Internet Architecture and Protocol Set# 03 Local Area Networks Delivered By: Engr Tahir Niazi
Layer Reference to Protocol Application Presentation Session Application FTP, Telnet, SMTP, HTTP, SNMP.. Transport Host-to-Host TCP, UDP Network Internet IP, ICMP Data Link Physical Network Access LAN Ethernet, Token-Ring, FDDI, Fast Ethernet...
LAN (Local Area Networks) A LAN is a computer network that covers a small area (home, office, building, campus) a few kilometers LANs have higher data rates (Mbps to Gbps) as compared to WANs LANs (usually) do not involve leased lines; cabling and equipments belong to the LAN owner. A LAN consists of Shared transmission medium now so valid today due to switched LANs (for wired LANs), but still valid for wireless LANs regulations for orderly access to the medium set of hardware and software for the interfacing devices
IEEE 802 Subgroups and their Responsibilities 802.1 Internetworking 802.2 Logical Link Control (LLC) 802.3 CSMA/CD 802.4 Token Bus LAN
IEEE 802 Subgroups and their Responsibilities (Cont.) 802.5 Token Ring LAN 802.6 Metropolitan Area Network 802.7 Broadband Technical Advisory Group 802.8 Fiber-Optic Technical Advisory Group
IEEE 802 Subgroups and their Responsibilities (Cont.) 802.9 Integrated Voice/Data Networks 802.10 Network Security 802.11 Wireless Networks 802.12 Demand Priority Access LANs
Ethernet Protocol Standards 10 Mbps IEEE 802.3 100 Mbps IEEE 802.3u 1 Gbps IEEE 802.3ab 10 Gbps IEEE 802.3ae
LAN Protocol Architecture Corresponds to lower two layers of OSI model Current LANs are most likely to be based on Ethernet protocols developed by IEEE 802 committee IEEE 802 reference model Logical link control (LLC) Media access control (MAC) Physical
IEEE 802 Protocol Layers vs. OSI Model
IEEE 802 Layers - Physical Signal encoding/decoding Preamble generation/removal for synchronization Bit transmission/reception Specification for topology and transmission medium
802 Layers OSI layer 2 (Data Link) is divided into two in IEEE 802 Logical Link Control (LLC) layer Medium Access Control (MAC) layer LLC layer Interface to higher levels flow control Based on Data Link Control Protocols MAC layer Prepare data for transmission Error detection Address recognition Govern access to transmission medium
LAN Protocols in Context
Generic MAC & LLC Format Actual format differs from protocol to protocol MAC layer receives data from LLC layer MAC layer detects errors and discards frames LLC optionally retransmits unsuccessful frames
LAN Topologies Bus Ring Star
Bus Topology - 1 Stations attach to linear medium (bus) Via a tap - allows for transmission and reception Transmission propagates in medium in both directions Received by all other stations Not addressed stations ignore Need to identify target station Each station has unique address Destination address included in frame header Terminator absorbs frames at the end of medium
Bus Topology - 2 Need to regulate transmission To avoid collisions If two stations attempt to transmit at same time, signals will overlap and become garbage To avoid continuous transmission from a single station. If one station transmits continuously, access is blocked for others Solution: Transmit Data in small blocks frames
Ring Topology Repeaters joined by pointto-point links in closed loop Links are unidirectional Receive data on one link and retransmit on another Stations attach to repeaters Data transmitted in frames Frame passes all stations in a circular manner Destination recognizes address and copies frame Frame circulates back to source where it is removed Medium access control is needed to determine when station can insert frame
Frame Transmission Ring LAN
Star Topology Each station connected directly to central node using a full-duplex (bi-directional) link Hub or Switch Central node can broadcast (hub) Only one station can transmit at a time; otherwise, collision occurs Central node can act as frame switch
Medium Access Control (MAC) Traditionally, in LANs data is broadcast there is a single medium shared by different users We need MAC sublayer for orderly and efficient use of broadcast medium This is actually a channel allocation problem Synchronous (static) solutions everyone knows when to transmit Asynchronous (dynamic) solution in response to immediate needs Two categories Round robin Contention
Static Channel Allocation Frequency Division Multiplexing (FDM) Channel is divided to carry different signals at different frequencies Efficient if there is a constant (one for each slot) amount of users with continous traffic Problematic if there are less or more users Even if the amount of users = # of channels, utilization is still low since typical network traffic is not uniform and some users may not have something to send all the time
Static Channel Allocation Time Division Multiplexing Each user is statically allocated one time slot if a particular user does not have anything to send, it remains idle and wastes the channel for that period A user may not utilize the whole channel for a time slot Thus, inefficient.
