Local rea Networks 4/11/40 page 1 Faculty of Engineering, Kasetsart University Local rea Networks 1-1
LN characteristics confined within geographical area relatively high data rate under single management LN Ingredients... Topology bus, star, ring hub hub hub stations stations station Transmission medium twisted pair, coaxial, fiber optics, wireless hub Medium access techniques protocol to coordinate the sharing of media router server 4/11/40 page 2 Communications network that serves users within a confined area is known as a Local rea Network (LN). Computers (server and workstations) are normally distributed around a single office or building, including and network device like hub, bridge or router. Messages are transmitted in LN as a series of packet with high data rate (small propagation delay). The packets may be fixed or variable length. Most LNs have broadcast capability. Faculty of Engineering, Kasetsart University Local rea Networks 1-2
Topology Topology defines how nodes/stations are connected LN typical topology : bus/tree : all nodes connected to a common medium star : all nodes are joined to a central nodes ring : nodes form a ring by point-to-point links to adjacent neighbors bus star ring 4/11/40 page 3 The topology of a network defines the structure of its logiacal layout. There are three main categories of LN topologies: bus/tree, star and ring. Faculty of Engineering, Kasetsart University Local rea Networks 1-3
Topology : Bus B C D D B and C pplication Presentation Session Transport Network Data Link Physical D pplication Presentation Session Transport Network Data Link Physical 4/11/40 page 4 In a bus network, one node s transmission traverses the entire network and is received and examined by every node The access method can be : (1) Contention scheme : multiple nodes attempt to access bus; only one node succeed at a time (e.g. CSM/CD in Ethernet) (2) Round robin scheme : a token is passed between nodes; node holds the token can use the bus (e.g.token bus) dvantages: (1) Simple access method (2) Easily add or remove stations Disadvantages: (1) Poor efficeincy with high network load (2) Relatively insecure, due to the shared medium Faculty of Engineering, Kasetsart University Local rea Networks 1-4
Topology : Ring nodes function as a repeater only destination copies frame to it, all other nodes have to discarded the frame Unidirectional link C B transmits frame addressed to B C C ignores frame B C B C B copies frame C absorbs returning frame 4/11/40 page 5 ring consists of a single cable in the shape of loop. Each station is only connected to each of its two nearest neighbors. Data in the form of pakests pass around the ring from one station to station in designated unidirection. dvantages : (1) ccess method supports heavy load without degradtion of performance because the medium is not shared. (2) Several packets can simulteneous circulate between different pairs of stations. Disadventages: (1) Comples and elaborate management. (2) Reinitialztion of the ring whenever a failure occurs. Faculty of Engineering, Kasetsart University Local rea Networks 1-5
Topology : Star Each station attaches to the central node Two point-to-point links for tranmitting and receiving Two alternatives operation broadcasting switching B C shared hub broadcast packet to every ports but only destination copies frame B C C transmitts frame addressed to B C switch hub retransmitts packet to the destination only 4/11/40 page 6 ll stations are attatched to a single central node. The main advantage of star topology is easy to manage with relative low cost. But the network is very vulnerable, since it depends entirely upon the operation of the central node. Faculty of Engineering, Kasetsart University Local rea Networks 1-6
Transmission media Twisted pair Cat 3 -- voice grade UTP, data rate up to 10Mbps Cat 5 -- data grade UTP, data rate 100 Mbps Coaxial cable baseband coaxial support data rates 20 Mbps at distance of up to 2 km Fiber optics support gigabit data rate and longer distances 4/11/40 page 7 Faculty of Engineering, Kasetsart University Local rea Networks 1-7
