Module 2 lan,data link layer

MODULE 2 MCA-402 Computer Networks ADMN 2012-‘15
Dept. of Computer Science And Applications, SJCET, Palai Page 1
LOCAL AREA NETWORK -LAN
Uses of LAN
As LAN provides the capability to route data between devices connected to a common network within a
relatively limited distance, numerous benefits can accrue to users of the network.
 Peripheral sharing: Peripheral sharing allows network users to access color laser printers and
other devices. Users of a LAN can obtain access to resources that would be too expensive to justify
for each individual workstation user.
 Common software access: The ability to access data files and programs from multiple
workstations can substantially reduce the cost of software and shared access to database
information allows network users to obtain access to updated files on a real-time basis.
 Internet access: Instead of supporting individual dial access to the Internet you could install a router
and leased line to an Internet Service Provider (ISP) that is a fraction of the cost associated with a
large number of individual Internet dial network accounts
Attributes of a LAN
i. Transmission technology
ii. Signalling methods
iii. Transmission medium
iv. Access Control
v. Topologies
i. Transmission Technology
 Two Transmission Technologies used by LANs are :
a. Broad cast: A single communication channel is shared by all the machines on the network
b. Point to Point: consists of many connections between individual pairs of machines.Packets have to follow
multiple routes of different lengths
ii. Signalling Methods
 Two signaling methods used by LANs are :
a. Broadband: The bandwidth of the transmission medium is subdivided by frequency to form two or
more sub-channels
b. Base-Band: only one signal is transmitted at any point in time
iii. Transmission Medium
 Transmission medium can be classified into 4 from physical cable connection to wireless systems.
a. Twisted Pair: consists of two individual insulated copper wires physically twisted together to
minimize unwanted electromagnetic signals from interfering with or radiating from
the pair
b. Coaxial cables: consists of a central copper core surrounded by a layer of insulating material.
c. Optical fiber cables: contains glass fiber and signals are transmitted in the form of light pulses.
d. Wireless transmission
iv. Access control
 Represents how the devices get permission to communicate on the network.
 Some of the access methods primarily employed in local area networks are :
 Polling
 Token-passing
 Slotted ring
 CSMA/CD

 Switching
v. Topology
 Refers to the way in which the end points, or stations, attached to the network are interconnected.
 The common topologies for LANs are:
i. Bus
ii. Tree
iii. Ring
iv. Star
LAN PROTOCOL ARCHITECTURE
 The LAN protocol architecture consists of layering of protocols that contribute to the basic functions of a
LAN
 The standardized LAN protocol architecture encompasses 3 layers
i. Physical layer
ii. Medium Access control layer (MAC)
iii. Logical Link control
IEEE 802 REFERENCE MODEL
 The layers of OSI Reference Model can be classified as Network Support Layers and User support
layers
 LAN protocols are concerned with the network support layers, mainly Physical and Data Link Layer
 IEEE 802 Reference Model is a modified reference model suitable for LANs built by IEEE.
 It corresponds to the lower 2 layers of OSI reference model
 Here the data link layer has been divided into two sub layers: logical link control (LLC) and media
access control (MAC)
Fig 2.1 IEEE 802 Protocol Layers Compared to OSI Model

Physical layer
 includes functions such as :
i. Encoding/decoding of signals
ii. Specification of the transmission medium and the topology
Logical Link Layer
 Acts as the interface between the Network layer and the MAC sub layer
 Functions:
i. Error Control
ii. Flow Control
iii. Sequencing & User Addressing Functions
 LLC standard is common to all LAN’s and offers three types of services for controlling the exchange
of data between two users.
i. Unacknowledged connectionless service: does not involve any of the flow- and error control
mechanisms and delivery of data is not guaranteed.
ii. Connection-mode service: A logical connection is set up between two users exchanging data, and
flow control and error control are provided.
iii. Acknowledged connectionless service: datagram are to be acknowledged, but no prior logical
connection is set up.
LLC Protocol Data Unit (PDU)
 which contains 4 fields
Fig 2.2 LLCPDU
 Destination Service Access Point (DSAP), Source Service Access Point (SSAP) : address fields
which specify the destination and source users of LLC.These identify the network protocol entities
which use the link layer service
 LLC control field: describes type of PDU(U,I and S) and includes other information such as
sequencing and flow control information
Medium Access Control
 It is lower sub-layer of data link layer closer to the physical layer
 Key parameters of MAC technique is where and how
a. Where: refers to whether control is exercised in a centralized or distributed fashion.
 Central: A controller is designated that has the authority to grant access to the network.
 Distributed: The stations collectively perform a MAC function to determine the order in which
stations transmit.
b. How: is constrained by the topology and competing factors like cost, performance, and
complexity.
 Basic functions of MAC Sub Layer are:
i. Media Access Control
ii. Error Detection
iii. Station Addressing
 Defines two medium access control techniques specific for each LAN.

