Local & Metropolitan Area Networks ACOE322 Lecture 3 LAN types
0. LAN types • The types of LANs we will examine in this section are as follows: • Ethernet (IEEE 802.3) • Token Ring (IEEE 802.5) • Fiber Distributed Data Interface (FDDI) • Wireless LANs (IEEE 802.11)
1. Ethernet LANs 1.1 Traditional Ethernet (CSMA/CD) 1.2 10Mbps Ethernet 1.3 100Mbps Ethernet (Fast Ethernet - FE) 1.4 1000Mbps Ethernet (Gigabit Ethernet - GE)
1.1 Ethernet (CSMA/CD) • The most widely used LANs today are based on Ethernet and have been standardized by the IEEE 802.3 standards committee • The Ethernet uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD) as the access method • Recall: • Layers specified by 802.3: • Ethernet Physical Layer • Ethernet Medium Access (MAC) Sublayer • Possible Topologies: • Bus • Branching non-rooted tree for large Ethernets
Network Interface Card (NIC) • Each station on an Ethernet network (such as a PC, workstation or printer) has its own NIC. • The NIC fits inside the station and itis the interface between the station and the network. • In most desktop computers, the NIC is an Ethernet card (10, 100 or 1000 Mbps) that is plugged into a slot on the computer motherboard.
How does Ethernet work? • The NIC provides the station with a 6-byte physical address, normally written in hex notation. This is the MAC address. • Using MAC addresses to distinguish between machines, Ethernet transmits frames of data across baseband cables using CSMA/CD (IEEE 802.3)
What is a MAC Address? • Media Access Control (MAC) Address is the physical address of any device, e.g. a NIC in a computer on the network. • The MAC address is 6-bytes long and has two parts of 3 bytes long. Example: 06 D3 BF 7A 2C 01 the second 3 bytes are the serial number of the NIC The first 3 bytes specify the company that made the NIC
Minimal Bus Configuration Coaxial Cable Transceiver Terminator Transceiver Cable Host
Typical Large-Scale Configuration Repeater Host Ethernet segment
Ethernet Physical Layer • Transceiver • Transceiver Cable • 4 Twisted Pairs • 15 Pin Connectors • Channel Logic • Manchester Phase Encoding • 64-bit preamble for synchronization
Ethernet Physical Configuration(for thick coaxial cable) • Segments of 500 meters maximum • Maximum total cable length of 1500 meters between any two transceivers • Maximum of 2 repeaters in any path • Maximum of 100 transceivers per segment • Transceivers placed only at 2.5 meter marks on cable
Manchester Encoding Recall: • 1 bit = high->low voltage signal • 0 bit = low->high voltage signal Ethernet uses Manchester Encoding scheme Data stream Encoded bit pattern 1 0 1 1 0 0
Ethernet Synchronization • 64-bit frame preamble used to synchronize reception • 7 bytes of 10101010 followed by a byte containing 10101011 • Manchester encoded, the preamble appears like a sine wave
Ethernet Cabling Options • 10Base5: Thick Coax • 10Base2: Thin Coax (“cheapernet”) • 10Base-T: Twisted Pair • 10Base-F: Fiber optic • Each cabling option carries with it a different set of physical layer constraints (e.g., max. segment size, nodes/segment, etc.)
Ethernet: MAC Layer • Data encapsulation • Frame Format • Addressing • Error Detection • Link Management • CSMA/CD • Backoff Algorithm
Multicast bit Destination (6 bytes) Source (6 bytes) Length(2 bytes) Data (46-1500 bytes) Pad FCS (4 bytes) Ethernet Frame Format 64 Bytes <Ethernet frame size< 1518 Bytes
Ethernet MAC Frame Address Field • Destination and Source Addresses: • 6 bytes each • Two types of destination addresses • Physical address: Unique for each user • Multicast address: Group of users • First bit of address determines which type of address is being used 0 = physical address 1 = multicast address
Communicating Within the LAN Unicast Broadcast Multicast
IEEE 802.3 frame format (1) 64 Bytes* <Ethernet frame size< 1518 Bytes* * Excluding Preamble and SFD
IEEE 802.3 frame format (2) • Preamble: a 7-byte pattern of alternating 1s and 0s to establish bit synchronization • Start frame delimiter (SFD):The sequence 10101011 indicates the actual start of the frame • Destination address (DA): specifies the station(s) for which the frame is intended • Source address (SA): specifies the station that sent the frame • Length/type: length of LLC data field in bytes or Ethernet type field • LLC data: data unit supplied by LLC • Pad: bytes added to ensure that the frame is long enough for proper CD operation. Filled when Length < 46. • Frame check sequence (FCS): a 4-byte CRC check, based on all fields except preamble, SFD and FCS.