Medium ccess Control Protocol for controlling access to transmission medium Defined as part of Data Link layer The protocol performs: perform functions related to medium access (MC sublayer) concerned with the transmission of a link-level between two nodes (LLC sublayer) Network Data Link Physical LLC MC Logical Link Control sublayer Medium ccess Control sublayer 4/11/40 page 8 The role of Data Link layer is to permit the trasnfer of data between the stations and detect trasnmission errors. IEEE divides this layer into separated sublayer : MC (Medium ccess Control) and LLC (Logical Link Layer) The MC sublayer control the access of medium using access method. The LLC provides interface to next upper layer. Faculty of Engineering, Kasetsart University Local rea Networks 1-8
LLC Sublayer Functions Provides a compatibility interface, irrespective of the MC layer used. Service ccess Points (SPs) interface sublayers to upper-layer functions Optional connection, flow control, and sequencing services 4/11/40 page 9 The LLC sublayer rests on top of the other 802 protocols to provide interface flexibility. Upper layers, for example IP at Layer 3, can operate autonomously without regards for the specific type of LN media. This layer remains consistent no matter what type of MC protocol (Token ring, Token bus or CSM/CD) is used.. From the perspective of these lower MC sublayers, the service access point (SP) process provides a convenient interface to the upper OSI layers. These SP entries simplify access to the shared channel up to the specified upper-layer service identified by LLC SP entities. LLC sublayer options include support for connections between applications running on the LN, flow control to the upper layer by means of ready/not ready codes, and sequence control bits. Faculty of Engineering, Kasetsart University Local rea Networks 1-9
MC frame format MC control Destination ddress Source ddress Type LLC PDU CRC MC Frame (802.3) SPs define a protocol for upper layer DSP SSP CTRL Data LLC PDU DSP: Destination Service ccess Point SSP: Source Service ccess Point Control: Command and response functions Data: LLC SDU 4/11/40 page 10 From the perspective of these lower MC sublayers, the service access point (SP) process provides a convenient interface to the upper layers. These SP entries simplify access to the shared channel up to the specified upper-layer service identified by LLC SP entities. LLC carries two 8-bit addresses, the destination SP and the source SP LLC sublayer options include support for connections between applications running on the LN, flow control to the upper layer by means of ready/not ready codes, and sequence control bits. Faculty of Engineering, Kasetsart University Local rea Networks 1-10
MC ddress 24 bits 24 bits Vendor Code Serial Number 0000.0c12. 3456 ROM RM MC address is burned into ROM on a network interface card 4/11/40 page 11 The MC sublayer defines a hardware address called the MC address. The MC address is unique for each LN interface. The MC address is burned into ROM-hence the term, burned-in address (BI) in most interface cards. When the network interface card initializes, this address is copied into RM. The MC address is a 48-bit addressed as 12 hexadecimal digits. The first 6 hexadecimal digits of a MC address contain a manufacture identification (vender code) also known as the Organizationally Unique Identifier (OUI). IEEE administers OUIs to ensure vender uniqueness. The last 6 hexadecimal digits are administered by each vendor and represent the interface serial number. Faculty of Engineering, Kasetsart University Local rea Networks 1-11
MC Strategies Where? centralized : a designated station has an authurity to grant access to the network distributed : stations can dynamically determined transmission order How? Sychronous : dedicated specific capacity to a conection synchronous : dynamic allocate capacity (3 categories available) Round-robin Reservation Contention 4/11/40 page 12 Faculty of Engineering, Kasetsart University Local rea Networks 1-12
MC techniques Contention Medium is free for all node senses the free medium and occupies it Example: Ethernet, IEEE 802.3 Round robin Give every body a turn Inefficient for lightly loaded network Example: Token Ring/IEEE 802.5, Token Bus/IEEE 802.4, FDDI Reservation schedule a time slot like TDM Example: DQDB 4/11/40 page 13 The access method corresponds to the protocol for managing the transmission and can be categorized as above. Faculty of Engineering, Kasetsart University Local rea Networks 1-13