Dynamic Channel Allocation Categories Round robin each station has a turn to transmit declines or transmits up to a certain data limit Performs well if many stations have data to transmit for most of the time otherwise passing the turn would cause inefficiency
Dynamic Channel Allocation Categories Contention All stations contend to transmit No control to determine whose turn is it Stations send data by taking risk of collision (with others packets/frames) however they understand collisions by listening to the channel, so that they can retransmit In general, good for bursty traffic which is the typical traffic types for most networks Efficient under light or moderate load Performance is bad under heavy load
ALOHA When station has frame, it sends collisions may occur Station listens for max round trip time If no collision, fine. If collision, retransmit after a random waiting time Collison is understood by listening
Slotted ALOHA Divide the time into discrete intervals (slots) equal to frame transmission time need central clock (or other sync mechanism) transmission begins at slot boundary Collided frames will do so totally or will not collide Algorithm If a node has a packet to send, sends it at the beginning of the next slot If collision occurred, retransmit at the next
Aloha & Slotted Aloha
CSMA (Carrier Sense Multiple Access) First listen for clear medium (carrier sense) If medium idle, transmit If busy, continuously check the channel until it is idle and then transmit If collision occurs Wait random time and retransmit (called back-off ) Collision probability depends on the propagation delay Longer propagation delay, worse the utilization
Ethernet 802.3 (CSMA/CD) Carriers Sense Multiple Access with Collision Detection is the underlying technology (protocol) for medium access control IEEE 802.3 standard (1983) Contention technique that has basis in famous ALOHA network
Access Method: CSMA/CD The Ethernet architecture does not have a regulated access to the medium. Whenever multiple users have unregulated access to a single line, there is a danger of signals overlapping and destroying each other. Such overlaps, which turn the signals into unusable noise, are called collisions. As traffic increases on a multiple-access link, so do collisions.
Access Method: CSMA/CD The access mechanism used in an Ethernet is called carrier sense multiple access with collision detection CSMA/CD In CSMA/CD system, any workstation wishing to transmit must first listen for existing traffic on the line. A device listens by checking for a voltage. If no voltage is detected, the line is considered idle and the transmission is initiated. During the data transmission, the station checks the line for the extremely high voltages that indicate a collision.
If a collision is detected, the station quits the current transmission and waits a predetermined amount of time for the line to clear, then sends its data again. Addressing Each station on an Ethernet network has its own network interface card (NIC). The NIC usually fits inside the station and provides the station with a six-byte physical address. The number on the NIC is unique. Data Rate Ethernet LANs can support different data rates between 1 and 1000Mbps.
Frame Format IEEE 802.3 specifies one type of frame containing seven fields:
Frame Format Preamble The preamble contains seven bytes (56 bits) of alternating 0s and 1s that alert the receiving system to the coming frame and enable it to synchronize its input timing. Start Frame delimiter SFD The SFD tells the receiver that everything that follows is data, starting with the addresses. Destination Addresses DA The DA field contains the physical destination address (six bytes) of the frame s next destination CRC The last field in the 802.3 frame contains the error detection information, in this case CRC-32.
Frame Format Source Address The SA field contains the physical address ( six bytes) of the last device to forward the frame. That device can be the sending station. Length/type of Protocol Data Unit (PDU) This field contains two bytes indicating the number of bytes in the PDU. If the length of the PDU is fixed, this field can be used to indicate type, or as a base for other protocols. Protocol Data Unit PDU The PDU is generated by the upper LLC sublayer, then linked to the 802.3 frame. The PDU can be anywhere from 46 to 1500 bytes long, depending on the type of frame and the length of the information field.
Implementation Ethernet LANs are configured as logical buses, although they may be physically implemented in bus, ring or star topologies. Each frame is transmitted to every station on the link but read only by the station to which it is addressed.
