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Exam 2 Review
Topic 6. Network and Transport Layer (Chapter 4, 5)
Review focuses:
1.
2.
3.
4.
5.
Protocol
OSI model (7 layers)
Internet (TCP/IP) Model (5 layers)
1) Application
2) Host-to-Host (transport)
3) Internet
4) Network Access
5) Physical
IPv4 vs IPv6
Three functions of the network layer (TCP/IP):
 Packetizing: breaking data into packets, numbering, error control, reassembling - TCP
 Addressing: determines the correct network layer and data link layer addresses - IP
 Routing: determines where the message should be sent next on its way to its final destination. – IP
6.
Addressing
1) Internet IP addresses:
- address assigning
- dynamic addressing: bootstrap Protocol, Dynamic Host Control Protocol (DHCP)
2) Address classes: A, B, C, D, E
3) Subnet and subnet mask
- Why need subnet mask and how to design a subnet mask
4) Three levels of addresses:
application layer address (domain name), network layer address, data link layer address
5) Address resolution: server name resolution, data link layer address resolution
6) DNS server
7) Four pieces of information for a client: its IP address, subnet mask, DNS server IP address,
gateway IP address
8) Broadcasting, Unicasting, Multicasting, Anycasting
7.
Routing
1) Dynamic routing: routing table update, RIP, ICMP, and OSPF
2) Connectionless vs. connection-oriented (virtual circuit)
3) Traffic types: real-time, elastic
4) QoS: Quality of Service (QoS) is the idea that transmission rates, error rates, and other
characteristics can be measured, improved, and, to some extent, guaranteed in advance.
5) QoS routing: a special connection-oriented dynamic routing in which different data flows are
assigned different priorities and classes of service.
6) Network congestion: what is it?
7) Network service types: Best effort, IntServ and DiffServ:
- How they work?
8) Internet flow control
9) Token bucket model
Key terms:
Address resolution, routing, application layer address, network layer address, data link layer address,
multicasting, subnet mask, virtual circuit, domain name, name server, QoS routing, routing table,
IntServ, DiffServ.
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Topic 7. LANs and Backbone Networks (Chapter 14, 15)
1.
Network components
a. Computers (server, client)
b. NIC
c. Cable
d. NOS
e. Hub
2. Tiered LAN
3. Network topology: Bus, start, tree, ring
a. Physical vs. logical
4. Ethernet
a. Standard: IEEE 802.3
b. CSMA/CD
5. Token ring
a. Standard: IEEE 802.5
b. Token passing protocol
6. Backbone devices
a. Hub
b. Bridge
c. Switch
d. Router
e. Gateway
f. Layer-3 switch: why need it?
7. Contrast between different kinds of networking devices
g. Switch vs. hub
h. Switch vs. bridge
i. Bridge vs. router
j. Router vs. gateway
k. Router vs. switch
8. LAN types
a. 10Base-T vs. 100Base-T Ethernet (3 versions)
b. Gigabit Ethernet
c. New types of Ethernet: 1000Base-T (1GbE), 10GbE, 40GbE
d. Fibre channel
e. Wireless LAN
f. FDDI
i. 100Mbps
ii. Dual-ring structure
iii. Token passing MAC protocol
9. Wireless LAN
a. WLAN and LAW
b. Standard: IEEE 802.11b and IEEE 802.11a
c. MAC protocol: CSMA/CA, (contrast with CSMA/CD)
d. Hidden node problem and how it is solved
10. Backbone design
a. Three layers of architecture: access layer, distribution layer, and core layer
b. Four architectures:
- Routed backbone – using routers
Advantage – clearly segment each part of the network
Disadvantage – Delay, and more management
- Bridged backbone – using bridges, not popular any more
Advantages – cheaper, simpler
Disadvantages – difficulties in management
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- Collapsed backbone –using switches, is most commonly used.
Advantages - Better performance, Fewer network devices are used
Disadvantages – switch problem may fail whole network, more cabling work
Two types
Rack-based collapsed backbone
Chassis-based collapsed backbone
- Virtual LAN (VLAN)
Key terms:
NOS, CSMA/CD, CSMA/CA, 10Base-T, 100Base-T, 1000Base-T, hidden node problem.
Questions/Answers (Not necessarily cover all topics):
1.
How is TCP different from UDP?
TCP is a connection-oriented protocol. UDP is a connection-less protocol. What are the differences
between connectionless and connection-oriented routing?
Connection-oriented routing sets up a virtual circuit between the sender and receiver. In this case, a
temporary virtual circuit is defined between the sender and receiver. The network layer makes one routing
decision when the connection is established, and all packets follow the same route. All packets in the same
message arrive at the destination in the same order in which they were sent. In this case, packets only need
to contain information about the stream to which it belongs; sequence numbers are not needed, although
many connection-oriented protocols include a sequence number to ensure that all packets are actually
received.
Connection-oriented routing has greater overhead than connectionless routing, because the sender must
first “open” the circuit by sending a control packet that instructs all the intervening devices to establish the
circuit routing. Likewise, when the transmission is complete, the sender must “close” the circuit.
Connection-oriented protocols also tend to have more overhead bits in each packet.
Connectionless routing means each packet is treated separately and makes its own way through the
network. It is possible that different packets will take different routes through the network depending upon
the type of routing used and the amount of traffic. Because packets following different routes may travel at
different speeds, they may arrive out of sequence at their destination. The sender’s network layer therefore
puts a sequence number on each packet, in addition to information about the message stream to which the
packet belongs. The network layer must reassemble them in the correct order before passing the message
to the application layer.
