Using dynamic optical networking for high-speed access
Malathi Veeraraghavana, Dimitris Logothetisb, Xuan Zhenga [email protected], [email protected], [email protected]
aPolytechnic University, bAtmel Corporation
Abstract
While LAN and wide-area network link capacities keep increasing, access links from enterprises are still bottle-necks. In this paper, we propose a networking solution to improve the access link rate seen by an enterprise user. The solution is developed in the context of existing constraints, such as the presence of ring based metropolitan area access and core networks, and Ethernet cards in end hosts. Our proposal enables the use of hybrid Ethernet-optical access cir-cuits set up and released on demand. Through the use of switched optical circir-cuits the number of hosts that can be sup-ported on a metropolitan area core ring network can be increased significantly at an acceptable grade of service. Through analysis, we demonstrate these increases. For example, at a traffic load of 50%, an increase in the number of hosts (or correspondingly the access rate seen by a user) of 131% is possible on a core OC48 ring at a negligible mean waiting time (1ms).
Keywords: Optical Circuit, Metropolitan Area Networks (MAN), Dial-Up, SONET/SDH, Access Networks.
1. Introduction
An end-to-end data communication path for an enterprise user typically consists of three types of segments: the local area network (LAN) segments within enterprise buildings, the access segments from enterprise buildings to service provider buildings, and the service provider(s) network seg-ments. Links in the LAN and service provider segments are typically of higher capacity than links in the access segments: compare GbE, 10GbE in LANs, and OC48 (2.5Gbps), OC192 (10Gbps) in WANs, to T1 (1.5Mbps) and T3 (45 Mbps) for access. This lag in access link data rates has been noted in various articles, such as [1] [2]. While LAN data rates should be higher to support both intra-LAN and external traffic, the access link rates are nevertheless relatively lower. A possible reason for these low rates is that access links are leased-line circuits, which are typically expensive because of a lack of resource sharing.
A simple but not very feasible solution to this problem of relatively low access link rates is to use an “optical dial-up” solution as is common today with 56kbps modems. In this solution, the net-work can allocate a high-speed optical circuit, at Synchronous Optical Netnet-work/Synchronous Digi-tal Hierarchy (SONET/SDH) rates, exclusively to one end host for its wide-area access link to an ISP’s Remote Access Server (RAS) on a dynamic basis as illustrated in Fig. 1. In the example shown in Fig. 1, end host 1 sends signaling messages to the SONET/SDH/WDM circuit switch to request a high-speed optical circuit to its ISP’s RAS. It releases the circuit after completing its wide-area access session allowing another end host, such an end host N in Fig. 1, to then request
and use the shared circuit. IP datagrams are carried on the dynamically setup SONET/SDH circuits using a data-link layer protocol such as PPP. The number of hosts sharing the link l capacity should be engineered to trade off service quality versus costs.
The above solution appears simple; however, it is difficult to implement for two reasons. First, it requires end hosts to be equipped with optical Network Interface Cards (NICs) and fiber drops to desktops, both of which proved to be difficult to realize with ATM technology. Second, the archi-tecture used in telephony based Internet access networks, which is the same as shown in Fig. 1 above, is a tree structure, while current optical access network deployments are rings. Hence, in this paper, we address the problem of low access link rates constrained by two boundary conditions: (i) the presence of Ethernet NICs with copper (not fiber) drops to desktops, and (ii) the presence of ring-based optical access networks.
While our end goal is to enable enterprise users to receive higher access bandwidth, this problem could also be described from a service provider perspective. As stated in [3], high-speed optical access links are not being deployed for cost reasons. From a service provider perspective, network-ing solutions that allow currently deployed rnetwork-ings to support more users, or solutions that allow for the deployment of rings of lower capacity than would be needed in the leased-line mode, should hence be useful. Our proposed architecture is a solution to both the enterprise user problem and the dual service provider problem.
Section 2 describes related work. Section 3 describes the current optical access network architec-ture, which serves both as a basis for comparison as well as sets the constraints of our problem. Sec-tion 4 describes our proposed switched architecture for usage in optical access networks. Section 5 presents an analysis of our solution using well-known techniques for modeling circuit-switched net-works. Thus the main contribution of this work is the architecture proposed in Section 4, rather than
Fig. 1 Tree access architecture: value of multiplexing data from a large number N of end hosts; dynamic optical networking End host 1 1. Setup 2. Setup 3. Success 3. Success End host N SONET/ or WDM circuit SDH switch Link l 5. Data flow End host 1 6. Release 7. Release 8. Complete 9.Complete End host N RAS End host 1 11. Setup 12. Setup 13. Success 14. Success End host N 15. Data flow RAS 10. Circuit disconnected RAS N
any new modeling techniques. The analysis of Section 5 is merely supportive, i.e., it helps us quan-tify the increase in the number of users that can be supported on a given core ring. Finally, we sum-marize the paper in Section 6.