Categories of traditional Ethernet
10BASE5: Thick Ethernet 10Base5 is a bus topology LAN that uses based signaling and has a maximum segment length of 500 meters. To reduce collisions, the total length of the bus should not exceed 2500 meters (five segments). Also, the standard demands that each station be separated from each neighbor by 2.5 meters. The physical components includes coaxial cable, network interface cards, transceivers, and attachment unit interface (AUI) cables. RG-8 cable is thick coaxial cable that provides the backbone of the IEEE802.3 standard. Transceiver Each station is attached by an AUI cable to an intermediary device called a medium attachment unit (MAU) or transceiver.
10BASE5: Thick Ethernet The transceiver performs CSMA/CD function of checking for voltages and collisions on the line and may contain an small buffer. It also serves as the connector that attaches a station to the thick coaxial cable itself via a tap. AUI cables Each station is linked to its corresponding transceiver by an attachment unit interface (AUI), also called a transceiver cable. AUIs are restricted to a maximum length of 50 meters. Transceiver tap Each transceiver contains a connecting mechanism, called a tap because it allows the transceiver to tap into the line at any point.
Topology of 10BASE5
10BASE2:Thin Ethernet Like 10Base5, 10Base2 is a bus topology LAN. The advantages of thin Ethernet are reduced cost and ease of installation. The disadvantages are shorter range (185 meters) The NICs in a thin Ethernet system provide all the same functionality as those in a thick Ethernet, plus the functions of the transceivers. The cable required to implement the 10Base2 standard is RG-58. These cables are relatively easy to install and move.
Topology of 10BASE2
10Base-T: Twisted-Pair Ethernet A star-topology LAN using unshielded twisted pair (UTP) cable instead of coaxial cable. It supports a data rate of 10Mbps and has a maximum length (hub to station) of 100 meters. It requires a hub with a port for each station. The hub fans out any transmitted frame to all to all its connected stations.
Topology 10BASE-T
10Base-FL: Fiber Link Ethernet 10Base-FL uses a star topology to connect stations to a hub. The standard is normally implemented using an external transceiver called fiber-optic MAU. The station is connected to the external transceiver by an AUI cable. The transceiver is connected to the hub by using two pairs of fiber-optic cables.
Switched Ethernet The 10Base-T Ethernet is a shared media network, which means that the entire media is involved in each transmission. Any frame produced by one station is retransmitted by the hub to all stations in the network. Thus, if two stations try to send frames simultaneously, there is a collision. If we replace the hub by a switch, a device that can recognize the destination address and can route the frame to the port to which the destination station is connected, the rest of the media are not involved in the transmission process.
Full-duplex switched Ethernet In Full-duplex switched Ethernet each station is connected to the switch through two links: one to transmit and one to receive. The speed passes from 10Mbps to 20Mbps. There is no need for the CSMA/CD method.
Fast Ethernet implementations
Fast Ethernet Fast Ethernet operates to 100 Mbps. Star topology 100Base-TX The 100Base-TX design uses two category 5 unshielded twistedpair (UTP) or two shielded twisted-pair (STP) cables to connect a station to the hub. One pair is used to carry frames from the station to the hub and the other to carry frames from the hub to the station. The distance between the station and the hub should be less than 100 meters.
Fast Ethernet 100Base-FX The 100Base-FX design uses two optical fibers, one to carry frames from the station to the hub and the other from the hub to the station. The distance between the station and the hub should be less than 2000 meters. 100Base-T4 It requires four pairs of category 3 (voice grade) UTP. Two of the four pairs are bidirectional; the other two are unidirectional. The 100 Mbps flow of data is divided into three 33.66-Mbps flows
Gigabit Ethernet It supports a data rate of 1 Gbps (1000 Mbps) Usually serves as a backbone to connect Fast Ethernet Networks. Four implementations have been designed for Gigabit Ethernet: 1000Base-LX, 100Base-SX, 1000Base-CX and 1000Base-T.
Token Bus Local area networks have a direct application in factory automation and process control. In this case the nodes are computers controlling manufacturing process. Ethernet (IEEE802.3) is not a suitable protocol for these purposes because the number of collisions is not predictable and the delay in sending data from a control center (a computer in the network) to other devices or computers along the assembly line is not a fixed value.
Token Bus Token Bus (IEEE 802.4) combines features of Ethernet (a bus topology) and Token ring. Token bus is a physical bus that operates as a logical ring using tokens. Stations are logically organized into a ring. A token is passed among the stations. If a station wants to send data, it must wait and capture the token. Token Bus is limited to factory automation and process control and has no commercial applications in data communication.