i. Synchronous techniques: a specific capacity is dedicated to a connection.
ii. Asynchronous: allocate capacity in an asynchronous (dynamic) fashion, more or less in response to
immediate demand.
 Asynchronous techniques can be classified into three:
i. Round Robin
ii. Reservation
iii. Contention
i. Round Robin
 Each station in turn is given the opportunity to transmit.
 When it is finished, relinquishes its turn, and the right to transmit passes to the next station in logical
sequence.
 Control of sequence may be centralized or distributed.
 Efficient when many stations have data to transmit over an extended period of time
ii. Reservation
 Time on the medium is divided into slots.
 A station wishing to transmit reserves future slots for an
extended or even an indefinite period.
 Reservations may be made in a centralized or distributed
fashion.
iii. Contention
 Useful for bursty type traffic
 No control is exercised to determine whose turn it is.
 Stations send data by taking risk of collision (with others’ packets).however they understand
collisions by listening to the channel, so that they can retransmit.
 Efficient under light or moderate load, bad under heavy load
 Round-Robin and Contention techniques are the most commonly used in LANs.
MAC- Frame Format
 The MAC layer is responsible for performing functions related to medium access and for transmitting
the data.
Fig 2.3 MAC Frame
 MAC Control: contains any protocol control information needed for the functioning of the MAC
protocol
 Destination MAC Address: The destination physical attachment point on the LAN for this frame.
 Source MAC Address: The source physical attachment point on the LAN for this frame.
 LLC PDU: The LLC data from the next higher layer.
 CRC: The Cyclic Redundancy Check field
IEEE 802 STANDARDS
LAN standards proposed by the IEEE committee have the following goals in mind:
• To promote compatibility
• Implementation with minimum efforts

• Accommodate the need for diverse applications
For the fulfillment of the above mentioned goals, the committee came up with a bunch of LAN
standards collectively known as IEEE 802.The various standards differ at the physical layer & MAC sub-
layer but are compatible at the data link layer.
Fig 2.4 IEEE 802 standards
MULTIPLE ACCESS PROTOCOLS
 A MAC (Media Access Control) protocol is a set of rules to control access to a shared
communication medium among various users.
Fig 2.5 Multiple access protocols
Random Access
 In random access or contention methods, no station is superior to another station and none is assigned
the control over another.

 Two features give this method its name:
 First, there is no scheduled time for a station to transmit.
 Second, no rules specify which station should send next. Stations compete with one another to access
the medium
ALOHA
 When the user simply transmits a frame, there are chances of collision
 The user could simply retransmit, but this would not help, other user involved in the collision will
also retransmit, resulting in another collision
 One way to avoid this is to wait a random amount of time before retransmitting which forms the basis
of ALOHA
 There are two versions of ALOHA: pure and slotted.
Pure ALOHA
The system is working as follows:
1. Let users transmit whenever they have data to be sent.
2. Collisions will occur.
3. Using a feedback mechanism to know about the status of frame.
4. The collided frames will be destroyed.
5. Retransmit the destroyed frame.
 The number of collisions rises rapidly with increased load.
 After a maximum number of retransmission attempts Kmax station must give up and try later.
Fig 2.6 flow chart for pure ALOHA