2. Related work
Most research work in optical networking has focused on broadcast-and-select architectures for LANs and on wavelength-routed architectures for wide-area networks [4]-[6]. Research papers [7] [8] analyze the network throughput of SONET/SDH/WDM access rings operated in the leased-line mode, but do not provide alternate solutions for access networks.
A packet based alternate solution has been proposed for access networks in [9]. This solution uses a MAC protocol called Spatial Reuse Protocol (SRP). Recently, a new group, 802.17, was established by the IEEE to formulate a standard for a Reliable Packet Ring (RPR). The motivation for creating this new group is that SONET/SDH rings are not ideally suited for bursty Internet data traffic given that these rings are circuit switched.
While packet-switched networks are best suited for bursty data traffic, there is an intermediate solution between the low-delay, low-utilization service of leased-line circuits, and the higher-delay, higher-utilization service of packet-switched networks as shown in Fig. 2. This solution is to
oper-ate the circuit-switched network in a dynamic mode, i.e., set up and release circuits on demand. When enterprise nodes request bandwidth on demand, utilization is improved relative to leased-line circuits. However, for the duration of a call, when the circuit is held open, since traffic will only use the circuit in a bursty mode, utilization will be lower than in packet-switched networks. Utilization can be improved by holding the circuits open for shorter periods of time, i.e., by having shorter delay timers that trigger the release of calls. However, this will result in higher call handling vol-umes. High call arrival/departure rates can be handled by signaling protocol processing engines implemented in hardware as we reported in [10]. Also with the trend moving toward bandwidth
Leased-line circuits Increasing bandwidth abundance
Switched-mode circuits Packet switching
Fig. 2 Trade off utilization for improved latency (dynamic networking)
Lower latency Higher utilization
abundance [11], it becomes possible to trade off utilization for decreased latency as shown in Fig. 2.
In this paper, we propose this intermediate solution of upgrading the currently deployed SONET/ SDH rings to support the switched (dynamic) mode of operation by adding signaling protocol pro-cessing engines to ADMs. This solution is a more gradual evolution of the currently deployed access rings than the SRP or other packet-switched solutions.
To support the dynamic mode of operation, ADMs need a signaling protocol. There appears to be industry-wide convergence on a set of signaling protocols for SONET/SDH networks known as “Generalized Multi-Protocol Label Switching (GMPLS)” [12]-[16], and the Optical Internetwork-ing Forum (OIF) User-Network Interface (UNI) specification [17]-[18]. Our proposed solution to provide switched optical circuits on access is an application of these signaling protocols.
Finally, since our goal is to bring the advantages of dynamic optical networking in a hybrid con-text, with Ethernet links from the hosts to a basement switch and optical circuits from enterprise basements to the wide-area network, we cite prior work on internetworking. In particular, reference [19] was one of the early proposals for an IP/ATM internetworking solution that led to the creation of MPLS (MultiProtocol Label Switching). The problem of internetworking occurs when data starts flowing from applications on end hosts in a connectionless mode, but somewhere in the network, a gateway device initiates the set up of a connection if this data is to be carried on a connection-ori-ented network. With IP-ATM internetworking, many research efforts addressed this question of how to identify a “flow” and initiate an ATM connection setup to handle the IP packets of a partic-ular flow. The same problem appears while internetworking IP/Ethernet with dynamically setup optical circuits. In this paper, we propose “pushing” the functionality that triggers the dynamic set up of an optical circuit all the way to end hosts. This is described in detail in Section 4.
3. Current access architecture
The current state of enterprise access networks is as follows. A “metro1 access ring” consists of SONET/SDH Add/Drop Multiplexers (ADMs), with or without Wavelength Division Multiplexing (WDM) capability. These ADMs are capable of adding/dropping signals at the T1, T3, OC1, OC3, and higher rates2. An access ring interconnects multiple enterprises to an access service provider HUB node as shown in Fig. 3. This HUB node also belongs to a “metro core ring” that intercon-nects multiple service provider nodes, such as Internet Service Provider (ISPs). Leased lines at