2.
How does TCP establish a connection?
TCP sets up a virtual circuit between the sender and the receiver. The transport layer software sends a
special packet (called a SYN, or synchronization characters) to the receiver requesting that a connection be
established. The receiver either accepts or rejects the connection, and together, they settle on the packet
sizes the connection will use. Once the connection is established, the packets flow between the sender and
the receiver, following the same route through the network.
3.
What is a subnet and why do networks need them?
Each organization must assign the IP addresses it has received to specific computers on its networks. In
general, IP addresses are assigned so that all computers on the same local area network have a similar
addresses. For example, suppose a university has just received a set of Class B addresses starting with
128.184.x.x. It is customary to assign all the computers in the same LAN numbers that start with the same
first three digits, so the Business School LAN might be assigned 128.184.56.x while the Computer Science
LAN might be assigned 128.184.55.x (see Figure 6-8). Likewise, all the other LANs at the university and
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the backbone network that connects them, would have a different set of numbers. Each of these LANs are
called a TCP/IP subnet because they are logically grouped together by IP number. Knowing whether a
computer is on your subnet or not it very important for message routing.
4.
How does TCP/IP perform address resolution for network layer addresses?
Server name resolution is the translation of application layer addresses into network layer addresses (e.g.,
translating an Internet address such as www.cba.uga.edu into an IP address such as 128.192.98.3). This is
done using the Domain Name Service (DNS). Throughout the Internet there are a series of computers
called name servers that provide DNS services. These name servers run special address databases that store
thousands of Internet addresses and their corresponding IP addresses. These name servers are in effect the
"directory assistance" computers for the Internet. Any time a computer does not know the IP number
for a computer, it sends a message to the name server requesting the IP number.
When TCP/IP needs to translate an application layer address into an IP address, it sends a special TCPlevel packet to the nearest DNS server. This packet asks the DNS server to send the requesting computer
the IP address that matches the Internet address provided. If the DNS server has a matching name in its
database, it sends back a special TCP packet with the correct IP address. If that DNS server does not have
that Internet address in its database, it will issue the same request to another DNS server elsewhere on the
Internet.
Once your computer receives an IP address it is stored in a server address table. This way, if you ever need
to access the same computer again, your computer does not need to contact a DNS server. Most server
address tables are routinely deleted whenever you turn off your computer.
5.
How does TCP/IP perform address resolution for data link layer addresses?
To send a message to a computer in its network, a computer must know the correct data link layer
address. In this case, the TCP/IP software sends a broadcast message to all computers in its subnet. A
broadcast message, as the name suggests, is received and processed by all computers in the same LAN
(which is usually designed to match the IP subnet). The message is a specially formatted TCP-level
request using Address Resolution Protocol (ARP) that says “Whoever is IP address xxx.xxx.xxx.xxx,
please send me your data link layer address.” The TCP software in the computer with that IP address
then responds with its data link layer address. The sender transmits its message using that data link
layer address. The sender also stores the data link layer address in its address table for future use.
6.
Explain the terms 10Base-2, 10BaseT, 100BaseT, 1000BaseT, 10GbE, and 10/100 Ethernet?
The original ethernet specification was a 10 Mbps data rate using baseband signaling on thick coaxial
cable, called 10Base5 (or “Thicknet”), capable of running 500 meters between hubs. Following 10Base5
was 10Base2 or thinnet as we used to say. Thinnet or RG-58 coaxial cable, similar to what is used for
cable TV was considerably cheaper and easier to work with, although it was limited to 185 meters between
hubs. The 10Base-2 standard was often called “Cheapnet.”
When twisted pair cabling was standardized for supporting Ethernet (app. 1988) the T replaced the 2 to
represent “twisted-pair”. Twisted pair is the most commonly used cable type for Ethernet. 10BaseT breaks
down as 10 Mbps, baseband, and the “T” means it uses twisted pair wiring (actually unshielded twisted
pair). It was the 10Base-T standard that revolutionized Ethernet, and made it the most popular type of LAN
in the world.
Eventually the 10BaseT standard was improved to support Fast Ethernet or 100BaseT that breaks down as
100Mbps baseband over twisted-pair cable. This eventually was improved even further to 1000BaseT or 1
Billion BITs per second baseband. There is currently a revised standard evolving which makes Ethernet
even faster. It is known as the 10GbE or 10 Billion BITs per second Ethernet. Though proven to work it
has yet to reach the marketplace. But it would be astute to consider that it will be here in the near future.
Finally, 10/100Mbps Ethernet refers to the standard that can autosense which speed it needs to run at
between the two speeds of 10Mbos or 100Mbps. It comes down to the type of NIC running at the
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individual node and the type of switch port that the node connects into. It is commonplace to run
10/100Mbps switches in LAN operating environments where there are older NICs already operating and
no real business case requirements for upgrading these nodes.
7.
Explain how the two approaches to media access control work in CSMA/CA?
The two approaches are Physical Carrier Sense Method (PCSM) and Virtual Carrier Sense Method
(VCSM). PCSM is based on the ability of the computers to physically listen before they transmit. After a
transmission is sent the receiving computer acknowledges (ACK) the transmission by sending an ACK
packet in reply. The source computer upon receipt of the ACK packet then knows it has a connection and
can continue transmission to the destination computer. VCSM does not rely on physical media. A
computer running this protocol first must send a Request to Transmit (RTS) packet to the AP. If all clear
the AO responds with a Clear to Send (CTS) packet back to the source computer. The source computer
may then begin transmission.
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