Overview of LN Standard 802.1 802.2 802.3 802.4 802.5 802.6 FDDI Overview Logical Link Control CSM/CD (Ethernet) Token Bus Token Ring Distributed Queue Dual BUS --MN standard Fiber Distributed Data Interface 802.3 CSM/CD 802.4 Token Bus 802.2 LLC 802.5 Token Ring 802.6 DQDB FDDI 4/11/40 page 14 Standard LN/MN Ethernet and IEEE 802.3 - The major LN technologies, it runs the largest number of LNs. Token Bus -- Bus topoly with logical ring connection Token Ring -- From IBM, it followed Ethernet and is now widely used in the large number of IBM networks. FDDI -- lso using tokens, it is now a popular campus LN. DQDB -- use in Metropolitan rea Network Faculty of Engineering, Kasetsart University Local rea Networks 1-14
Ethenet/802.3 Operation Every node can receive a transmission by all other nodes need addressing scheme to identify a destination only destination copies frame to it, all other nodes have to discarded the frame B C B C terminator C finds the bus is free C transmits frame addressed to B C B C B ignores frame copies frame signal is absorbed by the terminators 4/11/40 page 16 packet in an Ethernet network traverses the entire network and is received and examined by every node. When the signal reaches the ends of a segment, it is absorbed by terminators to prevent it from going back onto the segment. If two or more station try to tranmitt packets at the same time, a collision is occured and the packet can not be used. Faculty of Engineering, Kasetsart University Local rea Networks 1-16
Token Ring/802.5 Operation T T = 0 T = 0 T T = 1 T Data Token Ring LNs continuously pass a token or a Token Ring frame 4/11/40 page 17 Token-passing networks move a small frame, called a token, around the network. Possession of the token grants the right to transmit. If a station receiving the token has no information to send, it simply passes the token to the next station. If a station possessing the token has information to transmit, it claims the token by altering one bit of the frame, the T bit. The station then appends the information it wishes to transmit and sends the information frame to the next station on the Token Ring. The information frame circulates the ring until it reaches the destination station where the frame is copied by the station and tagged as having been copied. The information frame continues around the ring until it returns to the station that originated it, and is removed. Faculty of Engineering, Kasetsart University Local rea Networks 1-17
FDDI FDDI Dual Ring 4/11/40 page 18 FDDI is logically and physically a ring topology. lthough it operates at higher speeds, FDDI is similar to Token Ring. The two network types share many features such as token passing and predictable deterministic delays. ccess to the FDDI dual ring is determined by token possession. However, stations attach new tokens to the ends of their transmissions and a downstream station is allowed to add its frame to the existing frame. Thus, at any given time, several information frames can be circling the ring. Faculty of Engineering, Kasetsart University Local rea Networks 1-18
LN Frame Format Summary 802.2 LLC DSP 1 SSP 1 CTRL 1 Data >=0 FDDI FDDI uses 4-bits symbols Preamble 16 SD 2 FC 2 D 12 S 12 802.2 LLC FCS 8 ED 1 or 2 FS 1 802.3 802.5 SD 1 C 1 FC 1 D 6 S 6 802.2 LLC FCS 4 ED 1 FS 1 Preamble 8 D 6 S 6 Length 2 802.2 LLC FCS 4 Ethernet Preamble 8 D 6 S 6 Type 2 Data FCS 4 4/11/40 page 19 The 802.2 LLC uses a source service access point (SSP) and a destination service access point (DSP) to identify the source and destination protocols of the data being carried in the LLC subframe. These service access point (SP) fields are 8-bits each and are used to define only one of a few registered SPs. The FDDI frame format is similar to the Token Ring format. However, because of FDDI s 4B/5B reliability mechanism, data is described in groups of four bits called symbols. The fields are actually the same size as the fields used in Token Ring. The first three fields of the Ethernet frame and the 802.3 frame are identical. However, 802.3 has changed the fourth field the type field, into a length field and added the 802.2 LLC fields. Faculty of Engineering, Kasetsart University Local rea Networks 1-19