1. Short for “metropolitan.” 2. or E1, E3 and SDH rates.
SONET/SDH rate are provisioned from enterprise ADMs through the access service provider HUB node to an ISP or telephone service provider ADM located on the metro core ring. Embedded within these SONET/SDH rate signals are T1s, T3s, or Ethernet signals. For example, T1s from Pri-vate Branch Exchanges (PBXs) in offices/floors of an enterprise building3 carrying voice traffic, T1 or T3 wide-area connections from IP routers, as well as Ethernet links are multiplexed on to SONET/SDH signals as shown within enterprise building 1 in Fig. 3. A T1 is embedded into a Vir-tual Tributary VT1.5, which is then carried in an OC1 signal (an OC1 can fit 28 T1s). A T3 signal fits in an OC1 signal. Several specifications are being developed for how to carry Ethernet frames within SONET signals for 10Mbps, 100Mbps, 1Gbps and 10Gbps Ethernet [20][21]. The ADMs on the ring are capable of adding/dropping signals at T1 (VT1.5), T3 (OC1) and Ethernet signal levels. These ADMs are often referred to as “multiservice ADMs” because they integrate voice and data traffic on to SONET/SDH/WDM signals. Many vendors sell these multiservice ADMs [22]-[24]. For simplicity reasons, we will henceforth refer to these SONET/SDH/WDM multiservice ADMs as “ring nodes.”
Details of the “Metro Core Ring” are illustrated in the right part of Fig. 3. Nodes on the metro
3. It is possible for one “enterprise building” to house many different corporations. But for purposes of this paper, we use the term “office/floor” to apply generically to either offices/floors within a single corporation if the whole enterprise building is owned by one corporation or to multiple corpo-rations within the enterprise building. We use the term “enterprise” to refer to an “enterprise building,” which is a ring node on access rings.
Fig. 3 Ring based optical access architecture Access service provider
HUB PBX PBX Ethernet switch/ IP router PBX SONET/SDH/WDM multiservice ADM Optical fiber Enterprise building 1 Office/Floor 1 Office/Floor 2 Office/Floor 3 T1 T3 Ethernet .... SONET/SDH/WDM multiservice ADM Metro Access Ring 1 Metro Ring Enterprise buildings Enterprise buildings Core Ethernet switch/ IP router Ethernet switch/ IP router Access ring
Internet service provider (ISP)
SONET/SDH/WDM multiservice ADM T1, T3, Ethernet
IP router
Telephone service provider WAN access node
...
Access service provider T1 I IIcore ring are those of access service providers, Internet Service Providers (ISPs), telephone service providers, and Wide Area Network (WAN) access nodes. The ISP ring nodes demultiplex data T1s, T3s, and Ethernet signals from the SONET/SDH signals received on the ring; similarly, the tele-phone service provider nodes demultiplex teletele-phone T1s from the SONET/SDH ring signals. As an example, consider a T1 line from an IP router within enterprise building 1 in Fig. 3, that is leased all the way to an IP router located within an ISP building in the metro core ring. It will traverse multi-ple access ring nodes including the HUB, and then traverse the metro core ring to reach the ISP ring node. In general, T1s, T3s, or Ethernet signals can be carried on bandwidth leased through a combi-nation of metro access rings, metro core rings and perhaps even a WAN.
As stated in Section 1, we consider this current access architecture as a “boundary condition” for our problem. In other words, our solution to the problem of increasing enterprise user access link rates is constrained to work in this current architecture. We also use this current architecture for our analytical comparison.
4. Our proposed solution
This is the main section of the paper describing our proposal of how to use dynamic optical net-working in access networks for data traffic4 under the existing network constraints. As stated earlier the dominance of Ethernet makes it difficult to require PCs to be upgraded with optical NICs needed for an optical dial-up solution of the type used today in residential 56kbps modem access. Hence our first phase solution is a hybrid approach that uses Ethernet inside the enterprise buildings and optical circuits on the access segment. However, our last phase proposes extending the optical circuit all the way to the PC to enable a PC to request a high-capacity optical access link for its exclusive use.
As for the second constraint, optical access architectures are ring based as opposed to telephone line access architectures, which are tree-based. With ring architectures, if all the ADMs on the ring are equipped with signaling protocols to operate in a dynamic mode, a call setup will require a free channel to be available at multiple nodes. Fairness algorithms of the type described in the SRP pro-tocol at a packet level [9] will be required at a call level in these rings. Otherwise, the advantages gained by statistical multiplexing in a typical switched network will be lost for calls originating from nodes far away from the HUB (see Fig. 3). Therefore, we postpone proposing that all nodes on
4. We focus on data traffic because of the relative ease of introducing software, such as device drivers shown in Fig. 4, in data equipment. This is more difficult to do in telephones or in PBXs; furthermore, data traffic is expected to be the more significant component.
the ring be equipped to operate in a dynamic mode to later phases, and propose a simpler logical tree-like operation in the first phase by only equipping one ADM on the access segment with dynamic mode capability.
4.1 Phase 1
This solution combines the use of Ethernet within the LAN with dynamically set-up optical cir-cuits on the access segment. Here we assume that all data traffic is only carried as Ethernet signals on SONET, and ignore any T1 and T3 links that could be used for data traffic from IP routers in enterprises. The application of this solution to the T1 and T3 links is straightforward.