CSMA/CD revisited • Recall: • CSMA/CD is a “carrier sense” protocol. • If channel is idle, transmit immediately • If busy, wait until the channel becomes idle • CSMA/CD can detect collections. • Abort transmission immediately if there is a collision • Try again later according to a backoff algorithm • Carrier sense reduces the number of collisions • Collision detection reduces the impact of collisions
CSMA/CD inefficiency • Inefficiency of CSMA/CD • When two frames collide, the medium remains unusable for the duration of transmission of both damaged frames • For long frames, compared to propagation time, the amount of wasted capacity can be considerable. • This waste can be reduced if a station continues to listen to the medium while transmitting
CSMA/CD and Ethernet • Ethernet: • Short end-to-end propagation delay • Broadcast channel • Ethernet access protocol: • CSMA/CD with Binary Exponential Backoff Algorithm
Ethernet Backoff Algorithm:Binary Exponential Backoff • If collision, • Choose one slot randomly from 2k slots, where k is the number of collisions the frame has suffered. • One contention slot length = 2 x end-to-end propagation delay This algorithm can adapt to changes in network load.
Binary Exponential Backoff (cont’d) slot length = 2 x end-to-end delay = 15 ms A B @ t=0ms: Assume A and B collide (kA = kB = 1) A, B choose randomly from 21 slots: [0,1] Assume A chooses 1, B chooses 1 @ t=30ms: A and B collide (kA = kB = 2) A, B choose randomly from 22 slots: [0,3] Assume A chooses 2, B chooses 0 @ t=45ms: B transmits successfully @ t=75ms: A transmits successfully
Binary Exponential Backoff(cont’d) • In Ethernet, • Binary exponential backoff will allow a maximum of 15 retransmission attempts • If 16 backoffs occur, the transmission of the frame is considered a failure.
Ethernet Features and Advantages • Passive interface: • No active element • Broadcast: • All users can listen • Distributed control: • Each user makes own decision Simple Reliable Easy to reconfigure
Ethernet Disadvantages • Lack of priority levels • Cannot perform real-time communication • Security issues
Ethernet Switching • Recent development: Connect many Ethernet segments or subnets through an “Ethernet switch” to segment 4 to segment 1 to segment 3 to segment 2
Why Ethernet switching? • LANs may grow very large • The switch has a very fast backplane • It can forward frames very quickly from one segment to another • Cheaper than upgrading all host interfaces to use a faster network
1.2 10Mbps Ethernet Note: 10 refers to data rate in Mbps, BASE=Baseband 5= 50Ω coaxial cable 2= thinner 50Ω coaxial cable T= Unshielded twisted pair cable F = Fiber Optic cable • Physical configurations • 10BASE5 • 10BASE2 • 10BASE-T • 10BASE-F
10BASE-T medium specification • Unshielded twisted pair (UTP) cables are found prewired in office buildings as excess telephone cable and can be used for LANs • 10BASE-T defines a star topology • Stations attach to a multiport repeater via a point-to-point link • The link consists of two UTPs. • The data rate is 10Mbps using Manchester encoding • Length of link is limited to 100m • If an optical cable is used, the maximum length is 500m. • A 10BASE-T system can be mixed with 10BASE5 and 10BASE2 systems via repeaters • Maximum transmission path between any two stations is 5 segments and 4 repeater sets. • A segment is a point-to-point link or a coaxial cable • The maximum number of coaxial cable segments in a path is 3.
