octet.

Conversion Example C-16 Convert 26 to Binary

Alternative Method for Converting from Decimal to Binary

The dividing procedure just described works; it just takes a lot of time. Another way is to remember the bit position values within a byte—128, 64, 32, 16, 8, 4, 2, 1—and play with the bits until the sum adds up to the desired number. This method works when you convert integer values between 0 and 255, inclusive. Table C-5 shows these binary numbers and their decimal values.

For example, to convert 26, you know that it is a number smaller than 128, 64, and 32, so those 3 bits are 0 (000?????). Now you need to find a combination of 16, 8, 4, 2, and 1 that adds up to 26. This method involves using subtraction to compute the remaining number. Start with the largest number, and make the bit at 16 a 1 (0001????). The difference between 26 and 16 is 10. What combination of 8, 4, 2, and 1 gives 10? 1010. Therefore, the answer is 00011010. You might think this method involves too much guesswork, but it becomes second nature after some practice.

Conversion Example C-17 Convert 137 to Binary

Conversion Example C-18 Convert 211 to Binary

In addition to remembering the bit-position values (128, 64, 32, 16, 8, 4, 2, 1), it helps to remember network subnet mask values. Remembering them makes it easier to figure out whether you need to enable a bit. Table C-6 summarizes the binary subnet mask numbers and their decimal values.