Assume that there are access service provider ring nodes, ISP ring nodes, telephone service provider ring nodes, and WAN access ring nodes on a metro core ring. Further assume a partition matrix , where is the bandwidth provisioned between node and node on the metro core ring. The matrix has a dimension of , where . Note the diagonal elements of matrix are all zero. The bandwidth partitions are determined by the service providers based on the amount of traffic expected and the desired grade of service. The set of sets of hosts sharing a partition , where and , is pre-determined based on enterprise subscriptions. Finally, assume that enterprise building on an access ring has leased Ethernet signals from its basement ADM to the HUB node of access ring . In other words, there are sets of hosts within building of access ring , where each set of hosts is labeled where . Hosts within a set share a single Ethernet signal on the wide-area access link. For example, the number of Ethernet signals from enterprise building 1 on access ring 1 shown in Fig. 3 is 3, i.e., there are 3 offices/floors. The hosts connected by Ethernet switch in floor 1 form the set .
To implement this phase:
1. Equip each access service provider core ring ADM with a signaling protocol processing engine enabling these nodes to operate in the dynamic mode.
2. Each access service provider HUB (core ring node) is programmed with the addresses of hosts in sets , for , for all enterprise buildings on its access ring.
3. The bandwidth on metro core ring spans are partitioned according to the matrix . Each access service provider core ring node , , is programmed with the identities of the
nA nI nT nW P = [ ]pij pij i j P (nC×nC) nC = nA+nI+nT+nW P Bij pij 1≤ ≤i nA 1≤ ≤j nI k i nki i nki k i He ( )ki 1≤ ≤e nki ( )He ki n11 H1 ( )11 i He ( )ki 1≤ ≤e nki k P i 1≤ ≤i nA
sets , , over all enterprise buildings on access ring that share each
partition . These sets, which are sets of sets, are denoted corresponding to each
partition , and . In other words, .
When an access service provider core ring node receives a Setup request from a host belonging to one of the sets in set for a circuit to an ISP ring node , it should
only allocate available circuits from the partition .
4. All nodes on each access service provider ring other than the HUB nodes are operated in a leased line mode. This means that bandwidth partitions are leased on the access ring between each enterprise ring node and the HUB (access service provider core ring node). Thus,
Ethernet signals are embedded within SONET/SDH signals arriving at the
HUB on access ring .
5. Each ISP ring node is provisioned to drop Ethernet signals received on the core ring from partition , and to add Ethernet signals from its associated IP router to each partition for
and . The number of Ethernet interfaces needed on an ISP router is at least equal to the number of “Ethernet” signals carried on the core ring from all access service
provider ring nodes, i.e., it is equal to .
6. Add a device driver between the TCP/IP module and the Ethernet module in all end user hosts that participate in this dynamic mode of operation.
Besides the assumptions of Ethernet NICs and ring based access and core networks, we assume that the IP routers located in ISP buildings have Simple Network Management Protocol (SNMP) or some standard network management access to their IP routing tables and optionally, to their Address Resolution Protocol (ARP) tables.
Dynamic operation in this Phase 1 architecture is explained using Fig. 4:
1. When a user of a PC in set located in an enterprise building on access ring , He ( )ki 1≤ ≤e nki k i pij Bij pij 1≤ ≤i nA 1≤ ≤j nI Bij ( )He ki 1≤ ≤
∪
e nk i k ∀ ∈ring i (∪
) ⊆ i He ( )ki Bij j pij nki k ∀ ∈ring i (∑
) i j pij pji 1≤ ≤i nA 1≤ ≤j nI pij 1≤ ≤∑
i nA He ( )ki k i, initiates a communication application that uses TCP/IP, the newly added Device Driver (shown as “new DD” in Fig. 4) checks the destination IP address field of outgoing packets to determine whether the user has initiated a wide-area network access. If so, the device driver sends a Setup message to the access service provider HUB node.
2. Upon receiving this Setup request, the access service provider HUB determines whether another end host in set had previously requested an optical circuit that is still open; if
so, it simply returns a Success message. This is because the Ethernet signal from the Ethernet switch/IP router of the Setup-generating PC is already “connected” to an Ethernet link to an IP router of the ISP through embedded signals in the access and core rings. If, on the other hand, this is the first end host in set requesting an optical circuit, then the access
service provider ring node selects an available circuit on the appropriate partition . This
partition is determined from the identity of the enterprise building and the table that maps enterprise sets on access ring to ISP .
