Local Area Networks

Question 1

LAN

what’s a LAN?

Answer 1

• transmission system, usually private owned, speedy and secure, covering a geographical area in the range of km, comprising a shared transmission medium and a set of hardware and software for interfacing devices to the medium and regulating the orderly access to the medium

Question 2

Ring Topology

Answer 2

• repeaters joined by point to point links in a closed loop
• data in frames - circulates past all stations, destination recognizes address and copies frame, then frame circulates back to source where it is removed
• media access control determines when station can insert frame
• advantages:
• failure of a station does not affect ring transmission;
• can cover a wider area (LANs, MANs, WANs)
• disadvantages:
• limited number of nodes/ring;
• failure of repeater may cause failure of ring transmission
• more complex operation to insert new nodes and to manage network (clock synchronization)

Question 3

Star Topology

Answer 3

• each station connected directly to central node
• central node can broadcast
• central node can act as frame switch
• advantages:
• easy to extend topology
• less prone to problems with connecting devices
• more security (central access point - the switch)
• flexibility for customer needs
• disadvantages:
• central switch down => all network down
• local switch down => local network down
• wiring overhead

Question 4

Most used contention-based MAC techniques

Answer 4

• ALOHA - simplest protocol; each station checks the frame and if the address matches the receiver, sends ACK; frame may be damaged by noise or collision
• slotted ALOHA - more efficient due to the organized time in uniform slots, equal to frame transmission time; needs synch. mechanism
• CSMA/CD - stations listen while transmitting, detecting collision occurence

Question 5

FDDI

Answer 5

• use of token ring MAC algorithm
• station seizes token by aborting token transmission
• once token captured, one or more data frames transmitted
• new token released as soon as transmission finished
• allows for synch. & asynch. frame transmissions

Question 6

FDDI

frame

Answer 6

Topology - two FO rings: primary and back-up ring
Types of stations:
* DAS (Dual Attach. Station)
* DAC (Dual Attach. Concentrator)
* SAS (Single Attach. Station) - connected only to the primary ring

Question 7

Ring topology LANs

IEEE 802.5 Token Ring

Answer 7

• Differential Manchester encoding
• Each station connected using a repeater, which introduces a delay when active!
• Each repeater connects to two others via unidirectional transmission links in a single closed path
• Data transferred bit by bit, from one repeater to the next
• Repeater performs data insertion, data reception, data removal
• Packet removed by transmitter after one trip round ring

Repeater is in one of the following states:

• listen state:
• copy incoming bit and send to attached station
• modify bit as it passes to indicate a packet has been copied (acts as ACK)
• transmit state:
• repeater receives bits from station and puts them on ring
• bypass state
• signals propagate past repeater with no delay (other than propagation delay)