Fig 2.7 Frames in a pure ALOHA network
Suppose L: the average frame length,
R: rate,
X=L/R: frame time
1. Transmit a frame at t=t0 (and finish transmission of the frame at t0+X )
2. If ACK does not come after t0+X+2tprop or detect collision, wait for random time: B
3. Retransmit the frame at t0+X+2tprop+B
Fig 2.8 timing diagram for pure aloha
 Vulnerable period: t0-X to t0+X, if any other frames are transmitted during the period, the collision will
occur.
 Therefore the probability of a successful transmission is the probability that there is no additional
transmission in the vulnerable period.
 Therefore, if a station generates only one frame in this vulnerable time (and no other stations generate a
frame during this time), the frame will reach its destination successfully.
 Max channel utilization is 18% - very bad.
Slotted ALOHA
 Slotted ALOHA was invented to improve the efficiency of pure ALOHA.
 Here, we divide the time into slots and force the station to send only at the beginning of the time slot
 If a station misses this moment, it must wait until the beginning of the next time slot.

 There is still the possibility of collision if two stations try to send at the beginning of the same time
slot
Fig 2.9 frames in slotted ALOHA
Fig 2.10 timing diagram for slotted ALOHA
 Max channel utilization is 37%,doubles Normal ALOHA, but still low
Carrier Sense Multiple Access (CSMA)
 A station wishing to transmit first listens to the medium if another transmission is in progress (carrier
sense).
 If the medium is in use, station waits
 if the medium is idle, station may transmit
 Collision probability depends on the propagation delay

 Longer propagation delay, worse the utilization
 Collisions can occur only when more than one user begins transmitting within the period of
propagation delay.
 The vulnerable time for CSMA is the propagation time .
 If collision occurs
 Wait random time and retransmit
 Suppose tprop is propagation delay from one extreme end to the other extreme end of the medium.
When transmission is going on, a station can listen to the medium and detect it.
Fig 2.11 vulnerable period in CSMA
 After tprop, A’s transmission will arrive the other end; every station will hear it and refrain from the
transmission, so A captures the medium and can finish its transmission.
 Following are some versions of CSMA protocol Based on how to do when medium is busy
i. 1-Persistent CSMA
ii. Non-Persistent CSMA
iii. p-Persistent CSMA
1-persistent CSMA
 if the medium is idle, transmit.
 if the medium is busy, continue to listen until the channel is sensed idle; then transmit immediately.
 If more than one station are sensing, then they will begin transmission the same time when channel
becomes idle, so collision. At this time, each station wait for a random time, and then re-senses the
channel again.
 Problem with 1-persistent CSMA is “high collision rate”.
Fig 2.12 flow chart for 1-persistent CSMA

Nonpersistent CSMA
 If the channel is busy the station does not continually check it for detecting the end of ongoing
transmission. It waits for a random time then checks the channel. If the channel is idle, sends the
frame.
Fig 2.13 flow chart for non persistent CSMA
P-persistent CSMA
 If medium is idle, station transmits with a probability p. otherwise it defers to the next slot with
probability 1-p. the process repeat until either the frame has been transmitted or another station has
begun transmission.
Fig 2.14 flow chart for P-persistent CSMA
Fig 2.15 Behaviour of three persistence methods

CSMA/CD (CSMA with Collision Detection)
 Drawback of CSMA: when two frames collide, the medium remains unusable for the duration of
transmission of both damaged frames.
 CSMA/CD:
1. If the medium is idle, transmit; otherwise, go to step 2.
2. If the medium is busy, continue to listen until the channel is idle then transmit.
3. if a collision is detected during transmission, transmit a brief jamming signal
4. After transmitting a jamming signal, wait a random amount of time, then attempt to transmit.
5.
Fig 2.16 flow chart for CSMA/CD
CSMA/CD efficiency
 tprop = max prop between 2 nodes in LAN
 ttrans = time to transmit max-size frame
 Efficiency = 1/(1+5 * tprop / ttrans)
 For 10 Mbit Ethernet, tprop = 51.2 us, ttrans = 1.2 ms
 Efficiency is 82.6%!
 Much better than ALOHA,
 simple, and cheap
 Efficiency goes to 1 as tprop goes to 0
 Goes to 1 as ttrans goes to infinity

Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)
 In CSMA/CA once the channel is clear, it again waits for an additional time period before performing
the transmission.
 Before sending a frame, source senses the medium
 Backoff until the channel is idle.
 After the channel is found idle, the station waits for a period of time called the Distributed
Inter Frame Space (DIFS); then the station sends a control frame called Request To Send
(RTS).
 After receiving RTS, the destination waits for a period called Short Inter Frame Space (SIFS),
the destination station sends a control frame, called Clear To Send (CTS) to source. This
control frame indicates that the destination station is ready to receive data.
 Source sends data after waiting for SIFS
 Destination sends ACK after waiting for SIFS.
Fig 2.17 flowchart for CSMA/CA
 RTS frame indicates the duration of time that the source needs to occupy the channel.
 Stations that are affected by this transmission create a timer called a Network Allocation Vector
(NAV) that shows how much time must pass before these stations are allowed to check the channel
for idleness.

Fig 2.18 signals in CSMA/CA
Exponential Back off Algorithm
 used by a transmitting station to determine how long to wait following a collision before attempting
to retransmit the frame
 Each station generates a random number that falls within a specified range of values. This determines
the length of time it must wait before testing the carrier. The range of values increases exponentially
after each failed retransmission.
 After c collisions, the range is between 0 and 2c
– 1, it then waits that number of slot times before
attempting retransmission.
 If repeated collisions occur, the range continues to expand, until after 10 attempts when it reaches
1023. After that the range of values stays fixed .If a station is unsuccessful in transmitting after 16
attempts, then gives up if cannot transmit
 low delay with small amount of waiting stations
 large delay with large amount of waiting stations
CONTROLLED ACCESS
 In controlled access, the stations consult one another to find which station has the right to send
 A station cannot send unless it has been authorized by other stations.
 Three popular control access methods are:
i. Polling
ii. Reservation
iii. Token Passing
i. Polling
 Stations take turns accessing the medium
 Two models: Centralized and distributed polling
 Centralized polling
 One device is assigned as primary station and the others as secondary stations
 All data exchanges are done through the primary.
 If the primary wants to receive data, it asks the secondary's if they have anything to send; this
is called poll function.

 If the primary wants to send data, it tells the secondary to get ready to receive; this is called
select function
 Polling can be done in order (Round-Robin) or based on predetermined order
Fig 2.19 signals in polling
 ACK is the acknowledgment of the secondary's ready status.
 Secondary responds either with a NAK frame if it has nothing to send or with data (in the
form of a data frame) if it does
 Distributed polling
 No primary and secondary.
 Stations have a known polling order list which is made based on some protocol.
 station with the highest priority will have the access right first, then it passes the access right
to the next station (it will send a pulling message to the next station in the pulling list), which
will passes the access right to the following next station, …
ii. Reservation
 A station needs to make a reservation before sending data.
 Transmissions are organized into variable length cycles.
 Each cycle begins with a reservation interval that consists of (N) minislots. One minislot for each of
the N stations.
 When a station needs to send a data frame, it makes a reservation in its own minislot.
 By listening to the reservation interval, every station knows which stations will transfer frames, and
in which order.
 The stations that made reservations can send their data frames after the reservation frame.
Fig 2.20 reservation scheme

iii. Token Passing
 Here the stations in a network are organized in a logical ring.
 In this method, a special packet called a token circulates through the ring.
 token gives the station the right to access the channel and send its data
 When a station receives the token and has no data to send, it just passes the data to the next
station.
Fig 2.21 token ring
Fig 2.22 flow chart for token ring
LAN SYSTEMS
ETHERNET
 Ethernet is a dominant physical and data link layer technology for local area networks (LANs).
 It is a bus based broadcast network using co-axial cable operating at 10 or 100 Mbps.
 It uses a control method called Carrier Sense Multiple Access/Collision Detection (CSMA/CD) to
transmit data.