3. The access service provider HUB then maps the selected circuit on the core ring partition to the Setup-generating PC’s Ethernet signal embedded within an access ring SONET/SDH signal. This is where the dynamic cross-connect operation is performed at the level of an Ethernet signal. Since , where and , the HUB nodes need to
End hosts 1-N kernel space TCP/IP new DD Ethernet driver user space
Access service provider SONET/SDH/WDM SONET/SDH/WDM multiservice ADM Access ring multiservice ADM Ethernet (10, 100, 1G, 10G) 3. Response 1. Setup 4. Success ISP IP router 2. Set Success Enterprise building
Fig. 4 Hybrid Ethernet-optical circuit switched architecture
Ethernet switch Routing table Map 1-N IP addresses to newly-setup optical circuit ARP table Map 1-N IP & MAC addresses to newly-setup optical circuit entries leased lines Core ring partition L1 L2 HUB 1≤ ≤i nA i He ( )ki He ( )ki pij k Bij i j i Bij ≥pij 1≤ ≤i nA 1≤ ≤j nI
dynamically set up and release mappings between the Ethernet signals on their access rings and the Ethernet signals on the core rings to ISPs.
4. The next step is for the access service provider HUB to set up entries in the routing table of the IP router in the ISP network. This step is needed because at different instants in time, different IP subnets are reachable through the same Ethernet interface of the IP router. Typically routers provide a network management interface (using SNMP or similar protocol) to set entries in the routing table. We depict this network management exchange with “Set entries” and “Response” messages in Fig. 4. At the ISP’s IP router, IP addresses of all the hosts in the set are mapped to the Ethernet interface corresponding to the optical
circuit that was selected in step 2 (this could simply be one subnet address). This allows the router to route incoming packets to a particular office/floor by consulting the updated routing table.
5. A timer-based release procedure is used to release the optical circuit when no host on a given Ethernet switch uses the optical circuit. Detailed studies are needed to select these timer values to balance utilization with call handling load.
The setting of IP routing table entries is illustrated with an example shown in Fig. 5. Let the Office/floor 1 subnet address be 128.239.5 and the Office/floor 2 subnet address be 156.78.5 as
shown in Fig. 5. The dashed lines shown in Fig. 5 are the lines leased through the access ring, and are of “Ethernet capacity.” The link L2 shown in Fig. 5 is the partitioned capacity on the metro core ring set aside for traffic from/to the access ring shown in Fig. 5 to/from the ISP node. For this
i He ( )ki ISP ring node PC 11
Fig. 5 Dynamic linking of Ethernet-level tributaries PC 1n PC 12 .... Ethernet switch PC 21 PC 2n PC 22 .... Ethernet switch Office/floor 1 Office/floor 2 IP router 128.239.5 156.78.5 Access ring Core ring nodes 1 3 2 le0
DestinationIP routing tableInterface
128.239.5 le0
156.78.5 le0
Access ring HUB
L2
le1 le2
example, we describe the sharing of one Ethernet interfacele0, assuming the other interfaces le1 and
le2 are in use for other enterprises. If one of the PCs in Office/floor 1 starts an Internet access ses-sion, then at the access ring HUB, a connection is made from interface 1 to interface 3. The IP rout-ing table at the IP router shown in Fig. 5 is then written with the entry mapprout-ing destination 128.239.5 to interface le0 of the router. Packets arriving at the IP router for any of the PCs on the 128.239.5 subnet will then be sent on interface le0. If a sufficient time expires and no PC on the sub-net 128.239.5 sends/receives packets, then the connection made in the access ring HUB is dropped and the entry in the routing table is removed (as shown in Fig. 5). Later if a PC from Office/floor 2 initiates an Internet session, a connection is made from interface 2 to interface 3. The IP routing table at the IP router, as shown in Fig. 5, is then written with the entry mapping destination 156.78.5 to interface le0 of the router. Note that the connection setup and released is of Ethernet capacity, where the Ethernet signal is embedded in the SONET/SDH signals on the access ring and carried through to the core ring (or vice versa). The ARP table can be updated with data in the same manner as the IP routing table to avoid excessive ARP queries.
Finally, we note that a mechanism is needed to route TCP connections originating from the Inter-net (connected to the ISP IP router) and destined to a server located in one of the enterprises. To avoid having to equip deployed IP routers with the user-network interface of a signaling protocol (i.e., the ability to request a circuit on demand), we propose having leased lines for servers, such as web servers, located in these enterprises. If only client computers are connected to Ethernet switches and acquire bandwidth in the proposed dynamic mode, a request for a circuit always origi-nates from a client. This is similar to the Dynamic Host Configuration Protocol (DHCP) approach used by clients to obtain IP addresses for their communication sessions, which are released after usage. Such a dynamic allocation of addresses is not feasible for “servers” because other clients, on the Internet at large, will need to know their IP addresses.