Token Ring (IEEE 802.5) Token ring requires that stations take turns sending data. Each station may transmit only during its turn and may send only one frame during each turn. The mechanism that coordinates this rotation is called token passing.
Token Passing
Token Passing
Token Passing
Token Passing
Token Ring (IEEE 802.5) Access Method: Token passing Whenever the network is unoccupied, it circulates a simple threebyte token. This token is passed from NIC to NIC in sequence until it encounters a station with data to send. That station waits for the token to enter its network board. If the token is free the station may send a data frame. This data frame proceeds around the ring regenerated by each station.
Token Ring (IEEE 802.5) Each intermediate station examines the destination address, if the frame is addressed to another station, the station relays it to its neighbor. If the station recognizes its own address, copies the message, checks for errors, and changes four bits in the last byte of the frame to indicate address recognized and frame copied. The full packet then continues around the ring until it returns to the station that sent it.
Token Ring (IEEE 802.5) The sender receives the frame and recognizes itself in the source address field. It then checks the address- recognized bits. If they are set, it knows that the frame was received. The sender then discards the used data frame and releases the token back to the ring.
Priority and reservation A busy token can be reserved by a station waiting to transmit regardless of that station s location on the ring. Each station has a priority code. As a frame passes by, a station waiting to transmit it may reserve the next open token by entering its priority code in the access control (AC) field of the token or data frame. A station with a higher priority may remove a lower priority reservation and replace it with its own.
Token Ring Frame
Monitor stations Several problems may occur to disrupt the operation of a token ring network. 1. A station may neglect to retransmit a token 2. A token may be destroyed by noise 3. A sending station may not release the token once its turn has ended 4. A sending station may neglect to remove its used data frame from the ring To handle these situations, one station on the ring is designated as monitor station.
Addressing Token ring uses a six-byte address, which is imprinted on the NIC card. Data Rate Token ring supports data rates of up to 16 Mbps. Frame formats The token ring protocol specifies three types of frames: 1. Data/command Frames 2. Token Frames 3. Abort Frames
The data/command frame is the only one of the three types of frames that can carry a PDU and is the only addressed to a specified destination. The nine fields of the frame are start delimiter (SD), access control (AC), frame control (FC), destination address (DA), source address (SA), PDU, CRC, en delimiter (ED), frame status (FM). The AC field is one byte long and includes four subfields: Priority ( 3 bits), Token ( 1 bit), Monitor (1bit), reservation (3bits)
Token bit: 1 indicates that the frame is a data/command frame; 0 indicates a token or abort frame. The frame control (FC) field is one byte long and contains two fields: type and special information. Type is one bit field used to indicate the type of information contained in the PDU ( control information or data). Special information field is a 7 bits field. This field contains information used by the Token ring logic.
The last byte of the frame is the Frame status field (FS). It can be set by the receiver to indicate that the frame has been read, or by the monitor to indicate that the frame has already been around the ring. Token frame This frame includes only three fields: the SD, AC, and ED. The SD indicates that a frame is coming. The AC indicates that the frame is a token and includes the priority and reservation fields. Abort frame An abort frame carries no information at all, just starting and ending delimiters. It can be generated either by the sender to stop its own transmission or by the monitor to purge an old transmission from the line.
FDDI: Fiber Distributed Data Interface Supports transmission rates of up to 200 Mbps Uses a dual ring First ring used to carry data at 100 Mbps Second ring used for primary backup in case first ring fails If no backup is needed, second ring can also carry data, increasing the data rate up to 200 Mbps Supports up to 1000 nodes Has a range of up to 200 km
FDDI Architecture
The physical layer defines the electrical, mechanical, and logical characteristics for transmitting bits across the physical medium. Examples of physical media include twisted pair, coaxial, and fiber optic cable. Dual ring FDDI specifies fiber optic cable as the physical medium. The data link layer specifies the way a node accesses the underlying physical medium and how it formats data for transmission. FDDI specifies formatting data into frames, using a special set of symbols and following a special set of rules. The MAC sub layer within the data link layer specifies the physical address (MAC address) used for uniquely identifying FDDI nodes
Transmission Media FDDI uses optical fiber as the primary transmission medium, but it also can run over copper cabling. FDDI over copper is referred to as Copper-Distributed Data Interface (CDDI). FDDI defines two types of optical fiber: single-mode and multimode. Multimode: Multimode fiber uses LED as the light-generating device. Multimode fiber allows multiple modes of light to propagate through the fiber.. multimode fiber is generally used for connectivity within a building or a relatively geographically contained environment. Single-mode: single-mode fiber generally uses lasers. Single-mode fiber allows only one mode of light to propagate through the fiber. Therefore, single-mode fiber is capable of delivering considerably higher performance connectivity over much larger distances, which is why it generally is used for connectivity between buildings and within environments that are more geographically dispersed.
