Lecture 6 Ethernet, Wireless LAN & Internetworking
Textbook: Ch.8, 13 and 15
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IEEE 802.3 Ethernet
IEEE 802.11 Wireless LAN WAN & Internetworking Circuit-switched Network
Packet Switching
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Virtual Circuit Approach
Datagram Approach
Store-and-Forward operation
Delay in a datagram network
Main Topics
Delay in a circuit-switched network
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IEEE 802 Standard for LANs
• In 1985, the Computer Society of the IEEE started a project, called Project 802, to set standards to enable intercommunication among equipment from a variety of manufacturers.
• Project 802 is a way of specifying functions of the physical layer and the data link layer of major LAN protocols.
WCB/McGraw-Hill
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Standard Ethernet
Ethernet evolution through four generations
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Network Components of Ethernet
Communication controller card (NIC, network interface card) in the station (computer) contains:
MAC unit for such functions as encapsulation, error detection & execution of MAC algorithm
Transceiver : transmitter and receiver in one unit (also called MAU, medium attachment unit) – part of the NIC
send & receive data from cable detect occurrence of collisions
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MAC Protocol of Ethernet
Bus topology with a broadcast channel (usually a coaxial cable)
Access Method
1-persistent CSMA/CD Bus (IEEE 802.3)
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Frame Format of Ethernet
Figure 13.3 802.3 MAC frame SEHH2238 Lecture 6 7
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Ethernet Frame Format & Parameters
Ethernet Frame contains seven fields:
(with a little bit faster clock) Start of Frame Delimiter (SFD)
contains 7x(10101010) for bit synchronization
10101011 (also as a last chance for synchronization)
signals the beginning of the frame
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Ethernet Frame Fields
3. Length (or type)
the packet length in bytes (excluding
preamble and SFD)
4. Destination Address (DA)
6 bytes containing the physical address of the destination station or stations to receive the packet
5. Source Address (SA)
6 bytes containing the physical address of the
sender of the packet
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Ethernet Frame Fields
carries data encapsulated from the upper-layer
data length: a minimum of 46 bytes and a maximum of 1500 bytes
if length < minimum frame size, then dummy bytes are added (known as padding) in the data field
CRC-32 for error detection
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Ethernet Frame Length
Minimum length:
Remember in CSMA/CD, a minimum length
restriction is required for correct operation
Maximum length:
Reduce the size of buffer in memory
Prevent one station from monopolizing the shared channel (using the channel too long)
Frame length:
Minimum: 64 bytes (512 bits) Maximum: 1518 bytes (12,144 bits)
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Figure 13.5 Minimum and maximum lengths Payload: data + padding
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Wireless LAN - IEEE 802.11
IEEE 802.11 is the IEEE specifications for a wireless LAN.
Infrastructure (architecture): uplink and downlink via the access points (base stations)
Transmission media: Infrared or radio signal using spread spectrum techniques
Use CSMA/CA (collision avoidance) protocol to organize the transmissions from mobile stations
Wireless (radio) networks cannot use the CSMA/CD protocol (skip the details:15.1.3, p.438-439)
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Architecture of Wireless Network
Twokindsofservices:BSSandESS Basic Service Set (BSS)
It made up of stationary or mobile wireless stations. A BSS without an AP is called an ad hoc network;
a BSS with an AP is called an infrastructure network.
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Extended Service Set (ESS)
It made up of two or more BSSs with Access Points (AP).
BSSs are connected through a distribution system (usually Wired LAN)
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MAC layers in IEEE 802.11 standard
(Skip the details)
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CA: Collision Avoidance Key difference:
There is no collision detection
IFG is inter-frame gap
If channel is still idle
(Skip the details)
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CSMA/CA procedure
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Wide Area Network (WAN)
A network that links stations and LANs that are physically located in different geographic areas
Include both public data networks and enterprise wide private data networks
Three major concerns/functions in internetworking:
Congestion Control Flow Control
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Three major concerns on WANs
determine how packets are routed from source to
destination (i.e. select the best path)
Congestion Control
make sure the network is able to carry the offered
a global issue involves all stations and routers
Flow Control
make sure that a fast sender cannot continually
transmit data faster than the receiver can accept
a local issue between a given sender and a given
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A switched network consists of a series of interlinked nodes, called switches.
Switches are devices capable of creating temporary connections between two or more devices linked to the switch.
Figure 8.1 Switched network
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End Systems Switches
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Three Methods of Switching
Switching is a method in which communication devices are connected to one another efficiently.
Traditionally three methods of switching: circuit switching, packet switching, and message switching.
The first two are commonly used today. The third has been phased out in general communications but still has applications.
Packet switching can further be divided into two subcategories, virtual-circuit approach and datagram approach.
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Figure 8.2: Taxonomy of switched networks
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Circuit-switched Network
It consists of a set of switches connected by physical links.
A connection between two stations is a dedicated path made of one or more links.
However, each connection uses only one dedicated channel on each link (which is normally divided into n channels by using FDM or TDM).
In circuit switching, the resources need to be reserved during the setup phase.
The resources remain dedicated for the entire duration of data transfer until the teardown phase.