4.2 Subsequent phases
The quantitative benefits of introducing the dynamic mode in a controlled fashion as proposed for Phase 1 will be demonstrated in Section 5. However, we note that these benefits could be increased with even greater sharing of resources. In Table 1, we show one potential approach to introducing the dynamic mode of operation in phases. There are mainly two dimensions: the NICs used in end hosts, and the use of signaling protocols in core and access ring ADMs.
hosts to the last phase. This would allow each PC to request and receive any-rate optical access link that it can handle. For example, if a host can handle traffic at 2.5 Gbps, it can request an OC192 from itself to an ISP IP router. This is similar to the residential 56kbps modem access used today, except at much higher optical rates.
As for the introduction of signaling protocols into core and access ring ADMs, we propose doing this in two phases, Phases 2 and 3. In Phase 2 core ring ADMs are equipped with signaling proto-cols, and in Phase 3, access ring ADMs are also equipped with signaling protocols. To set up a call on a ring where all ADMs support dynamic circuit setup, the fairness problem mentioned earlier will need to be solved before bandwidth savings can be realized. This is because in ring architec-tures, calls have varying path lengths, and longer-path calls will have a lower probability of success (or longer waiting times) than shorter-path calls if no fairness algorithm is present. To model such networks, we will require call blocking models for paths consisting of multiple nodes. Such models can be solved with either exact or approximate techniques [25]-[26]. One popular approximate technique is the reduced load approximation technique [25]. We plan to present a call-level fairness algorithm with an analysis in a subsequent paper.
5. Comparison of architectures
Section 5.1 describes the analysis methodology and Section 5.2 presents our results. Table 1: Phased introduction of the dynamic mode
Phase Host NICs
Nodes with signaling protocols
Core ring bandwidth Access ring bandwidth
Phase 1 (see Sec-tion 4.1)
Ethernet Access service provider HUBs
Partitions , and shared
among enterprises in sets
Leased lines from enterprises to HUB nodes
Phase 2 Ethernet All core ring ADMs
Shared Leased lines from enterprises to HUB nodes Phase 3 Ethernet All access and
core ring ADMs
Shared Shared
Phase 4 Optical PPP NICs
All access and core ADMs Shared Shared pij 1 i n A ≤ ≤ 1≤ ≤j nI Bij
5.1 Analysis methodology
The goal of this section is to analyze the Phase 1 architecture described in Section 4.1, and to compare it to the current leased line architecture, described in Section 3. One approach to the analy-sis is to fix the access ring rate, which determines the number of sources5 that generate requests for circuits on the core ring partitions, and then compute the core ring bandwidth partitions needed to support this number of sources in the leased and switched architectures. This analysis would yield the capacity savings possible on the core ring enabled by the dynamic mode of operation. However, since our goal is to increase the effective access link rate of an enterprise user, we take an alternate approach to the analysis.
In this approach, we fix the core ring rate , and the bandwidth partition matrix . Given the size of the partitions , we then determine the number of sources that can be supported using the dynamic mode. Using the number of sources, we determine the corresponding access ring rate. As will be shown in the analysis below, this access ring rate is much higher than in the leased mode, which means for the same cost metro core ring, we can offer each enterprise user a much higher access rate.
SONET/SDH rings are typically of four types: Unidirectional Line Switched Rings (ULSRs), Unidirectional Path Switched Rings (UPSRs), 4-fiber Bidirectional Line Switched Rings (BLSRs), and 2-fiber BLSRs. Typically metro core rings are of the 4-fiber BLSR variety. We assume that a specific partition bandwidth units, where and , and a unit is of “Ether-net” signal rate. Set
(EQ 1) We determine , the number of sources that can be supported on the partition of capacity , if the access service provider core ring node (HUB) is operated in a switched mode. This ring node can be operated in one of two modes: (i) the Blocked Call Clearing (BCC) mode, and (ii) the Blocked Call Queueing (BCQ) mode.
In the BCC mode, calls are cleared if there are no available resources at the time of call arrival. Under this mode, users will experience a non-zero call blocking probability. Given that the number of sources competing for a limited set of resources is finite, the well-known Engset formula for finite population [27] can be used. Under an assumption of Poisson call arrival and departure
pro-5. Even though a PC generates the Setup request shown in Fig. 4, the number of “sources” is equal to the number of Ethernet signals from enterprise buildings carried on an access ring rather than the number of PCs.
pij R P pij pij = C 1≤ ≤i nA 1≤ ≤j nI M = Bij M C
cesses, the call blocking probability is given by:
(EQ 2)
where load , where is the per Ethernet switch call arrival rate (which is the cumulative arrival rate from all the PCs connected to an Ethernet switch), is the call departure rate, is the total number of sources generating calls competing for the bandwidth partition , and is the number of channels (in units of Ethernet signals) available on partition . We use the recursive algorithm described in [28] for our numerical computations.
