Networks: FDDI, ALOHA, VLAN, CSMA/CD, IEEE 802.3, 802.4, 802.5, and 802.11

Networks: FDDI, ALOHA, VLAN, CSMA/CD, IEEE 802.3, 802.4, 802.5, and 802.11

FDDI

FDDI (Fiber Distribution Data Interface) is similar to Token ring as it shares several characteristics including token passing and a ring architecture. FDDI uses a dual-ring connection architecture. Traffic on each ring in the interface flows in opposite directions (called counter-rotating). The dual rings has a primary and a secondary ring. The primary ring is used for data transmissions during normal operation while the secondary ring remains idle. The secondary ring is only used when the primary ring fails or to send some special information. The primary purpose of the dual rings is to make the network reliable and robust.

FDDI provides multiple ways to connect devices to the ring. FDDI defines three types of devices that can be connected: single attachment station (SAS) like PCs, dual attachment station (DAS) like routers, servers, and a concentrator.

ALOHA

Pure ALOHA

The original ALOHA protocol is called pure ALOHA. This is a simple, but an elegant protocol. The idea is that each station can send a frame whenever it wants a frame to send. However, since only one channel is shared among stations, there is the chance of collision between frames send from different stations. When a station sends a frame, it waits for the receiver to send an acknowledgment. If the receiver doesn't acknowledge for a time-out period, the station assumes that the frame (or the acknowledgment) has been destroyed at the middle and resends the frame.

A collision can occur between two or more stations. If all the stations resend their frames after the time-out, the frames will collide again. So, each station waits for a random amount of time before resending its frame. The randomness will help to avoid further collisions.

Slotted ALOHA

Slotted ALOHA is an improvement over the pure ALOHA. Pure ALOHA has no rule that defines when the station can send. A station may send anytime: after another station has started before another station has finished. In slotted ALOHA time is divided into the slot and the station can send only at the beginning of every time slot. As a station is only allowed to send at the beginning of the time slot; if a station misses this moment, it has to wait until the next time slot begins. But, still, there is the possibility of collision if two stations try to send at the beginning of the same time slot. However, the vulnerable time is reduced to one-half.

VLAN (Virtual LAN)

VLANs are the new type of LAN/BN architecture that uses intelligent, high-speed switches. VLAN assigns computers to LAN segments by using software. VLANs are designed in two ways Single-switch VLANs or Multi-switch VLANs. In Single switch VLANs, computers are assigned to VLANs using special software but physically connected together using a larger physical switch. Computer can be assigned to VLANs in different ways and they are:

• according to their VLAN switch port
• according to their data link layer address
• according to their IP address
• on the basis of the application that the computer uses

CSMA/CD

Carrier Sense Multiple Access with Collision Detection (CSMA/CD) is used to access computers of LAN in Ethernet. When a device tries to access the network, it checks to see if the network is free or not. If the network is not free, the device will wait for some random amount of time before retrying. If the network is free and two devices try to access the line at exactly the same time, their signals will collide. Then, when the collision is detected, they both back off and wait for a random amount of time and retry.

IEEE 802.3

IEEE 802.3 uses CSMA/CD for Multiple Access which senses carrier before transmitting and listens for a collision while transmitting. When it detects collision it backs off and wait for random time and retransmit.

Preamble (Pre) : 7 Bytes bit pattern 10101010… used for synchronization
Start-of-frame delimiter(SFD) : 10101011 indicates Start of Frame
DA,SA : 6 bytes Destination and Source MAC Addresses
Length/Type : Length of data Field in bytes (but in Ethernet II, this field identifies the Type of Network Layer Protocol used.)
Data : Upper Layer Data (min. 46 bytes, max.1500 bytes)
FCS : 4 Bytes error detection CRC Calculated over DA, SA, Length and Data Fields

IEEE 802.4

IEEE 802.4 was developed combining robustness of Linear Medium 802.3 and predictability of token passing. Stations are attached onto linear tree shaped cable. Each station knows the address of its left and right neighbors. A ring is first initialized by using a coordinator and the stations are inserted in the order of their address.

Start Delimiter: marks the beginning of frame.
Destination address: It specifies destination address.
Source address: It specifies bytes source address.
Checksum: to detect transmission errors.
End Delimiter: marks the end of frame.
Preamble : clock synchronization
Frame Control: Successors, predecessors, Entry of new station, Claim token, Token lost, station with token dead, Protocols to handle all issues, Useful for real-time traffic

IEEE 802.5

IEEE 802.5 uses token ring technique where a small frame called token is passed around the network. The node which passes the token can transmit data. If a node receiving the token has no information to send, it passes the token to the next end station. Each station can hold the token for a maximum period of time.

Start delimiter—Alerts each station of the arrival of a token (or data/command frame).
Access-control byte—Contains the Priority field (the most significant 3 bits) and the Reservation field (the least significant 3 bits),
End delimiter—Signals the end of the token or data/command frame.
Destination and source addresses—Consists of two 6-byte address fields that identify the destination and source station addresses.
Data—Indicates that the length of field is limited by the ring token holding time, which defines the maximum time a station can hold the token.
Frame-check sequence (FCS)— for error detection.
Frame Status—Is a 1-byte field terminating a command/data frame. The Frame Status field includes the address recognized indicator and frame-copied indicator.

IEEE 802.11

802.11 is the collection of standards set up for wireless networking. 802.11 lives in the physical layer and data link layer in the OSI. There are three popular standards: 802.11a, 802.11b, 802.11g and the latest one is 802.11n. Each standard uses a frequency to connect to the network and has a defined upper limit for data transfer speeds.

Frame Control: Contains following
Version: Protocol version Type : data, control or mgmt. Subtype : RTS or CTS
To/From DS: Going to or Coming from intercell distribution (eg. ethernet)
MF: More fragments to follow Retry: Retransmission of earlier frame
Pwr: used by base station to sleep or wake receiver
More: sender has more frames for receiver W: WEP Encryption
O : sequence of frames must be processed in order
Duration : time to occupy channel, used by other stations to manage NAV
Addresses : Two are source and design. Add. of sender and receiver, other two are that of base stations for intercell traffic.

Reference

• Tanenbaum, WetherRall, Andrew S. David J. Computer Networks. 5th ed. Boston : Pearson Education, Inc.
• W. Stallings, “Data and Computer Communication”, Macmillan Press, 1989.