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Phases in Circuit-switched Network
Setup phase
A channel is reserved on each link and the dedicated path is defined.
Data Transfer phase Two parties can
transfer data.
Teardown Phase
When one of the parties needs to disconnect, a signal is sent to each switch to release the resource.
Figure8.3 Atrivialcircuit-switchednetwork SEHH2238 Lecture 6 24
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Circuit Switching
A (temporary) dedicated path (just like a point-to- point link) between the source and the destination is provided for the duration of data transmission (called session). It is similar to a telephone call.
Advantage
Throughput and delay characteristics are predictable
Disadvantages:
Waste the capacity of the links (when no data
within a session)
Connection establishment and disconnection are relatively time-consuming
Possibility of blocking (stop new data input)
when traffic is heavy
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Delay in a circuit-switched network
Note that during data transfer the data are not delayed at each switch, as no waiting time is required inside each switch
Figure 8.6
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Packet Switching
Packetizing: the data message needs to be divided into packets of fixed or variable size
The size of the packet is determined by the network and the governing protocol
Encapsulating the payload in a network-layer packet at the source
Decapsulating the payload from the network- layer packet at the destination
Use Store-and Forward operation
Virtual-circuit approach and Datagram approach
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Datagram Networks
It is a connectionless service of packet switching
Each packet (of the same message) is sent out
independently
No connection set up is required
Does not guarantee delivery of error-free and sequenced data
Packets of the same message may travel along different paths via different intermediate nodes (thus re-sequencing is needed at the destination node)
Users must handle error & flow control themselves
Packets in this approach are referred to as datagrams
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Main Features of Datagram Packet Switching
Message is divided into packets of fixed (maximum) size
Thereisnoresourcereservation
Resources are allocated on demand
Packets (of the same message) are only reassembled at the destination node
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Store-and-Forward Operation
Each cable (channel/link) connects a pair of nodes (Point-to-Point Channel)
If no direct link between two nodes, they must communicate indirectly (via other nodes)
The packet is received at each intermediate node, be stored there until the output link is free, and then be forwarded to another node
A routing decision is made to select the next intermediate node before forwarding
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Forwarding Process & Routing Table
• A switch (router) uses a routing table (forwarding table) to determine the output port
• It is based on the destination address, (for datagram approach) which remains the same during the entire journey of the packet.
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Figure 18.4: Forwarding process in a router when used in a connectionless datagram network
SA DA Data
SA DA Data
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Delay in a Datagram Network
E.g. A packet travels two switches. There are 3 transmission times (3 Tx)
3 propagation delays (3 Tp)
2 waiting times (w1 and w2)
Thetotaldelay=3Tx +3Tp +w1 +w2
The waiting time includes the nodal processing time in this example
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Figure 8.9
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Circuit Switching and Datagram
Switching at the physical layer in the traditional telephone network uses the circuit-switching approach.
Switching in the Internet is done by using the datagram approach of packet switching at the network layer.
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Virtual Circuit (VC) Approach
• In a connection-oriented service (also called virtual- circuit approach), there is a relationship between all packets belonging to a message.
• Before all datagrams in a message can be sent, a virtual connection should be set up to define the path for the datagrams.
• After connection setup, the datagrams can all follow the same path using store-and-forward operation.
• In this type of service, not only must the packet contain the source and destination addresses, it must also contain a flow label, a virtual circuit identifier (VCI) that defines the virtual path the packet should follow.
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Figure 18.5: A virtual-circuit packet-switched network
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Virtual Circuit Approach
Virtual circuit approach (in packet switching) can be considered as a mix of circuit switched (CS) and packet switched (PS) networks.
Phase: Setup, data transfer, teardown (CS)
A “virtual” path is set up (to book the resource)
before data transfer
Data are packetized. Each packet carries an address (and VCI) in the header (PS)
All packets of the same message follow the exact (same) route (indicated by the VCI) (CS)
But the physical path is not dedicated (and may be shared by other connections) (PS)
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Virtual Circuit Approach
This “service” provides with a “perfect channel” and guarantees error-free and sequenced data
The packets may arrive at the destination with different delays
The complicated communication issues (e.g. error and flow control, re-sequencing of data packets) are handled by the (VC) service provider
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Figure 18.6: Forwarding process in a router when used in a virtual circuit network
label - The VCI
Outgoing label- The VCI
(Skip the details)
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Figure 18.9: Flow of one packet in an established virtual circuit
(Skip the details)
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E.g. A packet travels two switches.
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Figure 8.16
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Delay in a virtual-circuit network
The total delay = 3Tx +3 Tp + Setup delay + teardown delay
(The nodal processing time usually can be neglected. Also assume no waiting time at each node in this example)
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Packets travel along the same path Revision Quiz
IEEE 802.3 Ethernet - 1-persistent CSMA/CD Bus
IEEE 802.11 Wireless LAN – CSMA/CA
Circuit Switching - A (temporary) dedicated path
Packet Switching - Store-and-Forward operation
Datagram Approach - Connectionless, Datagrams
Virtual Circuit Approach - Connection-oriented
http://highered.mheducation.com/sites/0073376221/student_view0/ chapter8/quizzes.html
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