In the BCQ mode, buffers are provided at the ADMs operated in the dynamic mode, enabling them to queue calls if resources are not available when calls arrive. A zero call blocking probability can be achieved if there are queueing positions for calls. Under this assumption, using Lit-tle’s Law, and the average arrival rate across all the states of the system, , we obtain the average waiting time in the queue to be [29]:
(EQ 3) where is the average number of calls in the queue and is the average number of calls in the system (which includes calls being serviced and calls in the queue). The steady-state probability of being in state (i.e., there are calls are in the system) is given by:
, (EQ 4)
and (EQ 5)
(EQ 6) Equations (2) and (3) express the Grade of Service (GOS) as a function of number of sources ,
p p M–1 C ρC M–1 j ρj j=0 C
∑
---= ρ = λ µ⁄ λ µ M pij C pij M–C ( ) λ(M–L) Wq Lq λ(M–L) --- (n–C)pn n=C M∑
λ(M–L) ( ) ⁄ = = Lq L pn n n pn ρn M n p 0 0≤n<C ρn p0 M n n! Cn–CC! --- C≤ ≤n M = L npn n=0 M∑
= p0 ρn M n n=0 C–1∑
ρn M n n! Cn–CC! ---n=C M∑
+ 1 – = Mthe number of bandwidth units , and traffic load . We are looking at the reverse problem, i.e., to find the number of sources that be supported at a given GOS value, where the GOS measure can be call blocking probability or mean waiting time, when and are given.
After computing , the number of sources that can be supported on the partition of capacity , under these two modes of operation, we determine the corresponding access ring rate. For the same cost metro core ring (same rate ring), we show that a much higher bandwidth access ring rate can be supported, which means each enterprise user effectively receives a much higher access rate. This supports our goal of proposing a networking solution that effectively provides each enterprise user a higher access link rate.
5.2 Numerical results
For purposes of numerical computation, we assume that all data traffic from office/floors are car-ried in Ethernet links to the SONET/SDH/WDM multiservice ADM in the basement. Furthermore, we assume that all Ethernet links are 10Mbps, and while there are currently a few efforts to stan-dardize Ethernet over SONET/SDH as in [20][21], for our purposes, we assume that it is possible to “fit” 4 Ethernet signals in an OC1.
We obtain the values of that can be supported under the BCC mode (with equal to 0.005 and 0.01), and the BCQ mode (with equal to 0.001 sec and 0.005 sec) using (EQ 2) and (EQ 3), respectively. Fig. 6 shows the percent increase in number of sources, , that can be supported on the partition in the dynamic mode. The four graphs correspond to two types of metro core rings, the OC12 4-fiber BLSR and OC48 4-fiber BLSR, and to two different call blocking probabilities (0.005 and 0.1). The core ring partitions assumed in these graphs is 12 and 48 Ethernet signal bandwidth units on the OC12 and OC48 core rings, respectively.
The percent increase in number of sources that can be supported is good even at high loads. For example, at a traffic load of 0.9, we can support 79 sources at a call blocking probability of 0.5% on an OC48 ring using the Phase 1 approach. In the leased mode, we could only support 48 sources (equal to the partition on the core ring ). This increase in number of sources can be proportionally divided amongst the number of PCs competing for the bandwidth in a tree configuration.
C ρ M C ρ M C M p Wq 100((M–C)⁄C) ρ C
Given the constraints of ring access networks, the SONET hierarchy and the presence of Ether-net in LANs, we undertake a small demonstrative interpretation of how this increase in the number of sources can be translated into an increase in the effective access rate of an enterprise user. When the core ring is operated in a leased-line mode, the partition we considered above can only support 48 sources. If all enterprises on the access ring subscribe to the same ISP and share the core ring partition, then the access ring can only be an OC12 UPSR. This is because the network throughput of an OC12 UPSR is 12 OC1s in and 12 OC1s out of the HUB (which is equivalent to 48 Ethernet sources in and out of the HUB), and hence an OC12 access UPSR can fully utilize the core ring par-tition of 48 Ethernet units to the ISP.