FDDI Specification/Component FDDI is defined by four separate specifications: 1. Media Access Control (MAC)---Defines how the medium is accessed, including frame format, token handling, addressing, algorithm for calculating a cyclic redundancy check value, error recovery mechanism 2. Physical Layer Protocol (PHY)---Defines data encoding/decoding procedures, clocking requirement, framing and other function. 3. Physical Layer Medium (PMD)---Defines the characteristics of the transmission medium, including the fiber-optic link, power levels, bit error rates, optical components, and connectors. 4. Station Management (SMT)---Defines the FDDI station configuration, ring configuration, and ring control features, including station insertion and removal, initialization, fault isolation and recovery, scheduling, and collection of statistics.
FDDI Basic Principle Token circulates around a ring in network. A station first capture the token,send packet of data to network. After transmission token is released. Every station on the network will receive the transmission and repeat it. The transmission will travel around the ring until it is received by the station which originally sent it, which removes it from the ring. If a station does not receive its transmission back, it assumes that an error occurred some where. To solve this problem fault isolation techniques is used.
Cable Types: There are four cable types which can be used with FDDI. They are: Multimode Fiber Optic Cable Fiber optic cable, usually with a core size of 62.5 microns. It allows distances up to 2000 meters (6600 feet). Single-mode Fiber Optic Cable Fiber optic cable with a core size of 7 to 11 microns. It allows distances up to 10,000 meters (33,000 feet). Category 5 UTP An unshielded copper cable, usually with eight wires. The wires are twisted together in pairs, and the cable is rated at frequencies up to 100 MHz. It allows distances up to 100 meters (330 feet). IBM Type 1 STP A heavy, shielded copper cable. It consists of four wires, twisted in to two pairs. Each pair is covered with an individual shield, and an overall shield covers the entire cable. It allows distances up to 100 meters (330 feet).
FDDI Frame Format The FDDI frame format is similar to the format of a Token Ring frame. FDDI frames can be as large as 4,500 bytes.
The following descriptions summarize the FDDI data frame and token fields illustrated in the above figure. Preamble (16 bits)- Gives a unique sequence that prepares each station for an upcoming frame. Start delimiter (8 bits)- - Indicates the beginning of a frame by employing a signaling pattern that differentiates it from the rest of the frame. Frame control (8 bits)- Type and special information in PDU Destination address (48bits)- - Contains a unicast (singular), multicast (group), or broadcast (every station) address. As with Ethernet and Token Ring addresses, FDDI destination addresses are 6 bytes long. Source address (48 bits)- - Identifies the single station that sent the frame. As with Ethernet and Token Ring addresses, FDDI source addresses are 6 bytes long.
Data - Contains either information destined for an upper-layer protocol or control information. Frame check sequence (FCS) (32 bits)- - Is filed by the source station with a calculated cyclic redundancy check value dependent on frame contents (as with Token Ring and Ethernet). The destination address recalculates the value to determine whether the frame was damaged in transit. If so, the frame is discarded. End delimiter (16 bits)- - Contains unique symbols; cannot be data symbols that indicate the end of the frame. Frame status (16 bits)- - Allows the source station to determine whether an error occurred; identifies whether the frame was recognized and copied by a receiving station.
FDDI Benefits high bandwidth (10 times more than ethernet). larger distances between fddi nodes because of very low attenuation. improved signal-to-noise ratio because of no interference from external radio frequencies and electromagnetic noise Limitation of FDDI high cost of optical components required for transmission/reception of signals (especially for single mode fiber networks) more complex to implement.
Q & A IAP, University of Sargodha, CS & IT Dept