Fig 2.23 Ethernet standards
IEEE 802.3 MAC Frame
Fig 2.24 IEEE802.3 MAC frame
 Preamble: Alternating 0s and 1s; used for synchronizing; 7bytes (56 bits).
 Start Frame Delimiter (SFD): 10101011 indicates the start of the frame. Last two bits alerts that the
next field is destination address.
 Destination Address (DA): 6 bytes (48 bits) physical address of destination station(s)
 Source Address (SA): 6 bytes (48 bits) physical address of sender
 Length/Type: if less than 1500, it indicates the length of data field. If greater than 1536, it indicates
the type of PDU.
 Data: 46 to 1500 bytes;
 CRC: CRC-32 for error detection
Addressing
 Each station on an Ethernet network has its own network interface card( NIC) fits inside and provides
a 6-byte physical address
Eg:06:01:02:01:2C:4B.
 First three bytes from left specify the vendor. (Cisco 00-00-0C, 3Com 02-60-8C) and the last 24 bit
should be created uniquely by the company.
 A source address is always a unicast address where as destination address can be unicast, multicast,
or broadcast.
 Unicast: defines one recipient ,second digit from left is even

 Multicast: defines a group of recipients ,Second digit from left is odd
 Broadcast : defines a group of all stations in the same LAN ,All ones
 The transmission is left-to-right, byte by byte; however, for each byte, the least significant bit is sent
first and the most significant bit is sent last.
Physical Layer Implementation
 The Standard Ethernet defines several physical layer implementations; four of the most common, are
Fig 2.25 Ethernet standards
Physical Layer Signaling
 Uses Manchester encoding.
 At the sender, data are converted to a digital signal using the Manchester scheme; at the receiver, the
received signal is interpreted as Manchester and decoded into data.
 Helps synchronize sender and recvr.
Fig 2.26 Manchester encoding
10Base5: Thick Ethernet
 Use a bus topology with an external transceiver (transmitter/receiver) connected via a tap to a thick
coaxial cable.
 10-Mbps transmission speed and 5 represents 500 meters maximum cable segment length.
 The transceiver is responsible for transmitting, receiving, and detecting collisions

Fig 2.27 10base 5 Thick Ethernet
10Base2: Thin Ethernet
 Uses a bus topology, but the cable is much thinner and more flexible.
 The transceiver is normally part of the network interface card (NIC), which is installed inside the
station.
 Can transmit 10 Mbps digital signals over coaxial cable.
 More cost effective than 10Base5 because thin coaxial cable is less expensive than thick coaxial.
 The length of each segment cannot exceed 185 m (close to 200 m).
Fig 2.28 10 base2 thin Ethernet
10Base-T: Twisted-Pair Ethernet
 Uses a physical star topology.
 The stations are connected to a hub via two unshielded twisted pair cables; One for transmitting data,
and the other for receiving data
 Maximum length of the cable segment can be 100 meters.
Fig 2.29 10 base T twisted pair

10Base-F: Fiber Ethernet
 Although there are several types of optical fiber l0-Mbps Ethernet, the most common is called
10Base-F.
 It uses a star topology to connect stations to a hub.
 The stations are connected to the hub using two fiber-optic cables.
Fig 2.30 10BaseF
BRIDGED ETHERNET
 LAN can be divided using bridges. Bridges have two effects on an Ethernet LAN:
• They raise the bandwidth
• They separate collision domains
 Raising the Bandwidth
 Each network is independent.
 Suppose there are 12 stations. And bandwidth is 10 Mbps.
 If we divide the network into 2 networks using bridge, each network has a capacity of
10 Mbps.
 The 10 Mbps capacity is shared between 7 stations, 6+1(bridge acts as a station in each
segment), not 12 stations.
Fig 2.31 network with bridge and without bridge