To support 79 sources each at the Ethernet rate on the core ring partition in the dynamic mode, we require 20 OC1s out of/into the access ring. By using one 4-fiber BLSR OC12 access ring, we can support 20 OC1s. A 4-fiber BLSR has a network throughput of OC1s into and OC1s out of the HUB. Thus a 4-fiber OC12 BLSR can support 24 OC1s in and out of the HUB. Since only 20OC1s can be allowed to share the partition of 48 units on the core ring (in fact, one Ethernet
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 p = 0 . 0 0 5 ; 4 - f i b e r b i d i r e c t i o n a l O C 1 2 p = 0 . 0 1 ; 4 - f i b e r b i d i r e c t i o n a l O C 1 2 p = 0 . 0 0 5 ; 4 - f i b e r b i d i r e c t i o n a l O C 4 8 p = 0 . 0 1 ; 4 - f i b e r b i d i r e c t i o n a l O C 4 8
Fig. 6% increase in number of sources supported in the BCC
% i n cr e a se i n n u m b er o f so u rc e s ρρ: traffic load 2R 2R
signal less) at the required grade of service, the remaining 4 OC1s can be used for voice leased lines and leased lines to servers (for the reverse direction traffic as described in Section 4.1).
Under an assumption of a “centralized traffic pattern,” in which each node sends traffic to/from the HUB node and to no other node on the access ring, the demand from any node to/from the HUB is
(EQ 7) for rings of rate with nodes. This means that in the leased line mode, since only one OC12 UPSR can be used as the access ring, each enterprise can send only 6 Ethernet signals in a 9-node ring. While in the dynamic mode, since 20OC1s can be assigned to carry Ethernet traffic and com-pete for the core ring partition, any single enterprise building can be assigned 10 Ethernet signals on a 9-node ring. This effectively means each individual enterprise user can be provided more band-width for access. For example, some of the offices/floors of Fig. 3 can be allowed to send two Ethernet signals instead of one to the basement ADM. This means half the number of user PCs will share each Ethernet signal on those offices/floors, and hence the effective rate that an enterprise user receives is increased.
The cost trade-off is between the metro core ring bandwidth leased line costs and the cost of add-ing a signaladd-ing protocol processadd-ing engine in the HUB nodes. We believe that because leased lines costs are high, our solution of adding extra nodal hardware will prove to be cheaper. In the long run, if bandwidth costs decrease dramatically as predicted in [11], then we should move in the direction of leased line circuits in Fig. 2 instead of in the direction of packet-switched access and core rings.
Fig. 7 shows the percent increase in the number of sources that can be supported on a partition in the core ring under the BCQ dynamic mode of operation. The same assumption about the core ring partition is made as in the BCC case. Two values of , the mean waiting time, 1 msec and 5msec, are used. We also vary the mean call holding time. In the six plots where , the mean holding time is 1sec. In 1 sec, a 10Mbps Ethernet switch can send 833 maximum-sized frames. In the two plots where , we study the effect of holding the circuit for 20 minutes (as is common with modem dial-up connections). The mean waiting time (measure of service) is 0.1sec or 1sec. Even with these long holding times, the percentage increase in number of sources that can be sup-ported is significant at low loads
dUPSR R n–1 ---= d4F–BLSR 2R n–1 ---= R n Wq µ = 1 µ = 1 1200⁄
.
6. Summary and conclusions
We identified the access link from an enterprise as a bottleneck in an end-to-end Internet com-munication session. To improve the data rates of these links, we proposed a service concept in which optical access circuits are set up and released dynamically. Owing to the dominance of Ethernet technology inside enterprise buildings, our solution proposes a hybrid Ethernet-optical access circuit for the first phase. Analysis of this phase 1 solution shows that a significant increase in number of sources that can be supported on a given bandwidth partition is possible through the use of switched optical circuits. These increases range from over 800% at low traffic loads to 25% at high traffic loads with long call holding times. If the number of hosts is held the same, this roughly translates into a proportional increase in the access link rate experienced by a user. Three subsequent phases in which the access link is increasingly shared were also described. We plan to provide a more detailed explanation and analysis of these phases in a future paper.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 100 200 300 400 500 600 700 800 900 1000
W = 0 . 0 0 1 s e c ; m u = 1 ; 4 - fiber Bidirectional OC12 W = 0 . 0 0 5 s e c ; m u = 1 ; 4 - fiber Bidirectional OC12 W = 0 . 0 0 1 s e c ; m u = 1 ; 4 - fiber Bidirectional OC48 W = 0 . 0 0 5 s e c ; m u = 1 ; 4 - fiber Bidirectional OC48 W = 0 . 0 0 1 s e c ; m u = 1 ; 4 - fiber Bidirectional OC192 W = 0 . 0 0 5 s e c ; m u = 1 ; 4 - fiber Bidirectional OC192 W = 0.1sec; mu = 1/1200; 4-fiber Bidirectional OC12 W = 1 s e c ; m u = 1 / 1 2 0 0 ; 4 - fiber Bidirectional OC12
Fig. 7% increase in number of sources supported in the BCQ
% i n cr ea se i n n u m b er o f so u rc es ρρ: traffic load
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