10BASE-F medium specification • Added to IEEE 802.3 in 1993 • Advantages of the distance and transmission on optical fiber • 10BASE-FP (passive): passive-star topology for stations/repeaters with up to 1 km per segment; makes synchronous retransmission • 10BASE-FL (link): a point-to-point link for connecting stations/repeaters at up to 2 km; asynchronous signaling; any timing distortions are propagated through a series of repeaters • 10BASE-FB (backbone): a point-to-point link for connecting repeaters at up to 2 km; a cascade up to 15 repeaters in sequence to activate greater length.
High speed LANs • Standard Ethernet LANs and MANs (up to 10Mbps) are based on one copper wire. • This is OK for low speed and short distances but not suitable for high speed and longer distances -> use fiber instead. • Fast and Gigabit Ethernet • FDDI (Fiber Distributed Data Interface) • HIPPI (High Performance Parallel Interface)
1.3 Fast Ethernet • Operates at 100 Mbps • Standardized in IEEE 802.3 as 100BASE-T and 100BASE-F • Basic idea behind Fast Ethernet (FE) was simple: keep all the old packet format, interfaces, procedural rules and cables, but just reduce the bit time from 100ns to 10ns.
Configuration and operation • In its simplest form, a 100BASE-T network is configured in a star-wire topology, with all stations connected directly to a central point (multiport repeater). • The repeater is responsible for detecting collisions, not the attached devices. Its functions are: • A valid signal appearing on any input is repeated on all output links • If two inputs occur at the same time, a jam signal is transmitted on all links
Collision domain • Used to define a single CSMA/CD network • This means that if two stations transmit at the same time, a collision will occur. • Stations separated by a simple multiport repeater are within the same collision domain • Stations separated by a bridge are in different collision domains Bridged Ethernet Separates collision domains
1.4 Gigabit Ethernet (GE) • Operates at 1Gbps • Same strategy as FE • CSMA/CD protocol and frame format, same as in 10Mbps and 100Mbps Ethernet • Compatible with 100BASE-T and 10BASE-T
Protocol architecture • The MAC layer is an enhanced version of the basic 802.3 MAC algorithm • A separate Gigabit Medium-Independent Interface (GMII) is optional except for the UTP
GE Physical layer • 1000BASE-LX • Long-wavelength option • Supports duplex links up up to 550m for multimode fiber or up to 5km for single mode fiber • 1000BASE-SX • Short-wavelength • Supports duplex links up to 550m for multimode fiber • 1000BASE-CX • Supports 1Gbps among devices located within a single room using copper jumpers (STP cable up to 25m long) • 1000BASE-T • Makes use of 4 pairs CAT5 UTP cables to support devices over a range of up to 100m
GE Medium Access Layer (1) • Same CSMA/CD frame format and MAC protocol as in 10Mbps Ethernet and FE.
GE Medium Access Layer (2) • For hub operation (half duplex) there are two enhancements to the basic CSMA/CD scheme • Carrier extension • Appends a set of special symbols to the end of short MAC frames so that the resulting block is at least 4096 bit times in duration, up from the minimum 512 bit times imposed at standard Ethernet and FE. • Frame bursting • Allows for multiple short frames to be transmitted consequentially without releasing control for CSMA/CD between frames • Avoid the overhead of carrier extension when a single station has a number of small frames ready to send
2. Token Ring • Widely used although not as popular as Ethernet • Based on IEEE 802.5 standard • We will now examine the Medium Access Layer and then the Physical layer aspects of this specification
Token ring operation • Based on the use of a small frame, called a token, that circulates when all stations are idle • Whenever a station wishes to send a frame, it first waits for the token • On receipt of the token, it initiates transmission of the frame, which includes the address of the intended recipient at its header • The frame is repeated by all stations in the ring, until it circulates back to the initiating station, where it is removed • The intended recipient also retains a copy of the frame
Token Passing (1) Station A captures the token and sends its data to D Token is traveling along the ring