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Figure 7.1 |
An illustration of one reason computer networks use packets. While one pair of computers communicate, others must wait. |
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Figure 7.2 |
Illustration of multiplexing with packets. The sources take turns using the shared communication channel. (a) Computer 1 uses the resource to send a packet, and then (b) computer 2 uses the resource to send a packet. |
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Figure 7.3 |
An example frame that uses character soh to mark the beginning of the frame and eot to mark the end. The format is simple and unambiguous -- a receiver can tell when the entire frame has arrived, even if there are delays between characters. |
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Figure 7.4 |
An example of byte stuffing. For each occurrence of a character listed in the left column in the data, the sender transmits the two characters in the right column. |
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Figure 7.5 |
Illustration of byte stuffing, where (a) is an example of data that includes characters such as soh, and (b) is the frame after byte stuffing. The dashed lines show the locations in the original data where characters have been replaced or new characters added. |
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Figure 7.6 |
An example 16-bit checksum computation for a string of 12 ASCII characters. Characters are grouped into 16-bit quantities, added together using 16-bit arithmetic, and the carry bits are added to the result. |
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Figure 7.7 |
Illustration of how a checksum can fail to detect transmission errors. Reversing the value of the second bit in each data item produces the same checksum. |
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Figure 7.8 |
(a) A diagram of hardware that computes an exclusive or, and (b) the output value for each of the four combinations of input values. Such hardware units are used to calculate a CRC. |
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Figure 7.9 |
A shift register (a) before and (b) after a shift operation. During a shift, each bit moves left one position, and the output becomes equal to the leftmost bit. |
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Figure 7.10 |
A diagram of the hardware used to compute a CRC. After bits of a message have been shifted into the unit, the shift registers contain the 16-bit CRC for the message. |
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Figure 7.11 |
A modification of the frame format from Figure 7.3 that includes a 16-bit CRC. |
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Figure 8.7 |
Conceptual flow of bits across an Ethernet. While transmitting a frame, a computer has exclusive use of the cable. |
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Figure 8.9 |
The conceptual flow of bits during a transmission on a token ring network. Except for the sender, computers on the network pass bits of the frame to the next station. The destination makes a copy. |
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Figure 9.1 |
Organization of the hardware in a computer attached to a LAN. Because it is powerful and independent, the network interface hardware does not use the CPU when transmitting or receiving bits of a frame. |
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Figure 9.2 |
The general format of a frame sent across a LAN. The header contains information such as the addresses of the sender and the recipient. |
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Figure 9.3 |
Illustration of the frame format used with Ethernet. The number in each field gives the size of the field measured in 8-bit octets. |
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Figure 9.4 |
Examples of frame types used with Ethernet (type values are given in hexadecimal). The table lists only a few examples; many other types have been assigned. |
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Figure 9.5 |
Illustration of how type information can be included in a frame's data area if the frame header does not include a type field. .SX "frame" "type |
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Figure 9.6 |
An example of the 8-octet IEEE LLC/SNAP header, which is used to specify the type of data. The SNAP portion specifies an organization and a type defined by that organization. |
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Figure 11.4 |
Six computers connected to a pair of bridged LAN segments. The bridge, which uses the same type of connection as a computer, always sends and receives complete frames. |
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Figure 11.6 |
A bridge connecting LAN segments in two buildings. An optical fiber is used to connect the bridge to a remote LAN segment. |
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Figure 11.9 |
An example of bridges connected in a cycle. A problem occurs if all bridges forward broadcast frames. |
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Figure 12.6 |
Illustration of an STS-1 SONET frame with 810 octets divided into 9 rows of 90 columns. Octets at the beginning of each row provide clock synchronization and maintenance information. |
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Figure 13.1 |
A packet switch with two types of I/O connectors: one type is used to connect to other packet switches, and the other is used to connect to computers. |
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Figure 13.2 |
A small WAN formed by interconnecting packet switches. Connections between packet switches usually operate at a higher speed than connections to individual computers. |
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Figure 13.3 |
Example of hierarchical addresses in a WAN. Each address consists of two parts: the first part identifies a packet switch, and the second part identifies a computer connected to the switch. |
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Figure 13.4 |
(a) A network consisting of three packet switches, and (b) the next-hop forwarding information found in switch 2. Each switch has different next-hop information. |
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Figure 13.6 |
The network from Figure 13.2 and the corresponding graph. Each node in the graph corresponds to a packet switch, and each edge between two nodes represents a connection between the corresponding packet switches. |
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Figure 16.4 |
The nested protocol headers that appear in a frame as the frame travels across a network if the full ISO stack is used. Each layer of protocol software adds a header to an outgoing frame. |
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Figure 16.6 |
A 4-packet window sliding through outgoing data. The window is shown (a) when transmission begins, (b) after two packets have been acknowledged, and (c) after eight packets have been acknowledged. The sender can transmit all packets in the window. |
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Figure 16.7 |
Messages required to send a sequence of four packets using (a) stop-and-go flow control, and (b) a 4-packet sliding window. Time proceeds down the page, and each arrow shows one message sent from one computer to the other. |
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Figure 16.8 |
A graph that represents a network of six packet switches. Such networks can experience congestion. |
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Figure 19.7 |
