The OSI Reference Model 21
Examples of data-link layer technologies are the following:
•Frame Relay
•Asynchronous Transport Mode (ATM)
•Synchronous Data Link Control (SDLC)
•High-level data-link control (HDLC)
•IEEE 802.3z and IEEE 802.3ab (Gigabit Ethernet)
•IEEE 802.3u (Fast Ethernet)
•Ethernet version 2
•Integrated Services Digital Network (ISDN)
•Point-to-Point Protocol (PPP)
•Token Ring
•Spanning-Tree Protocol (STP)
Network Layer (OSI Layer 3)
The network layer is concerned with the routing of 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 in this layer.
The CCIE lives and dies in this layer.
Examples of network layer specifications are the following:
•Internet Protocol (IP)
•Routing Information Protocol (RIP)
•Open Shortest Path First (OSPF)
•Enhanced Interior Gateway Routing Protocol (EIGRP)
•Internetwork Packet Exchange (IPX)
•Connectionless Network Protocol (CLNP)
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
22 Chapter 2: Networking Concepts Review
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 the following:
•Transmission Control Protocol (TCP)
•Real-Time Transport Protocol (RTP)
•Sequenced Packet Exchange (SPX)
•AppleTalk’s Transaction Protocol (ATP)
•User Datagram Protocol (UDP) (provides unreliable transport at this layer with less
overhead than TCP)
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. Communi-
cation sessions consist of service requests and responses that occur between applications on dif-
ferent devices. The management of sessions involves the synchronization of dialog control by
using checkpoints in the data stream.
Examples of specifications that operate at the session layer are the following:
•NetBIOS
•Real-Time Control Protocol (RTCP)
•Session Control Protocol (SCP)
•AppleTalk’s Zone Information Protocol (ZIP)
•DECnet’s Session Control Protocol (SCP)
•H.323, H.245, H.225
•Real-Time Control Protocol (RTCP)
Presentation Layer (OSI Layer 6)
The presentation layer provides data representation with a variety of coding and conversion
functions. These functions ensure that data sent from a sending application on one system is
readable by the application layer on another system. This layer provides the conversion of
character representation formats, data compression schemes, and encryption schemes. Voice
coding schemes are specified at this layer.
The OSI Reference Model 23
Examples of specifications that operate at the presentation layer are the following:
•Abstract Syntax Notation 1 (ASN.1)
•ASCII
•EBCDIC
•Motion Picture Experts Group (MPEG)
•Graphics Interchange Format (GIF)
•Joint Photographic Experts Group (JPEG)
•Tagged Image File Format (TIFF)
•G.711, G.729a, G.726, G.728
Application Layer (OSI Layer 7)
The application layer provides 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 that reside on separate systems.
Examples of application layer specifications are the following:
•Telnet
•File Transfer Protocol (FTP)
•Simple Mail Transfer Protocol (SMTP)
•Simple Network Management Protocol (SNMP)
•Network File System (NFS)
•Association Control Service Element (ACSE)
•Remote Operations Service Element (ROSE)
Example of Layered Communication
If you use a Telnet application, Telnet maps into the top three layers of the OSI model. Figure
2-3 shows that 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 Request For Comments (RFC) 854, ASCII is the default code format used (presen-
tation layer). There is no session layer defined for Telnet; it is not an OSI protocol. Per the RFC,
Telnet uses TCP for connectivity (transport layer). The TCP segment gets placed into an IP
packet (network layer) with a destination IP address of Host 2. The IP packet gets placed into
an Ethernet frame (data-link layer), which is converted into bits, and sent onto the wire (phys-
ical layer).
24 Chapter 2: Networking Concepts Review
Figure 2-3 Telnet Example
When the frame arrives to 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 to the packet,
which makes it a serial frame, and forwards the frame to the serial link (data link), which sends
it as bits.
Router 2 receives the bits, converts to a frame, removes the serial encapsulation headers, checks
the destination IP address (network), adds an Ethernet header to the packet, which makes it a
frame, and places a 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
forwards the segment to TCP port 23 (Telnet), which is the application.
Numeric Conversion
This section focuses on the techniques used to convert between decimal, binary, and hexadecimal
numbers. Although there might not be a specific question on the exam that asks you to convert
a binary number to decimal, you need to know how to convert these numbers to complete
problems on the test. A diagram might show a Token Ring with a decimal number but the RIF
might be shown as hexadecimal. An IP address might be shown as binary or in dotted decimal
format. Some show commands have output information in hexadecimal or binary formats. As
a CCIE candidate, you must be prepared.
Router 1 Serial Link
Host 2
Host 1
Router 2
Ethernet 1 Ethernet 2
DataTCP
DataTCPIP Packet
Segment
DataTCPIP802.3
DataTCPIP DataTCPIPHDLC DataTCPIP
DataTCPIP802.3
DataTCP
DataTCPIP Packet
Segment
Destination IP? Destination IP?
Numeric Conversion 25
Hexadecimal Numbers
The decimal numeric system that is commonly used has 10 numeric digits, 0 through 9. After
9, you use 2 digits starting at 10 and then cycle the right-most digits from 0 through 9 again.
The hexadecimal numeric system follows this same concept, but instead of 10 digits, there are
16 digits. Table 2-1 shows the hexadecimal digits and their decimal equivalent.
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 use an h subscript to the right of the number, such as 10h. It is also
common to use the term hex when speaking of hexadecimal. You will use hex in much of the
text that follows.
Table 2-1 Hexadecimal Digits
Hexadecimal Digits Decimal Value
00
11
22
33
44
55
66
77
88
99
A10
B11
C12
D13
E14
F15
10 16