 Separating collision domains:
 Collisions domains become much smaller and possibility of collision is reduced.
 With bridging, lesser number of channels competes for access to the medium.
Fig 2.32 separating domains using bridge
SWITCHED ETHERNET
 The heart of the system is a switch containing room for typically 4 to 32 plug-in cards, each
containing one to eight connectors that allow faster handling of packets.
 When a station wants to transmit a frame, it outputs a frame to switch.
 Half duplex
Fig 2.33 Switched Ethernet
 All ports on the same card are wired together to form a local on-card LAN.
 Collisions on this on-card LAN are detected and handled using CSMA/CD protocol.
 One transmission per card is possible at any instant. All the cards can transmit in parallel.
 With this design each card forms its own collision domain.
FULL-DUPLEX SWITCHED ETHERNET
 Each station is connected to the switch through two links: one to transmit and one to receive.
 Increases the capacity of each domain from 10 to 20 Mbps.
 no chances of collision, so CSMA/CD is not used

Fig 2.34 full duplexed switched Ethernet
FAST ETHERNET
 IEEE 802.3 u.
 same frame format, media access, and collision detection rules as 10 Mbps Ethernet
 Data transfer rate of 100 Mb/s .
 Compatible with Standard Ethernet.
MAC Sub Layer
 The only two changes made in the MAC layer are the data rate and the collision domain
 A new feature added called Auto negotiation;allows two devices to negotiate the mode or data rate of
operation.
 For example, a device with a maximum capacity of 10 Mbps can communicate with a device with a
100 Mbps capacity.
Physical Layer
 Implementation
Fig 2.35 Fast Ethernet classification
Fig 2.36 fast Ethernet description

 Topologies
Fig 2.37 fast Ethernet topologies
GIGABIT ETHERNET
 IEEE 802.3z.
 All configurations of gigabit Ethernet are point to point.
 Point-to-point, between two computers or one computer – to –switch.
 Compatible with 100BASE-T and 10BASE-T
MAC Sublayer
Fig 2.38 gigabit Ethernet access methods
 It supports two different modes of medium access: full duplex mode and half duplex mode.
 Half duplex is used when computers are connected by a hub. Collision in hub is possible and so
CSMA/CD is required.
 Full duplex is used when computers are connected by a switch. No collision is there and so
CSMA/CD is not used.
 Carrier Extension tells the hardware to add its own padding bits after the normal frame to extend the
frame to 512 bytes.
Fig 2.39 carrier extension
 Frame Bursting allows a sender to transmit a concatenated sequence of multiple frames in a single
transmission. If the total burst is less than 512 bytes, the hardware pads it again.

Fig 2.40 frame bursting
Physical Layer
 Topology.
Fig 2.41 gigabit Ethernet topology
Fig 2.42 gigabit Ethernet physical layer implementations
Fig 2.43 gigabit Ethernet physical layer implementation description

LAN CONNECTING DEVICES
Fig 2.44 classification of LAN connecting devices
Fig 2.45 network connecting devices at each layer
REPEATERS
 A repeater (or regenerator) is an electronic device that operates on only the physical layer of the OSI
model.
 A repeater installed on a link receives the signal before it becomes too weak or corrupted, regenerates
the original pattern, and puts the refreshed copy back on the link.
Fig 2.46 repeater in network

 A repeater does not actually connect two LANS; it connects two segments of the same LAN.
 A repeater forwards every frame; it has no filtering capability
Fig 2.47 function of repeater
HUBS
Passive Hubs
 A passive hub is just a connector which connects the wires coming from different branches
Active Hub
 A Hub is a multiport repeater. used to create connections between stations in a physical star topology.
 Connection to the hub consists of two pairs of twisted pair wire one for transmission and the other for
receiving.
 it copy the received frame onto all other links
Fig 2.48 hub in network
BRIDGES
 Bridges operate in both the physical and the data link layers of the OSI model.
 Bridges can divide a large network into smaller segments.
 When a frame (or packet) enters a bridge, the bridge not only regenerates the signal but checks the
destination address and forwards the new copy only to the segment the address belong.
 This is done by a bridge table (forwarding table) that contains entries for the nodes on the LAN
 The bridge table is initially empty and filled automatically by learning from frames
movements in the network
 A bridge runs CSMA/CD before sending a frame onto the channel