Illustration of an ARP message encapsulated in an Ethernet frame. The entire ARP message travels in the data area of the frame; the network hardware neither interprets nor modifies contents of the ARP message. |
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Figure 19.8 |
Illustration of the type field in an Ethernet header used to specify the frame contents. A value of 0x806 informs the receiver that the frame contains an ARP message. |
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Figure 21.1 |
An IP datagram encapsulated in a hardware frame. The entire datagram resides in the frame data area. In practice, the frame format used with some technologies includes a frame trailer as well as a frame header. |
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Figure 21.2 |
An IP datagram as it appears at each step during a trip across an internet. Whenever it travels across a physical network, the datagram is encapsulated in a frame appropriate to the network. |
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Figure 21.3 |
An example of a router that connects two networks with different MTU values. A frame that travels across network 1 can contain 1500 octets of data, while a frame that travels across network 2 can contain at most 1000 octets of data. |
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Figure 23.2 |
Two levels of encapsulation that occur when an ICMP message is sent. The ICMP message is encapsulated in a datagram, which is encapsulated in a frame for transmission across a physical network. |
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Figure 25.2 |
Example of retransmission. Items on the left correspond to events in a computer sending data, items on the right correspond to events in a computer receiving data, and time goes down the figure. The sender retransmits lost data. |
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Figure 26.3 |
An example NAT translation table for the mapping illustrated in Figure 26.2. An entry specifies the direction of packet flow and the changes that should occur. |
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Figure 40.2 |
Illustration of the location of a packet filter. The filter software is configured to discard specified packets as they pass from one network to another. |
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Figure 40.3 |
The architecture of a firewall with a secure host bracketed by two packet filters. One filter restricts incoming packets, and the other restricts outgoing packets. |
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Animation 05_1 |
Multiple computers using a shared communication link |
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Animation 05_2 |
Using special characters for framing |
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Animation 05_3 |
"Byte-stuffing" for transmitting framing characters |
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Animation 10_1 |
Packet switches can be linked together into a Wide Area Network. Data packets delivered to one switch are forwarded through other switches to the destination. |
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Animation 06_1 |
Ethernet is a broadcast LAN technology; a computer transmits data by sending the data across the entire Ethernet and the data is received by every NIC attached to the network; only the NIC whose address appears in the destination field of the Ethernet frame delivers the frame to the attached computer; the other NICs discard the frame. |
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Animation 06_3 |
Token ring is a broadcast technology using ring topology; the each computer passes bits from its upstream neighbor to its downstream neighbor and make a local copy if its is the recipient of the frame; the token is used to arbitrate use of the ring among the computers attached to the network. |
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Animation 06_4 |
After transmitting a frame, the sender passes the token to the next station. After receiving the token, the next station can transmit a frame. |
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Animation 06_5 |
If two frames are sent simultaneously on an Ethernet, they are said to sollide. When the transmitting computers sense the collision, they immediately stop transmitting and retrnasmit the frame later. |
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Animation 09_2 |
Bridges interconnect Ethernet segments by receiving and retransmitting entire frames, employing CSMA/CD technology to avoid collisions and avoiding the propagation of collisions between segments; filtering bridges can reduce traffic by only forwarding frames on the path from the source to the destination. |
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Animation 09_3 |
Routers interconnect Ethernet segments by receiving and retransmitting IP datagrams carried in hardware frames; routers can limit the scope of hardware broadcasts and can interconnect network segments that use dissimilar hardware technologies. |
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Animation 10_1 |
Packet switches can be linked together into a Wide Area Network. Data packets delivered to one switch are forwarded through other switches to the destination. |
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Data file 1 |
Trace of all IP traffic on Ethernet segment. Contains approximately 87,000 packets and 6.5Mb. Trace includes packet headers only. |
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Data file 2 |
Anonymous FTP session with dir, get and put. Contains approximately 930Kbytes and 2300 packets. |
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Data file 3 |
Anonymous FTP session using mput in both ascii and binary modes. Contains approximately 33Kbytes and 340 packets. |
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Data file 4 |
Anonymous FTP session using mget in both ascii and binary modes. Contains approximately 37Kbytes and 370 packets. |
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Data file 5 |
TELNET session (headers only). Contains approximately 45Kbytes and 560 packets. |
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Data file 6 |
SMTP session with delivery of one mail message from SMTP client to SMTP server. Contains approximately 3,000 bytes and 30 packets. |
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Data file 7 |
WWW browser session accessing multiple URLs from multiple WWW servers. Contains approximately 590Kbytes and 1,270 packets. |
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Data file 8 |
X Window System application protocol messages from several clients, including xterm, emacs, xspread and xpaint to an X server. Contains approximately 760Kbytes and 5,500 packets. |
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Photo 6_019 |
A 24-port 10/100 Mbps Ethernet interface board for a Cisco Catalyst 5000 switch. The circuitry at the left of the photo handles the transmission and reception of Ethernet frames. The circuitry at the right communicates with the rest of the Catalyst 5000 switch. The circuitry in the middle of the board performs the frame switching function. |
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Photo 6_020 |
A 24-port 10/100 Mbps Ethernet interface board for a Cisco Catalyst 5000 switch. The circuitry at the left of the photo handles the transmission and reception of Ethernet frames. The circuitry at the right communicates with the rest of the Catalyst 5000 switch. The circuitry in the middle of the board performs the frame switching function. |