Fig 2.49 bridge in a network
Types of Bridges
1. Simple Bridge
2. Multiport Bridge
3. Transparent Bridge
1. Simple Bridge
 The address table must be entered manually
 Whenever a new station is added or removed, the table must modify.
 Installation and maintenance of simple bridges are time-consuming and potentially more.
2. Multiport bridges
 A multiport bridge can be used to connect more than two LANs.
3. Transparent Bridges
 A transparent, or learning, bridge builds its table of station addresses on its own as it performs its
bridge functions.
 The stations are completely unaware of the bridge’s existence.
 A transparent bridge must meet three criteria:
1. Frames must be forwarded from one station to another.
2. The forwarding table is automatically made by learning frame movements in the network.
3. Loops in the system must be prevented.
Fig 2.50 transparent bridge table updation

Loop Problem
 Multiple paths of bridges and local-area networks (LANs) exist between any two LANs in the
internetwork.
 Having more than one transparent bridge between a pair of LAN segments can create loops in the
system.
 Bridging Loops Can Result in Inaccurate Forwarding and Learning in Transparent Bridging
Environments.
Fig 2.51 loop problem in network
Step 1. Station-A sends a frame to Station-B. Both the bridges forward the frame to LAN Y and update the
table with the source address of A.
Step 2. Now there are two copies of the frame on LAN-Y. The copy sent by Bridge-a is received by Bridge-
b and vice versa. As both the bridges have no information about Station B, both will forward the frames to
LAN-X.
Step 3. Again both the bridges will forward the frames to LAN-Y because of the lack of information of the
Station B in their database and again Step-2 will be repeated, and so on.
So, the frame will continue to loop around the two LANs indefinitely.
 Looping problem Is avoided by using Blocking ports (no frame is send out of these ports).
SPANNING TREE ALGORITHM
 IEEE 802.1d
 In graph theory, a spanning tree is a graph in which there is no loop
 In a bridged LAN, this means creating a topology in which each LAN can be reached from any other
LAN through one path only
 A LAN can be depicted as a graph, whose nodes are bridges and LAN segments (or cables), and
whose edges are the interfaces connecting the bridges to the LAN segments
Steps
 Every bridge has a built-in ID and the bridge with smallest ID is selected as the root bridge
 The algorithm tries to find the shortest path (a path with the shortest cost) from the root bridge to
every other bridge or LAN
 The combination of the shortest paths creates the shortest tree
 Based on the spanning tree, we mark the forwarding ports and blocking ports
 The forwarding ports are shown as solid lines, whereas the blocked ports are shown as dotted lines.

Fig 2.52 Spanning tree of a network of bridges
TWO-LAYER SWITCHES
 Two-layer switch performs at the physical and data link layers.
 Is a bridge, with many ports and allows better (faster) performance.
 Able to allocate a unique port to each station, with each station on its own independent entity.
 It makes a filtering decision based on the MAC address of the frame it received.
 It can have a buffer to hold the frames for processing.
 More than one station transmitting at a time.
 It can have a switching factor that forwards the frames faster.
BACK BONE NETWORKS
 A backbone network allows several LANs to be connected.
 In a backbone network, no station is directly connected to the backbone; the stations are part of a
LAN, and the backbone connects the LANs.
 The backbone is itself a LAN that uses a LAN protocol such as Ethernet and each connection to
the backbone is itself another LAN.
 The two most common architectures are the bus backbone and the star backbone.
Bus Backbone
 In a bus backbone, the topology of the backbone is a bus.
 Bus backbones are normally used as a distribution backbone to connect different buildings in an
organization.
Fig 2. 53 bus backbone

Star Backbone
 The topology of the backbone is a star; the backbone is just a switch.
 Mostly used as a distribution backbone inside a building.
Fig 2.54 star backbone
CONNECTING REMOTE LANS
Another common application for a backbone network is to connect remote LANs. This type of backbone
network is useful when a company has several offices with LANs and needs to connect them. The
connection can be done through bridges sometimes called remote bridges. The bridges act as connecting
devices connecting LANs. These bridges have ports that have data rate and signal levels compatible to
telephone network standard. The bridges establish a data link connection through the leased circuit and
then carry out bridge operation
Fig 2.55 Connecting remote LANs with bridges

Module 2 lan,data link layer

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